ScienceCBSE Class 10 · Chapter 2

Acids, Bases and Salts Class 10: Concepts, Equations and Board Answers

Q: Do I need to learn the logarithm definition of pH?

No. The 2026-27 syllabus says the definition of pH relating to logarithm is not required. You need to know that pH is a number from about 0 to 14 showing acidic or basic nature, and that a lower pH means more hydrogen ions.

Q: Are the Do You Know boxes in this chapter asked in the board exam?

Information in NCERT text boxes is not assessed in the board exam. You may read them for interest, but put your revision time into the main text, the activities and the equations.

Q: What is the difference between baking soda and baking powder?

Baking soda is sodium hydrogencarbonate alone. Baking powder is a mixture of baking soda and tartaric acid, a weak edible acid. When baking powder is heated or mixed with water, carbon dioxide is released and makes cakes soft and spongy.

Q: Why is copper sulphate blue but turns white on heating?

Blue copper sulphate crystals contain five molecules of water of crystallisation per formula unit. Heating drives this water off and the salt turns white. Adding a few drops of water brings the blue colour back.

Q: Is every salt neutral?

No. A salt of a strong acid and a strong base, like sodium chloride, is neutral with pH 7. A salt of a strong acid and a weak base is acidic, and a salt of a strong base and a weak acid is basic, like sodium acetate.

Q: Why does the chlor-alkali process have that name?

It is named after two of its products: chlor for chlorine gas, given off at the anode, and alkali for sodium hydroxide, formed near the cathode. Hydrogen gas, the third product, is given off at the cathode.

Updated 14 October 2026 · Fact-checked

Acids, Bases and Salts is Chapter 2 of CBSE Class 10 Science. Acids give hydrogen ions in water and bases give hydroxide ions. The chapter covers indicators, the pH scale and important salts. The board asks for balanced equations, reasons, pH comparisons, identify-the-compound chains and case studies on the chlor-alkali process, baking soda and Plaster of Paris.

What Acids, Bases and Salts is about

This chapter explains why sour things behave alike and why bitter, soapy things behave alike. Acids release hydrogen ions, $H^{+} (a q)$ , in water. Bases release hydroxide ions, $O H^{-} (a q)$ . Once you see this, the rest follows: indicators, reactions with metals and carbonates, neutralisation, the pH scale, and the salts made when acids and bases cancel each other. The chapter ends with useful chemicals made from common salt: sodium hydroxide, bleaching powder, baking soda and washing soda, plus Plaster of Paris.

It builds directly on Chapter 1 (Chemical Reactions and Equations). Neutralisation is a double displacement reaction, heating baking soda is a decomposition, and every answer needs balanced equations with state symbols. It also prepares you for Metals and Non-metals, where metal oxides being basic and non-metal oxides being acidic is used again.

Board questions take several shapes. You will see 1-mark MCQs on pH values and indicator colours, assertion-reason items on dilution or ions, and 3-mark give-reason questions. There are also identify X, Y, Z chains (often on the chlor-alkali process or lime water) and case-based passages on pH in daily life. Practical-based questions draw on pH paper tests and on the reactions of HCl and NaOH with litmus, zinc and sodium carbonate.

Weightage: This chapter is part of Unit I, Chemical Substances: Nature and Behaviour, which carries 25 of the 80 theory marks together with Chemical Reactions and Equations, Metals and Non-metals and Carbon and its Compounds.Source: CBSE Science syllabus 2026-27, Unit I Chemical Substances: Nature and Behaviour

In real board papers: asked in 21 of the 21 board papers we hold (2023 to 2026), on average 6.6 marks per paper. See the questions

Key concepts

Acid-base indicators

NCERT section 2.1.1 Acids and Bases in the Laboratory

An indicator tells you whether a substance is acidic or basic by showing a change. Usually the change is a colour, but for some indicators it is a smell. You never taste or touch laboratory chemicals, because alkalis are corrosive. Indicators are the safe way to test them.

Litmus (natural): acids turn blue litmus red, and bases turn red litmus blue.
Turmeric (natural): a yellow curry stain turns reddish-brown when soap, which is basic, is rubbed on it. It turns yellow again once the soap is washed away with plenty of water.
Synthetic indicators: methyl orange and phenolphthalein. In Activity 2.6, phenolphthalein turns pink in NaOH solution. The pink disappears once enough acid is added.
Olfactory indicators: substances whose smell changes in acidic or basic media. In Activity 2.2, cloth strips that were kept overnight with chopped onion are tested. The onion smell fades on the strip with dilute NaOH but stays on the strip with dilute HCl. So onion works as an olfactory indicator. A basic solution such as baking soda solution removes its smell, while an acidic one such as lemon juice does not. Vanilla essence and clove oil are tested in the same way.

Quick check: a substance is useful as an indicator only if its colour or smell is different in an acid and in a base. If it looks and smells the same in both, it tells you nothing.

Distilled water is neutral, so it changes neither red nor blue litmus. Board questions often give you only one indicator and ask you to sort unknown liquids. The trick is to identify one liquid first and then use it to test the others.

Worked example

Three unlabelled test tubes P, Q and R contain dilute hydrochloric acid, dilute sodium hydroxide solution and distilled water, in some order. You are given only phenolphthalein solution. Explain how you will identify the contents of each test tube. (3 marks)

Try it first, then show the solutionSolution

Take a little liquid from each tube into three clean test tubes and add two drops of phenolphthalein to each. The one that turns pink contains sodium hydroxide, because phenolphthalein turns pink in a basic solution. (1)
The other two stay colourless, so phenolphthalein alone cannot separate the acid from water. Keep the pink mixture (NaOH + phenolphthalein) and divide it into two parts. (1)
Add the remaining liquids drop by drop, one to each part. Where the pink colour disappears, that liquid is hydrochloric acid, because it neutralises the base. Where the pink colour stays, that liquid is distilled water. (1)

Answer: The liquid that turns phenolphthalein pink is NaOH. The liquid that removes the pink colour of the NaOH–phenolphthalein mixture is dilute HCl. The liquid that leaves the pink unchanged is distilled water.

Reactions of acids and bases with metals, carbonates and hydrogencarbonates

NCERT section 2.1.2 How do Acids and Bases React with Metals? and 2.1.3 How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?

With metals: a metal pushes hydrogen out of an acid and takes its place. The result is a salt and hydrogen gas: Acid + Metal → Salt + Hydrogen. In Activity 2.3, zinc granules in dilute sulphuric acid give bubbles of gas. When the gas fills soap bubbles and a burning candle is brought near, the gas burns with a pop. That pop is the test for hydrogen.

Some bases also react with metals. Zinc warmed with sodium hydroxide solution gives sodium zincate and hydrogen. Here the negative ion of the salt is made of the metal and oxygen. This does not happen with all metals.

With carbonates and hydrogencarbonates: these react with acids to give a salt, carbon dioxide and water. You see brisk effervescence.

Testing the gas: carbon dioxide turns lime water milky because insoluble white calcium carbonate forms. If you keep passing carbon dioxide, the milkiness disappears because soluble calcium hydrogencarbonate forms.

Limestone, chalk and marble are all forms of calcium carbonate. The two gas tests tell you at once whether the solid was a metal or a carbonate.

Worked example

Dilute hydrochloric acid is added to two test tubes. Tube A contains zinc granules and tube B contains sodium hydrogencarbonate.
(a) Name the gas evolved in each tube and give one test for each. (2)
(b) Write balanced chemical equations, with state symbols, for the reactions in A and B. (1)
(c) The gas from B is passed through lime water for a long time. State what is seen at first and later, and write equations for both changes. (2)
(5 marks)

Try it first, then show the solutionSolution

(a) Tube A gives hydrogen gas (½). A burning candle or matchstick brought near it makes the gas burn with a pop sound (½).
(a) Tube B gives carbon dioxide gas (½). It turns lime water milky (½).
(b) A: $Z n (s) + 2 H C l (a q) \to Z n C l_{2} (a q) + H_{2} (g)$ (½)
(b) B: $N a H C O_{3} (s) + H C l (a q) \to N a C l (a q) + H_{2} O (l) + C O_{2} (g)$ (½)
(c) At first the lime water turns milky, because a white precipitate of calcium carbonate forms: $C a (O H)_{2} (a q) + C O_{2} (g) \to C a C O_{3} (s) + H_{2} O (l)$ (1)
(c) With excess carbon dioxide the milkiness disappears, because soluble calcium hydrogencarbonate forms: $C a C O_{3} (s) + H_{2} O (l) + C O_{2} (g) \to C a (H C O_{3})_{2} (a q)$ (1)

Answer: A gives hydrogen (burns with a pop); B gives carbon dioxide (turns lime water milky). Lime water first turns milky (CaCO₃ forms), then clears with excess CO₂ (soluble Ca(HCO₃)₂ forms).

Neutralisation and the nature of oxides

NCERT section 2.1.4 How do Acids and Bases React with each other?

When an acid and a base react, each cancels the other's effect and a salt and water form: Base + Acid → Salt + Water. This is neutralisation.

In Activity 2.6, phenolphthalein is pink in dilute NaOH. As dilute HCl is added drop by drop, the pink fades and vanishes once the base is used up. Adding a few drops of NaOH again brings the pink back, because base is once more in excess. So the colour tracks which of the two is left over.

Metal oxides: a metal oxide reacts with an acid to give a salt and water, exactly as a base does. So metal oxides are basic oxides. When copper oxide is stirred into dilute HCl, it dissolves and the solution turns blue-green because copper(II) chloride forms.

Non-metal oxides: carbon dioxide reacts with calcium hydroxide, a base, to give a salt (calcium carbonate) and water. This is just like acid + base, so non-metallic oxides are acidic.

When the same acid and base solutions are used, the volumes needed scale together. Double the base needs double the acid.

Worked example

(a) A small amount of copper oxide is added to dilute hydrochloric acid in a beaker with stirring. State two observations and write the balanced chemical equation. (2)
(b) Is copper oxide an acidic or a basic oxide? Give one reason. Also state why carbon dioxide is called an acidic oxide. (1)
(3 marks)

Try it first, then show the solutionSolution

(a) Observations: the copper oxide dissolves (½), and the solution turns blue-green (½).
(a) Equation: $C u O (s) + 2 H C l (a q) \to C u C l_{2} (a q) + H_{2} O (l)$ . The blue-green colour is due to copper(II) chloride. (1)
(b) Copper oxide is a basic oxide, because it reacts with an acid to give a salt and water, just as a base does. (½)
(b) Carbon dioxide reacts with a base, calcium hydroxide, to give a salt and water, just as an acid does. So it is an acidic oxide. (½)

Answer: CuO dissolves and the solution turns blue-green: CuO + 2HCl → CuCl₂ + H₂O. CuO is a basic oxide; CO₂ is an acidic oxide because it neutralises a base.

Hydrogen ions, hydroxide ions and dilution

NCERT section 2.2.1 What Happens to an Acid or a Base in a Water Solution?

What do all acids share? Activity 2.8 answers this. Dilute HCl and dilute H₂SO₄ make a bulb glow, but glucose and alcohol solutions do not, even though they also contain hydrogen. A glowing bulb means ions are carrying current. So acids are not acids just because they contain hydrogen. They are acids because they release $H^{+}$ ions in solution.

Activity 2.9 shows that water is essential. Dry HCl gas does not change dry blue litmus, but it turns moist blue litmus red. The $H^{+}$ ion separates only in water. It cannot exist alone, so it joins a water molecule as $H_{3} O^{+}$ (hydronium), written as $H^{+} (a q)$ .

Bases give $O H^{-}$ ions in water, and bases that dissolve in water are called alkalis. Neutralisation is then simply $H^{+} + O H^{-} \to H_{2} O$ .

Dissolving a concentrated acid or base in water is highly exothermic. So always add acid slowly to water with constant stirring. If water is added to the acid, the heat can splash the mixture out and cause burns, and the glass can crack. Dilution lowers the concentration of ions per unit volume.

Worked example

Give reasons for the following:
(a) Dry HCl gas does not change the colour of dry blue litmus paper, but turns moist blue litmus paper red. (1)
(b) Dilute sulphuric acid conducts electricity but a glucose solution does not, although both contain hydrogen. (1)
(c) While diluting concentrated sulphuric acid, the acid is added to water and not water to the acid. (1)
(3 marks)

Try it first, then show the solutionSolution

(a) $H^{+}$ ions form from HCl only in the presence of water: $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$ . Dry gas gives no $H^{+}$ ions, so it shows no acidic behaviour. Moist paper provides water, so ions form and the litmus turns red. (1)
(b) Sulphuric acid releases ions ( $H^{+}$ and $S O_{4}^{2 -}$ ) in water, and these ions carry the current. Glucose does not release ions, so its solution does not conduct. (1)
(c) Mixing the acid with water is highly exothermic. Adding water to the concentrated acid can make the mixture splash out and cause burns, and the container may break from local heating. Adding acid slowly to water with stirring spreads the heat safely. (1)

Answer: Ions form only in water. Acids conduct because they release ions, but glucose does not. Acid is added to water because the process is highly exothermic, and water added to acid can splash and cause burns.

pH scale and strength of acids and bases

NCERT section 2.3 How Strong are Acid or Base Solutions?

A universal indicator is a mixture of several indicators. It shows a different colour for each level of hydrogen ion concentration. Paper soaked in it (pH paper) is used to read the pH, a number usually from 0 (very acidic) to 14 (very alkaline). The p comes from the German word 'potenz', meaning power. For your exam, treat pH simply as a number that shows acidic or basic nature. You do not need any logarithm definition.

pH 7 is neutral.
Below 7 is acidic. The more hydronium ions there are, the lower the pH. So pH 1 is more acidic than pH 4.
From 7 up to 14, the $O H^{-}$ concentration rises, so the alkali gets stronger.

Strength depends on how many ions are produced. Take HCl and acetic acid at the same concentration. HCl gives more $H^{+}$ ions, so it is a strong acid. Acetic acid gives fewer, so it is a weak acid. This is why magnesium fizzes more vigorously in HCl than in acetic acid of equal amount and concentration. Strong and weak bases are defined in the same way, using $O H^{-}$ ions.

Worked example

Five solutions P, Q, R, S and T showed pH values of 3, 12, 7, 6 and 8 respectively with universal indicator.
(a) Which solution is neutral, which is strongly alkaline and which is weakly acidic? (1½)
(b) Arrange the solutions in increasing order of hydrogen ion concentration. (1)
(c) Which solutions will turn colourless phenolphthalein pink? (½)
(3 marks)

Try it first, then show the solutionSolution

(a) Neutral: R (pH 7) (½). Strongly alkaline: Q (pH 12) (½). Weakly acidic: S (pH 6) (½).
(b) Lower pH means higher $H^{+}$ concentration, so arrange from highest pH to lowest: Q (12) < T (8) < R (7) < S (6) < P (3). (1)
(c) Phenolphthalein turns pink in basic solutions, so Q and T (pH above 7). (½)

Answer: (a) R neutral, Q strongly alkaline, S weakly acidic. (b) Q < T < R < S < P. (c) Q and T.

Importance of pH in everyday life

NCERT section 2.3.1 Importance of pH in Everyday Life

Living things survive only within a narrow pH range, so pH matters everywhere.

Body: our body works within pH 7.0 to 7.8.
Acid rain: rain with pH below 5.6 is acid rain. When it flows into rivers, it lowers their pH and aquatic life struggles to survive.
Soil: plants need a specific soil pH. Activity 2.12 tests a soil filtrate with universal indicator paper. If soil is too acidic, a farmer can add bases such as quick lime, slaked lime or chalk.
Digestion: the stomach makes hydrochloric acid, which helps digestion. Too much acid during indigestion causes pain and irritation. Antacids, mild bases such as magnesium hydroxide (milk of magnesia), neutralise the excess.
Tooth decay: it starts when the mouth's pH falls below 5.5. Bacteria break down sugar and leftover food to form acids. Enamel, made of calcium hydroxyapatite, is the hardest substance in the body but is corroded below pH 5.5. Cleaning your mouth after eating and using basic toothpaste prevents this.
Stings: a bee sting leaves an acid, and a mild base like baking soda gives relief. Nettle hairs inject methanoic acid.

Many sour foods owe their taste to natural acids. Learn these pairs:

vinegar: acetic acid
orange and lemon: citric acid
tamarind: tartaric acid
tomato: oxalic acid
sour milk (curd): lactic acid
ant sting and nettle sting: methanoic acid

The lactic acid pair explains why the pH of milk falls as it turns into curd.

Every item is neutralisation or pH balance applied.

Worked example

Assertion (A): Tooth enamel is the hardest substance in the body, yet food left in the mouth after eating can damage it.
Reason (R): Bacteria in the mouth break down sugar and food particles to form acids, and enamel is corroded when the pH in the mouth falls below 5.5.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
(1 mark)

Try it first, then show the solutionSolution

Check A: enamel (calcium hydroxyapatite) is the hardest substance in the body and does not dissolve in water, but it can still be damaged. A is true.
Check R: bacteria form acids from leftover sugar and food, and enamel is corroded below pH 5.5. R is true.
Link: hardness does not protect enamel from acid. The acid made from leftover food lowers the pH below 5.5, and that is exactly why the enamel is damaged. So R explains A. (1)

Answer: (a) Both A and R are true and R is the correct explanation of A.

Families of salts and the pH of salts

NCERT section 2.4.2 pH of Salts

A salt forms when an acid and a base react. It takes its positive part from the base and its negative part from the acid. Salts that share the same positive or negative radical belong to one family. For example, NaCl and Na₂SO₄ are in the sodium family, and NaCl and KCl are in the chloride family. In Activity 2.13 you write formulae such as $K_{2} S O_{4}$ , $C a S O_{4}$ , $M g S O_{4}$ , $N a N O_{3}$ , $N a_{2} C O_{3}$ and $N H_{4} C l$ , and trace each to its parent acid and base.

A salt is not always neutral. Its pH depends on how strong its parents are. The stronger parent 'wins':

strong acid + strong base → neutral salt, pH 7 (sodium chloride, from HCl and NaOH)
strong acid + weak base → acidic salt, pH below 7
strong base + weak acid → basic salt, pH above 7

The book describes baking soda and washing soda as basic salts. In Activity 2.14 salts are dissolved in distilled water only, so that impurities in tap water do not change the reading.

Worked example

(a) Write the chemical formulae of potassium sulphate and ammonium chloride, and name the acid and the base from which each is obtained. (1)
(b) Group NaCl, Na₂SO₄, KCl and K₂SO₄ into families, naming each family. (1)
(c) Predict whether solutions of sodium chloride and sodium acetate are acidic, basic or neutral, with reason. (1)
(3 marks)

Try it first, then show the solutionSolution

(a) Potassium sulphate $K_{2} S O_{4}$ , from sulphuric acid and potassium hydroxide; ammonium chloride $N H_{4} C l$ , from hydrochloric acid and ammonium hydroxide. (½ + ½)
(b) Sodium family: NaCl, Na₂SO₄; potassium family: KCl, K₂SO₄; chloride family: NaCl, KCl; sulphate family: Na₂SO₄, K₂SO₄. (1)
(c) Sodium chloride comes from a strong acid (HCl) and a strong base (NaOH), so it is neutral with pH 7. (½)
(c) Sodium acetate comes from a strong base (NaOH) and a weak acid (acetic acid), so it is basic with pH above 7. (½)

Answer: K₂SO₄ (H₂SO₄ + KOH), NH₄Cl (HCl + NH₄OH); sodium, potassium, chloride and sulphate families; NaCl neutral (pH 7), sodium acetate basic (pH above 7).

Sodium hydroxide and bleaching powder from common salt

NCERT section 2.4.3 Chemicals from Common Salt

Common salt (sodium chloride) is separated from seawater. It is also mined as rock salt: large crystals, often brown from impurities, left behind when ancient seas dried up. It is a raw material for many everyday chemicals.

Sodium hydroxide (chlor-alkali process): brine, a solution of sodium chloride in water, is broken down by passing electricity through it. At the anode chlorine bubbles out. At the cathode hydrogen bubbles out, and NaOH collects in the liquid there. The name joins two of the products: 'chlor' for chlorine and 'alkali' for sodium hydroxide.

All three products are useful (NCERT Fig. 2.8):

Sodium hydroxide: making soaps and detergents, paper and artificial fibres; removing grease from metals
Chlorine: treating drinking water and swimming pools; making disinfectants, PVC, pesticides and bleaching powder
Hydrogen: fuels, margarine, and ammonia for fertilisers
Hydrogen and chlorine together give hydrochloric acid, used for cleaning steel and in medicines and cosmetics

Bleaching powder: the chlorine from this process is passed over dry slaked lime, $C a (O H)_{2}$ . Bleaching powder is written as $C a (C l O)_{2}$ , though its real composition is complex.

What bleaching powder does, in short:

it bleaches: cotton and linen in textile mills, wood pulp in paper mills, and clothes in laundries
it acts as an oxidising agent in many chemical industries
it kills germs in drinking water

Board questions often chain these steps: brine → gases → bleaching powder.

Worked example

When electricity is passed through a concentrated aqueous solution of compound P, which is used in food, a solution Q forms near one electrode. Gas R is given off at the anode and gas S at the cathode. Gas R is passed over dry slaked lime to give compound T, which is used to make drinking water germ-free.
(a) Identify P, Q, R, S and T. (2½)
(b) Name the process and state why it is so named. (½)
(c) Write balanced chemical equations for the formation of Q and of T. (2)
(5 marks)

Try it first, then show the solutionSolution

(a) P = sodium chloride, NaCl (½); Q = sodium hydroxide, NaOH (½); R = chlorine, $C l_{2}$ (½); S = hydrogen, $H_{2}$ (½); T = bleaching powder, $C a (C l O)_{2}$ (½).
(b) Chlor-alkali process, named after its products: chlorine ('chlor') and sodium hydroxide ('alkali'). (½)
(c) $2 N a C l (a q) + 2 H_{2} O (l) electricity 2 N a O H (a q) + C l_{2} (g) + H_{2} (g)$ (1)
(c) $2 C a (O H)_{2} + 2 C l_{2} \to C a (C l O)_{2} + C a C l_{2} + 2 H_{2} O$ (1)

Answer: P NaCl, Q NaOH, R Cl₂ (anode), S H₂ (cathode), T bleaching powder Ca(ClO)₂; the chlor-alkali process; both equations as written above.

Baking soda, washing soda, water of crystallisation and Plaster of Paris

NCERT section 2.4.4 Are the Crystals of Salts really Dry?

Baking soda is sodium hydrogencarbonate, $N a H C O_{3}$ . It is made from NaCl, water, carbon dioxide and ammonia. It is a mild, non-corrosive basic salt, so it can neutralise acids. On heating it gives sodium carbonate, water and $C O_{2}$ . Where you meet it:

in baking powder (baking soda mixed with tartaric acid), where the $C O_{2}$ given off makes cakes soft and spongy
in antacids, to cancel extra stomach acid
in soda-acid fire extinguishers
in the kitchen, for crispy pakoras

Washing soda is $N a_{2} C O_{3} \cdot 10 H_{2} O$ . You make it by heating baking soda to get sodium carbonate and then recrystallising that. It is basic. It is used in making glass, soap and paper, in making borax, for cleaning at home, and for removing permanent hardness of water.

Water of crystallisation is the water that is part of a salt's crystal, in a fixed number of molecules for each formula unit. Blue copper sulphate ( $C u S O_{4} \cdot 5 H_{2} O$ ) turns white on heating as it loses this water, and turns blue again when moistened. The crystals look dry because this water is locked inside the crystal.

Gypsum ( $C a S O_{4} \cdot 2 H_{2} O$ ) heated at 373 K becomes Plaster of Paris, $C a S O_{4} \cdot \frac{1}{2} H_{2} O$ . Two formula units share one water molecule, hence the half. With water it sets back into hard gypsum. Doctors use it to hold fractured bones in place. It is also used for toys, decorations and smoothing surfaces. Store it in a moisture-proof container.

Worked example

A student heats a few blue crystals of salt X in a dry boiling tube. The crystals turn white and droplets appear on the cooler part of the tube. After cooling, she adds 2–3 drops of water to the white solid.
(a) Name X and write its chemical formula. (1)
(b) Explain the colour change on heating and the source of the droplets. (1)
(c) State what happens when water is added, and explain why. (1)
(3 marks)

Try it first, then show the solutionSolution

(a) X is hydrated copper sulphate (½); formula $C u S O_{4} \cdot 5 H_{2} O$ (½).
(b) On heating, the crystals lose their water of crystallisation (five water molecules per formula unit), so the salt becomes white. The droplets are this water, which condenses on the cooler part of the tube. (1)
(c) The blue colour returns (½), because the white salt takes up water again and the hydrated, blue form is restored (½).

Answer: X is copper sulphate pentahydrate, CuSO₄·5H₂O. Heating removes its water of crystallisation, making it white, and the droplets are that water. Adding water restores the blue colour.

Science in real life: Acids, Bases and Salts

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Acid-base indicators

Suppose a little turmeric dal spills on your white school shirt. Your mother scrubs the yellow stain with soap and it turns reddish-brown. After she rinses it in plenty of water, it turns yellow again.

(a) What do the colour changes tell you about the nature of soap? (b) Why does the stain become yellow again after rinsing? (c) If a few drops of washing soda solution fell on another turmeric stain, what would you expect, and why?

Try it first, then show the solutionSolution

Turmeric is a natural acid-base indicator.
It turns reddish-brown in a basic medium, so the soap must be basic.
Rinsing with plenty of water washes the soap away, so the stain is no longer in a basic medium and returns to its yellow colour.
Washing soda is a basic salt, so it would also turn the turmeric stain reddish-brown.

Answer: Soap is basic. The stain turns yellow again because the soap is washed away. Washing soda would turn the stain reddish-brown because it is basic.

Memory hook: Turmeric blushes reddish-brown when a base touches it.

Reactions of acids and bases with metals, carbonates and hydrogencarbonates

Imagine some vinegar spills on a marble kitchen counter and you notice tiny bubbles fizzing on the surface. Marble is a form of calcium carbonate, and vinegar contains acetic acid.

(a) Name the gas causing the bubbles and describe how you would confirm it. (b) Write the word equation and the balanced chemical equation for the reaction. (c) Why would leaving acidic liquids on marble slowly damage its surface?

Try it first, then show the solutionSolution

Calcium carbonate is a metal carbonate; with an acid it gives a salt, carbon dioxide and water.
The gas is carbon dioxide. Passing it through lime water turns the lime water milky.
Word equation: calcium carbonate + acetic acid → calcium acetate + water + carbon dioxide.
Balanced: $C a C O_{3} (s) + 2 C H_{3} C O O H (a q) \to (C H_{3} C O O)_{2} C a (a q) + H_{2} O (l) + C O_{2} (g)$ . Check the atoms: Ca 1 = 1, C 5 = 5, H 8 = 8, O 7 = 7.
The acid uses up the calcium carbonate of the marble in this reaction, so the surface is slowly eaten away.

Answer: The gas is carbon dioxide (turns lime water milky). The reaction is CaCO₃ + 2CH₃COOH → (CH₃COO)₂Ca + H₂O + CO₂. Acid reacts with and uses up the marble, so the surface is damaged.

Memory hook: Acid on marble fizzes because carbonate plus acid gives CO₂.

Neutralisation and the nature of oxides

Suppose your uncle has acidity after overeating at a wedding. In the kitchen there is lemon juice, vinegar and baking soda.

(a) Which of the three would you dissolve in water and suggest as a remedy, and why? (b) Why would the other two not help? (c) Write the equation for the reaction of your remedy with the hydrochloric acid in the stomach.

Try it first, then show the solutionSolution

Acidity means the stomach has made too much hydrochloric acid.
Excess acid is removed by a mild base through neutralisation.
Baking soda (sodium hydrogencarbonate) is a mild, non-corrosive basic salt, so its solution is the right choice.
Lemon juice (citric acid) and vinegar (acetic acid) are acids; they would add acid, not neutralise it.
$N a H C O_{3} + H C l \to N a C l + H_{2} O + C O_{2}$

Answer: Baking soda solution. It is a mild base that neutralises excess stomach acid: NaHCO₃ + HCl → NaCl + H₂O + CO₂. Lemon juice and vinegar are acids and would make it worse.

Memory hook: Too much acid? Send in a mild base.

Importance of pH in everyday life

Imagine a student who eats a sweet after lunch every day at school and brushes only at bedtime. After some months, the dentist finds early tooth decay.

(a) Below what pH does tooth decay begin, and what produces the acid? (b) Name the substance of the teeth that is attacked and state one of its properties. (c) Suggest two habits that would protect the teeth, with the chemistry behind each.

Try it first, then show the solutionSolution

Tooth decay starts when the pH in the mouth falls below 5.5.
Bacteria in the mouth break down sugar and food particles left after eating to form acids.
Tooth enamel, made of calcium hydroxyapatite, is attacked. It is the hardest substance in the body and does not dissolve in water, but it is corroded below pH 5.5.
Habit 1: clean the mouth after eating, so that less sugar is left for bacteria to turn into acid.
Habit 2: brush with toothpaste, which is generally basic and neutralises the excess acid, keeping the pH above 5.5.

Answer: Decay begins below pH 5.5, from acid formed by bacteria acting on sugar. Enamel (calcium hydroxyapatite, the hardest substance in the body) is corroded. Rinsing after meals and brushing with basic toothpaste protect the teeth.

Memory hook: Five-point-five or above keeps your teeth alive.

Sodium hydroxide and bleaching powder from common salt

Suppose your school decides to disinfect the water in its overhead tank and the caretaker uses a white powder that smells of chlorine.

(a) Name the powder and its chemical formula as given in NCERT. (b) From which industrial process does its key raw material come? Write the equation for that process. (c) Write the equation for making the powder, and give two other uses of it.

Try it first, then show the solutionSolution

The powder is bleaching powder, represented as $C a (C l O)_{2}$ ; one of its uses is making drinking water free from germs.
Its key raw material is chlorine, obtained from the chlor-alkali process (electrolysis of brine).
$2 N a C l (a q) + 2 H_{2} O (l) electricity 2 N a O H (a q) + C l_{2} (g) + H_{2} (g)$
Chlorine acts on dry slaked lime: $2 C a (O H)_{2} + 2 C l_{2} \to C a (C l O)_{2} + C a C l_{2} + 2 H_{2} O$
Other uses: bleaching cotton and linen in the textile industry; acting as an oxidising agent in chemical industries.

Answer: The powder is bleaching powder, Ca(ClO)₂. It is made from chlorine produced by the chlor-alkali process, reacted with dry slaked lime. It is also used for bleaching cotton and linen and as an oxidising agent.

Memory hook: Brine gives chlorine, chlorine plus lime gives bleach.

Baking soda, washing soda, water of crystallisation and Plaster of Paris

Imagine your aunt is baking a cake for a birthday. She adds baking powder to the batter, and in the oven the cake rises and becomes soft and spongy.

(a) What is baking powder made of? (b) Which gas makes the cake spongy, and why does including tartaric acid help release it even when the batter is only mixed with water? (c) Write the equation for what happens if baking soda alone is heated.

Try it first, then show the solutionSolution

Baking powder is a mixture of baking soda (sodium hydrogencarbonate) and tartaric acid, a weak edible acid.
When heated or mixed in water, the hydrogencarbonate reacts with the acid's hydrogen ions: $N a H C O_{3} + H^{+} \to N a^{+} + C O_{2} + H_{2} O$ (the $N a^{+}$ ends up in the sodium salt of the acid).
The carbon dioxide bubbles make the cake rise, so it becomes soft and spongy.
Tartaric acid supplies the $H^{+}$ ions, so carbon dioxide is released through the hydrogencarbonate + acid reaction, not only by heating.
Baking soda alone on heating: $2 N a H C O_{3} heat N a_{2} C O_{3} + H_{2} O + C O_{2}$

Answer: Baking powder is baking soda plus tartaric acid. Carbon dioxide makes the cake spongy; the acid reacts with the hydrogencarbonate to release it. Baking soda alone on heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.

Memory hook: Soda plus acid gives CO₂, and CO₂ lifts the cake.

Topics in this chapter

Acids and Bases: Definitions and IndicatorsComing soon
Chemical Properties of Acids and Bases
pH Scale and Its Importance
Salts: Family and pH of SaltsComing soon
Chemicals from Common Salt: NaOH and Bleaching PowderComing soon
Baking Soda, Washing Soda and Plaster of Paris

Acids, Bases and Salts: formulas and key facts

Acid + metal: $Z n (s) + H_{2} S O_{4} (a q) \to Z n S O_{4} (a q) + H_{2} (g)$; General form: Acid + Metal → Salt + Hydrogen. Use the same pattern for Mg, Al and Fe, for example $2 A l + 3 H_{2} S O_{4} \to A l_{2} (S O_{4})_{3} + 3 H_{2}$ . Hydrogen burns with a pop.
Base + metal (sodium zincate): $2 N a O H (a q) + Z n (s) \to N a_{2} Z n O_{2} (s) + H_{2} (g)$; Happens on warming. It shows that some bases also give hydrogen with metals, but not all metals react this way.
Metal carbonate + acid: $N a_{2} C O_{3} (s) + 2 H C l (a q) \to 2 N a C l (a q) + H_{2} O (l) + C O_{2} (g)$; Remember the 2HCl. For limestone, chalk or marble: $C a C O_{3} + 2 H C l \to C a C l_{2} + H_{2} O + C O_{2}$ .
Metal hydrogencarbonate + acid: $N a H C O_{3} (s) + H C l (a q) \to N a C l (a q) + H_{2} O (l) + C O_{2} (g)$; One HCl only. This is the reaction used in soda-acid fire extinguishers and baking powder.
Lime water test for carbon dioxide: $C a (O H)_{2} (a q) + C O_{2} (g) \to C a C O_{3} (s) + H_{2} O (l)$; The milkiness is the white precipitate of $C a C O_{3}$ . Also an example of a non-metal oxide reacting with a base.
Excess carbon dioxide: $C a C O_{3} (s) + H_{2} O (l) + C O_{2} (g) \to C a (H C O_{3})_{2} (a q)$; Explains why the milkiness disappears: calcium hydrogencarbonate is soluble in water.
Neutralisation: $N a O H (a q) + H C l (a q) \to N a C l (a q) + H_{2} O (l)$; General: Base + Acid → Salt + Water. In ionic form: $H^{+} (a q) + O H^{-} (a q) \to H_{2} O (l)$ .
Metal oxide + acid: $C u O (s) + 2 H C l (a q) \to C u C l_{2} (a q) + H_{2} O (l)$; The solution turns blue-green. This is why metal oxides are called basic oxides.
Ions in water: $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$; Acids give $H_{3} O^{+}$ , also written $H^{+} (a q)$ , only in water. Bases give hydroxide ions: $N a O H (s) H_{2} O N a^{+} (a q) + O H^{-} (a q)$ .
pH scale (key fact): pH 7 neutral; below 7 acidic; above 7 basic; lower pH means more $H^{+}$ ions; The scale usually runs 0 to 14. The logarithm definition is not required. Use this for comparison and ordering questions.
pH of salts (key fact): Strong acid + strong base pH 7
strong acid + weak base pH below 7
strong base + weak acid pH above 7; Predicts the nature of a salt solution, for example NaCl is neutral.
Chlor-alkali process: $2 N a C l (a q) + 2 H_{2} O (l) electricity 2 N a O H (a q) + C l_{2} (g) + H_{2} (g)$; Chlorine forms at the anode, hydrogen at the cathode, and NaOH near the cathode.
Bleaching powder: $2 C a (O H)_{2} + 2 C l_{2} \to C a (C l O)_{2} + C a C l_{2} + 2 H_{2} O$; Chlorine acts on dry slaked lime. Used for bleaching, as an oxidising agent and to disinfect drinking water.
Baking soda: preparation and heating: $N a C l + H_{2} O + C O_{2} + N H_{3} \to N H_{4} C l + N a H C O_{3}; 2 N a H C O_{3} heat N a_{2} C O_{3} + H_{2} O + C O_{2}$; The second equation explains rising in cooking and is the first step towards washing soda.
Washing soda: $N a_{2} C O_{3} + 10 H_{2} O \to N a_{2} C O_{3} \cdot 10 H_{2} O$; Made by recrystallising sodium carbonate. It is a basic salt and removes permanent hardness of water.
Plaster of Paris: $C a S O_{4} \cdot 2 H_{2} O 373 K C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O; C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O \to C a S O_{4} \cdot 2 H_{2} O$; Gypsum heated gives Plaster of Paris; Plaster of Paris with water sets back to gypsum. Water counts per formula unit: CuSO₄ has 5, washing soda 10, gypsum 2, Plaster of Paris ½.

Strong acid + strong base	pH 7
strong acid + weak base	pH below 7
strong base + weak acid	pH above 7

Acids, Bases and Salts: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Reaction of zinc granules with dilute sulphuric acid and testing hydrogen gas (Fig. 2.1)

Draw a labelled diagram of the set-up to show that a metal reacting with an acid gives hydrogen, and how the gas is tested.

Draw

A test tube held in a stand, containing dilute sulphuric acid with zinc granules at the bottom; show small bubbles on the zinc.
A cork in the mouth of the test tube with a bent delivery tube coming out of it.
The other end of the delivery tube dipping into soap solution in a second container; draw gas-filled bubbles rising.
A burning candle held near a gas-filled bubble, with a small mark to show the pop sound.

Label

Stand
Test tube
Dilute sulphuric acid
Zinc granules
Cork
Delivery tube
Soap solution
Bubbles filled with hydrogen gas
Burning candle
Hydrogen gas burning with a pop sound

Watch out: The delivery tube must start above the acid level inside the test tube. If it dips into the acid, the liquid will be pushed out instead of the gas.

Passing carbon dioxide through lime water (Fig. 2.2)

Draw a labelled diagram to show that a metal carbonate or hydrogencarbonate gives carbon dioxide with an acid.

Draw

A test tube with sodium carbonate (or sodium hydrogencarbonate) and dilute hydrochloric acid, showing effervescence.
A cork with a delivery tube leading to a second test tube.
The second test tube containing lime water, with the delivery tube dipping into it.
Show the lime water turning milky.

Label

Test tube
Sodium carbonate / sodium hydrogencarbonate
Dilute HCl
Cork
Delivery tube
Carbon dioxide gas
Lime water (calcium hydroxide solution)
Lime water turns milky

Watch out: In the reaction tube the delivery tube stays above the liquid, but in the lime water tube it must dip into the liquid. Do not draw both the same way.

Acid solution in water conducts electricity (Fig. 2.3)

Describe or draw the activity that shows acids give ions in water, or that glucose and alcohol are not acids.

Draw

A 100 mL beaker containing dilute HCl.
A cork placed in the beaker with two nails fixed in it, dipping into the solution.
Wires from the two nails to a 6 volt battery through a bulb and a switch, making a complete circuit.
Show the bulb glowing.

Label

Beaker
Dilute HCl solution
Cork
Nails
Bulb (glowing)
Switch
6 V battery
Connecting wires

Watch out: The nails must not touch each other, and the circuit must be closed through the switch. Mention that the bulb does not glow with glucose or alcohol solution.

Preparation of HCl gas (Fig. 2.4)

Draw the set-up to prepare HCl gas and test it with dry and wet blue litmus paper.

Draw

A clean, dry test tube with solid sodium chloride, and concentrated sulphuric acid added to it.
A cork with a delivery tube leading the gas out.
Strips of dry and wet blue litmus paper held at the mouth of the delivery tube.
Optional: a guard tube with calcium chloride in the path of the gas to dry it in humid weather.

Label

Test tube
Solid sodium chloride
Concentrated sulphuric acid
Cork
Delivery tube
HCl gas
Dry blue litmus paper (no change)
Wet blue litmus paper (turns red)
Guard tube with calcium chloride

Watch out: Write clearly which litmus paper changes colour. Only the wet one turns red, because $H^{+}$ ions form only in the presence of water.

Removing water of crystallisation from copper sulphate (Fig. 2.9)

Draw a labelled diagram to show that copper sulphate crystals contain water of crystallisation.

Draw

A dry boiling tube held at an angle by a test tube holder.
Blue copper sulphate crystals at the bottom of the tube.
A burner heating the bottom of the tube.
Water droplets on the cooler upper part of the tube.

Label

Boiling tube
Test tube holder
Copper sulphate crystals
Burner / flame
Water droplets

Watch out: Keep the mouth of the tube slightly downward or show the droplets on the cooler part. Say the crystals turn white on heating and blue again when water is added.

How the board asks Acids, Bases and Salts

The question patterns to expect, and the method that works for each.

Question type	How it is asked	Method
MCQ on pH, indicators and reactions	A 1-mark question with four options. It may give a pH value and ask for the nature of the solution, ask which indicator changes colour, or ask which gas turns lime water milky.	Turn every pH into words first (below 7 acid, 7 neutral, above 7 base). For gas questions, recall that a metal gives hydrogen and a carbonate gives carbon dioxide. Eliminate options that break one of these rules.
Assertion-reason	Statements on dilution (acid to water), dry versus moist HCl, conduction by glucose, olfactory indicators or the pH of salts.	Judge A and R separately as true or false. Only then ask whether R is the actual cause of A. A common trap is an R that is true but describes a different property.
Identify X, Y, Z chains	A passage describes electrolysis of brine, a gas burning with a pop, a gas used for disinfecting water, or lime water turning milky and then clear. You must name each substance and write equations.	Anchor on the clearest clue (a pop means hydrogen; lime water milky means carbon dioxide; brine electrolysis means chlorine, hydrogen and NaOH). Name every letter with its formula and write each equation balanced.
Give reasons (3 marks)	Three short 'why' parts. Examples: Plaster of Paris stored moisture-proof, dry HCl not acidic, acid solution conducts, milkiness disappears with excess CO₂, fizzing more vigorous in HCl than in acetic acid.	Write one cause-and-effect sentence per part, using the key term (ions, exothermic, water of crystallisation, strong acid). Add an equation wherever one exists, because it often carries the mark.
Long answer on salts	5 marks: preparation of NaOH, bleaching powder, baking soda, washing soda or Plaster of Paris, with equations and uses.	Set out the answer in order: name, raw material, equation with condition, then two uses. Learn the uses in your own words so that you can recall any two of them quickly.
Case-based on pH in daily life	A passage on antacids, tooth decay, acid rain, soil or stings, followed by 1- and 2-mark sub-parts.	Pick out the numbers in the passage (pH 5.5 for teeth, 5.6 for rain, 7.0–7.8 for the body). Explain every remedy as neutralisation: an acid problem needs a mild base, and a base problem needs a mild acid.
Practical and activity-based	Observations from pH paper tests on HCl, NaOH, ethanoic acid, lemon juice, water and hydrogencarbonate solution. Reactions of HCl and NaOH with litmus, zinc and sodium carbonate. Ethanoic acid with sodium hydrogencarbonate.	State what is done, what is seen, and what it shows, plus one precaution (handle acids carefully, use clean pH paper, add acid to water). For ethanoic acid with NaHCO₃, expect brisk effervescence of carbon dioxide.

Tricks to remember Acids, Bases and Salts

Short memory aids. Say each one aloud once, then use it on a question the same day.

Always Add Acid
Think 'AAA': Always Add Acid to water, slowly, with stirring. Never pour water into concentrated acid.
Use it when: Assertion-reason and give-reason questions on dilution, plus practical precautions.
Anode gets the Anion
Chloride ions are negative, so chlorine appears at the Anode. Hydrogen comes out at the Cathode, and NaOH forms Near the Cathode.
Use it when: Chlor-alkali questions that ask which product forms at which electrode.
5-10-2-½ water count
Copper sulphate 5, washing soda 10, gypsum 2, Plaster of Paris ½. Say it as 'Copper five, Washing ten, Gypsum two, PoP half'.
Use it when: Writing formulae of hydrated salts without slips.
Milky then clear
A little CO₂ makes lime water milky (insoluble CaCO₃). A lot of CO₂ clears it (soluble Ca(HCO₃)₂).
Use it when: Lime water observations and their two equations.
Strong parent wins
Salt pH follows the stronger parent: strong acid gives an acidic salt, strong base gives a basic salt, and two strong parents give a neutral salt (pH 7).
Use it when: Predicting the pH or nature of a salt solution.
Low pH, lots of H⁺
pH and H⁺ move in opposite directions: when one goes down, the other goes up. pH 1 has far more H⁺ than pH 6.
Use it when: Arranging solutions in order of hydrogen ion concentration.

Acids, Bases and Salts: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Example 1: Ordering pH values from a practical · 1 mark · MCQBoard level

In a practical, a student uses pH paper to test four samples: lemon juice, distilled water, dilute sodium hydrogencarbonate solution and dilute sodium hydroxide solution. Which shows the samples in increasing order of pH?
(a) Lemon juice < distilled water < sodium hydrogencarbonate < sodium hydroxide
(b) Sodium hydroxide < sodium hydrogencarbonate < distilled water < lemon juice
(c) Lemon juice < sodium hydrogencarbonate < distilled water < sodium hydroxide
(d) Distilled water < lemon juice < sodium hydroxide < sodium hydrogencarbonate

Try it first, then show the solutionSolution

Lemon juice is acidic (it contains citric acid), so its pH is below 7. It is the lowest.
Distilled water is neutral, pH 7.
Sodium hydrogencarbonate is a mild basic salt, so its pH is a little above 7.
Sodium hydroxide is a strong alkali, so its pH is the highest.

Answer: (a) Lemon juice < distilled water < sodium hydrogencarbonate < sodium hydroxide

Example 2: Do all metals react with bases? · 1 mark · Assertion-reasonBoard level

Assertion (A): Zinc gives hydrogen gas with dilute hydrochloric acid and also when warmed with sodium hydroxide solution.
Reason (R): All metals react with bases to give hydrogen gas.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.

Try it first, then show the solutionSolution

Zinc reacts with dilute HCl to give zinc chloride and hydrogen, and with warm NaOH to give sodium zincate and hydrogen. So A is true.
The NCERT text points out that such reactions with bases are not possible with all metals. So R is false.

Answer: (c) A is true but R is false.

Example 3: Equations for reactions with metals and carbonates · 3 marks · Short answerBoard level

Write the word equation and the balanced chemical equation, with state symbols, for each of the following:
(a) Magnesium ribbon reacts with dilute sulphuric acid.
(b) Zinc granules are warmed with sodium hydroxide solution.
(c) Dilute hydrochloric acid is added to sodium carbonate.
(3 marks)

Try it first, then show the solutionSolution

(a) Word: magnesium + sulphuric acid → magnesium sulphate + hydrogen (½)
$M g (s) + H_{2} S O_{4} (a q) \to M g S O_{4} (a q) + H_{2} (g)$ (½)
(b) Word: zinc + sodium hydroxide → sodium zincate + hydrogen (½)
$Z n (s) + 2 N a O H (a q) \to N a_{2} Z n O_{2} (s) + H_{2} (g)$ (½)
(c) Word: sodium carbonate + hydrochloric acid → sodium chloride + water + carbon dioxide (½)
$N a_{2} C O_{3} (s) + 2 H C l (a q) \to 2 N a C l (a q) + H_{2} O (l) + C O_{2} (g)$ (½)

Answer: (a) $M g (s) + H_{2} S O_{4} (a q) \to M g S O_{4} (a q) + H_{2} (g)$
(b) $Z n (s) + 2 N a O H (a q) \to N a_{2} Z n O_{2} (s) + H_{2} (g)$
(c) $N a_{2} C O_{3} (s) + 2 H C l (a q) \to 2 N a C l (a q) + H_{2} O (l) + C O_{2} (g)$

Example 4: Reasons based on pH in daily life · 3 marks · Short answerHarder board level

Give reasons for the following:
(a) A person with indigestion is given milk of magnesia.
(b) Fish and other aquatic animals find it hard to survive when acid rain flows into a river.
(c) Applying baking soda on the skin gives relief from a honey-bee sting.
(3 marks)

Try it first, then show the solutionSolution

(a) During indigestion the stomach produces too much hydrochloric acid. Milk of magnesia (magnesium hydroxide) is a mild base, an antacid, that neutralises the excess acid. (1)
(b) Acid rain has pH below 5.6. It lowers the pH of river water, and living organisms can survive only within a narrow range of pH, so aquatic life is harmed. (1)
(c) A bee sting leaves an acid that causes pain and irritation. Baking soda is a mild base that neutralises this acid. (1)

Answer: Each case is neutralisation or a pH change: (a) a mild base neutralises excess stomach acid; (b) acid rain lowers river pH outside the narrow range aquatic life can bear; (c) a mild base neutralises the acid of the sting.

Example 5: Baking soda: making, heating and uses · 5 marks · Long answerHarder board level

(a) Give the chemical name and formula of baking soda, and write the equation for its preparation from sodium chloride.
(b) Write the equation for the reaction that takes place when baking soda is heated during cooking.
(c) What is baking powder? Explain, with an equation, how it makes a cake soft and spongy.
(d) Give two other uses of baking soda.
(5 marks)

Try it first, then show the solutionSolution

(a) Sodium hydrogencarbonate, $N a H C O_{3}$ . (½)
$N a C l + H_{2} O + C O_{2} + N H_{3} \to N H_{4} C l + N a H C O_{3}$ (1)
(b) $2 N a H C O_{3} heat N a_{2} C O_{3} + H_{2} O + C O_{2}$ (1)
(c) Baking powder is a mixture of baking soda and tartaric acid, a weak edible acid. (½)
When heated or mixed with water: $N a H C O_{3} + H^{+} \to N a^{+} + C O_{2} + H_{2} O$ (the $N a^{+}$ ends up in the sodium salt of the acid). (½)
The carbon dioxide produced makes the bread or cake rise, so it becomes soft and spongy. (½)
(d) It is an ingredient in antacids, neutralising excess stomach acid; it is used in soda-acid fire extinguishers. (1)

Answer: Baking soda is $N a H C O_{3}$ , made by $N a C l + H_{2} O + C O_{2} + N H_{3} \to N H_{4} C l + N a H C O_{3}$ . On heating, $2 N a H C O_{3} \to N a_{2} C O_{3} + H_{2} O + C O_{2}$ . In baking powder its reaction with tartaric acid releases $C O_{2}$ , which makes the cake rise. Other uses: antacids and soda-acid fire extinguishers.

Example 6: Water of crystallisation, washing soda and Plaster of Paris · 5 marks · Long answerHarder board level

(a) What is meant by water of crystallisation? Write the formulae of hydrated copper sulphate and washing soda.
(b) How is washing soda obtained from baking soda? Write the equations involved.
(c) How is Plaster of Paris made from gypsum? What happens when it is mixed with water? Write both equations. Why is only half a water molecule shown in its formula?
(d) Why should Plaster of Paris be stored in a moisture-proof container?
(5 marks)

Try it first, then show the solutionSolution

(a) Water of crystallisation is the water that forms part of a salt's crystal, with a set number of water molecules for each formula unit. (½)
Hydrated copper sulphate: $C u S O_{4} \cdot 5 H_{2} O$ ; washing soda: $N a_{2} C O_{3} \cdot 10 H_{2} O$ . (½)
(b) Baking soda is heated to get sodium carbonate: $2 N a H C O_{3} heat N a_{2} C O_{3} + H_{2} O + C O_{2}$ (1)
Sodium carbonate is recrystallised: $N a_{2} C O_{3} + 10 H_{2} O \to N a_{2} C O_{3} \cdot 10 H_{2} O$ (½)
(c) Gypsum is heated at 373 K: $C a S O_{4} \cdot 2 H_{2} O 373 K C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O$ (½)
With water it sets back to gypsum, a hard mass: $C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O \to C a S O_{4} \cdot 2 H_{2} O$ (1)
Half a water molecule is shown because two formula units of $C a S O_{4}$ share one water molecule. (½)
(d) Plaster of Paris reacts with moisture and turns into hard gypsum, so it would become useless if exposed to damp air. (½)

Answer: Water of crystallisation is water held inside a salt's crystal, a set number of molecules per formula unit ( $C u S O_{4} \cdot 5 H_{2} O$ , $N a_{2} C O_{3} \cdot 10 H_{2} O$ ). For washing soda, heat baking soda and then recrystallise the sodium carbonate. Plaster of Paris is made by heating gypsum at 373 K. It sets back to gypsum with water, so it must be kept away from moisture.

Example 7: Find and correct the errors · 3 marks · Short answerHOTS / competency

A student wrote these three statements in a test. Find the error in each, correct it and give a reason.
(i) Every compound that contains hydrogen is an acid.
(ii) When an acid solution is diluted with water, the concentration of hydronium ions per unit volume increases.
(iii) A salt of a strong acid and a weak base gives a solution with pH more than 7.
(3 marks)

Try it first, then show the solutionSolution

(i) Wrong. Glucose and alcohol contain hydrogen, but their solutions do not conduct electricity and do not show acidic character. An acid is a substance that produces $H^{+} (a q)$ ions in water. (1)
(ii) Wrong. Dilution spreads the ions through a larger volume, so the $H_{3} O^{+}$ concentration per unit volume decreases. (1)
(iii) Wrong. A salt of a strong acid and a weak base is acidic, with pH less than 7. It is a salt of a strong base and a weak acid that has pH above 7. (1)

Answer: (i) Only substances that give $H^{+} (a q)$ in water are acids; glucose and alcohol are not. (ii) Dilution decreases $H_{3} O^{+}$ concentration. (iii) Strong acid + weak base gives an acidic salt, pH below 7.

Example 8: Neutralisation with phenolphthalein: data and reasoning · 3 marks · Short answerHOTS / competency

A student takes 15 mL of dilute NaOH solution and adds two drops of phenolphthalein. She then adds dilute HCl drop by drop and finds that exactly 10 mL of the acid neutralises the base.
(a) State the colour of the mixture at the start and at the point of neutralisation. Why does it change?
(b) How much of the same HCl would be needed to neutralise 45 mL of the same NaOH solution?
(c) She now adds a few more drops of NaOH to the neutral mixture. What will she see and why?
(3 marks)

Try it first, then show the solutionSolution

(a) At the start the solution is pink, because phenolphthalein turns pink in a base. At neutralisation it becomes colourless, because the acid has neutralised all the base: $N a O H (a q) + H C l (a q) \to N a C l (a q) + H_{2} O (l)$ . (1)
(b) 45 mL is 3 times 15 mL, so acid needed $= 3 \times 10 = 30$ mL. (1)
(c) The pink colour reappears, because the extra NaOH makes the solution basic again. An acid and a base cancel each other's effect, so whichever is in excess decides the colour. (1)

Answer: (a) Pink to colourless, as the acid neutralises the base. (b) 30 mL. (c) The pink colour returns because the solution becomes basic again.

Example 9: Milk, curd and baking soda · 3 marks · Short answerHarder board level

(a) Fresh milk has a pH of about 6. How will its pH change as it turns into curd? Explain. (1)
(b) A milkman adds a very small amount of baking soda to fresh milk. Why does this shift the pH of the milk from 6 to slightly alkaline? (1)
(c) Why does this milk take longer to set as curd? (1)
(3 marks)

Try it first, then show the solutionSolution

(a) Curd (sour milk) contains lactic acid. As milk sets into curd, lactic acid forms, so the $H^{+}$ concentration rises and the pH falls below 6. (1)
(b) Baking soda is a mild basic salt. It neutralises the slight acidity of the milk, so the pH moves to just above 7. This slows the souring of the milk. (1)
(c) Curd sets only when enough lactic acid builds up to lower the pH. The acid formed at first is neutralised by the baking soda, so it takes longer for the pH to drop and the milk sets late. (1)

Answer: (a) The pH falls below 6, because lactic acid forms. (b) Baking soda is a mild base that neutralises the acid, making the milk slightly alkaline. (c) The lactic acid formed at first is neutralised, so the pH drops slowly and setting is delayed.

Example 10: Matching chlor-alkali products, electrodes and uses · 1 mark · MCQHOTS / competency

A student writes four statements about the chlor-alkali process:
I. Hydrogen is given off at the cathode and can be used as a fuel.
II. Chlorine is given off at the anode and is used to make margarine.
III. Sodium hydroxide forms near the cathode and is used for making soaps and detergents.
IV. Chlorine from the process reacts with dry slaked lime to give bleaching powder.
Which statements are correct?
(a) I and II only
(b) I, III and IV only
(c) II and IV only
(d) I, II, III and IV

Try it first, then show the solutionSolution

I: hydrogen forms at the cathode and is used in fuels. Correct.
II: chlorine does form at the anode, but margarine is made using hydrogen, not chlorine. Wrong.
III: NaOH collects near the cathode and is used for soaps and detergents. Correct.
IV: chlorine acts on dry slaked lime to give bleaching powder. Correct.

Answer: (b) I, III and IV only

Example 11: Identify the salts: a four-step chain · 5 marks · Long answerHarder board level

A white powder P is used by doctors to hold fractured bones in place. P is made by heating a salt Q at 373 K. A blue crystalline salt R turns white on heating and blue again when water is added. A white salt S, used in antacids, gives off a gas T on heating that turns lime water milky.
(a) Identify P, Q, R, S and T, with formulae. (2½)
(b) Write the balanced equations for the formation of P from Q, and for the action of heat on S. (2)
(c) Which of P, Q and R has the most water of crystallisation per formula unit? (½)
(5 marks)

Try it first, then show the solutionSolution

(a) P: Plaster of Paris, $C a S O_{4} \cdot \frac{1}{2} H_{2} O$ (½); Q: gypsum, $C a S O_{4} \cdot 2 H_{2} O$ (½); R: hydrated copper sulphate, $C u S O_{4} \cdot 5 H_{2} O$ (½); S: baking soda, $N a H C O_{3}$ (½); T: carbon dioxide, $C O_{2}$ (½).
(b) $C a S O_{4} \cdot 2 H_{2} O 373 K C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O$ (1)
(b) $2 N a H C O_{3} heat N a_{2} C O_{3} + H_{2} O + C O_{2}$ (1)
(c) R, with 5 water molecules per formula unit (Q has 2 and P has ½). (½)

Answer: P Plaster of Paris, Q gypsum, R CuSO₄·5H₂O, S NaHCO₃, T CO₂. Both equations as above. R has the most water of crystallisation (5).

Example 12: Using pH data to identify salts · 3 marks · Short answerHOTS / competency

A student tests four salt solutions, W, X, Y and Z, with pH paper and records the pH values as 7, 9, 5 and 7 respectively. The salts are potassium sulphate, sodium acetate, ammonium chloride and sodium chloride, in some order. Use this information: HCl and $H_{2} S O_{4}$ are strong acids, acetic acid is a weak acid, NaOH and KOH are strong bases, and $N H_{4} O H$ is a weak base.
(a) Identify X and Y, giving reasons. (2)
(b) Which two salts could W and Z be? Why can pH paper alone not tell W and Z apart? (1)
(3 marks)

Try it first, then show the solutionSolution

(a) X has pH 9, so it is basic. A basic salt comes from a strong base and a weak acid, so X is sodium acetate (from NaOH and acetic acid). (1)
(a) Y has pH 5, so it is acidic. An acidic salt comes from a strong acid and a weak base, so Y is ammonium chloride (from HCl and $N H_{4} O H$ ). (1)
(b) W and Z are sodium chloride and potassium sulphate. Both come from a strong acid and a strong base, so both are neutral with pH 7 and give the same colour on pH paper. (1)

Answer: (a) X is sodium acetate (strong base + weak acid, pH above 7); Y is ammonium chloride (strong acid + weak base, pH below 7). (b) W and Z are NaCl and K₂SO₄; both are neutral (pH 7), so pH paper cannot distinguish them.

Case-study practice (Section E style)

Read the situation, then answer every part. In the board paper this is a 4-mark question split into 1 + 1 + 2 marks.

Testing pH around a school

Suppose the science club of a school in a town near a group of factories tests a few samples with universal indicator paper during a monsoon week. Their notebook records these approximate pH values:

Rainwater collected on the school roof: 5.0
Filtrate of soil from the school garden (2 g soil shaken with 5 mL water and filtered): 5.5
Water from a nearby pond: 6.5
Lemon juice: 2.5
Baking soda solution: 8.5

The club notices that some plants in the garden are not growing well. One member is also stung by a honey-bee while collecting the soil sample, and the teacher asks the club to use their results to decide what to do.

(i) · 1 mark
Can the rainwater sample be called acid rain? Give a reason.
Answer
Answer: Yes, it is acid rain.
Rain is called acid rain when its pH is less than 5.6. The sample has pH 5.0, which is below 5.6.
(ii) · 1 mark
Which tested sample could be applied to give relief from the bee sting, and why?
Answer
Answer: The baking soda solution.
A bee sting leaves an acid. Baking soda solution (pH 8.5) is a mild base, so it neutralises the acid and relieves pain and irritation.
(iii) · 2 marks
The garden soil is acidic. Name two substances that could be added to the soil and explain how they help the plants.
Answer
Answer: Quick lime (calcium oxide), slaked lime (calcium hydroxide) or chalk (calcium carbonate); any two.
These substances are basic and neutralise the excess acid in the soil, raising its pH (1). Plants need a specific pH range for healthy growth, so bringing the soil pH back into that range helps them grow well (1).

Electrolysis of brine

Imagine your teacher sketches a cell on the board. Two electrodes dip into brine, a concentrated solution of common salt in water, and electricity is passed. A gas X bubbles out at one electrode and a gas Y at the other, while a solution Z collects near the electrode where Y forms. When a burning splinter is brought near Y, it burns with a pop. X is passed over dry slaked lime to make a white powder A, which is used to make drinking water free from germs.

(i) · 1 mark
Identify gases X and Y and name the electrode at which each is given off.
Answer
Answer: X is chlorine, given off at the anode; Y is hydrogen, given off at the cathode.
A gas that burns with a pop is hydrogen (½). The other gas from brine, which makes bleaching powder with slaked lime, is chlorine; chlorine forms at the anode and hydrogen at the cathode (½).
(ii) · 1 mark
Name A and write the balanced equation for its formation.
Answer
Answer: A is bleaching powder, $C a (C l O)_{2}$ .
Chlorine acts on dry slaked lime (½): $2 C a (O H)_{2} + 2 C l_{2} \to C a (C l O)_{2} + C a C l_{2} + 2 H_{2} O$ (½).
(iii) · 2 marks
Name Z, write the balanced equation for the whole process, and give one use each of Z and Y.
Answer
Answer: Z is sodium hydroxide. Z is used for making soap; Y (hydrogen) is used as a fuel.
Z is sodium hydroxide, which collects near the cathode (½). $2 N a C l (a q) + 2 H_{2} O (l) electricity 2 N a O H (a q) + C l_{2} (g) + H_{2} (g)$ (1). Uses: NaOH for soaps and detergents, hydrogen as a fuel (½).

Carbon dioxide at work: baking powder and fire extinguishers

Suppose a school science fair has two stalls. At the first, students bake small cakes. One batch uses baking soda alone, and another uses baking powder, which is baking soda mixed with tartaric acid. At the second stall, students build a model soda-acid fire extinguisher. A wash-bottle holds sodium hydrogencarbonate solution, with a small ignition tube of dilute sulphuric acid hung inside. When the bottle is tilted, the acid mixes with the solution and a discharge comes out of the nozzle, which puts out a burning candle.

(i) · 1 mark
Name the gas that both stalls depend on and give one test for it.
Answer
Answer: Carbon dioxide; it turns lime water milky.
A hydrogencarbonate gives carbon dioxide with an acid and on heating (½). Passed through lime water, it turns the lime water milky (½).
(ii) · 1 mark
Write the balanced equation for the reaction in the fire extinguisher.
Answer
Answer: $2 N a H C O_{3} + H_{2} S O_{4} \to N a_{2} S O_{4} + 2 H_{2} O + 2 C O_{2}$
A metal hydrogencarbonate and an acid give a salt, water and carbon dioxide. Atom check: Na 2 = 2, S 1 = 1, C 2 = 2, H 4 = 4, O 10 = 10.
(iii) · 2 marks
A student claims that baking powder works only because baking soda breaks down on heating. Use equations to explain why baking powder releases more carbon dioxide than the same amount of baking soda heated alone.
Answer
Answer: With tartaric acid, every $N a H C O_{3}$ gives one $C O_{2}$ ; heated alone, two $N a H C O_{3}$ give only one $C O_{2}$ .
Heated alone: $2 N a H C O_{3} heat N a_{2} C O_{3} + H_{2} O + C O_{2}$ , so only one $C O_{2}$ forms from two units of baking soda (1). In baking powder, tartaric acid supplies $H^{+}$ : $N a H C O_{3} + H^{+} \to N a^{+} + C O_{2} + H_{2} O$ (the $N a^{+}$ ends up in the sodium salt of the acid), so each unit gives one $C O_{2}$ , even on just mixing with water. More gas makes the cake rise more and become soft and spongy (1).

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Write balanced chemical equations to show (a) how Plaster of Paris is prepared from gypsum, and (b) what happens when Plaster of Paris is mixed with water. (2 marks) (2 marks)

Step	Marks
Gypsum heated at 373 K loses water and becomes calcium sulphate hemihydrate (Plaster of Paris): $C a S O_{4} \cdot 2 H_{2} O 373 K C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O$	1
Plaster of Paris takes up water and changes back to gypsum, a hard solid mass: $C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O \to C a S O_{4} \cdot 2 H_{2} O$	1

A person who has overeaten complains of pain and irritation in the stomach.
(a) What causes this pain?
(b) How does an antacid give relief? Name one antacid.
(c) Write the ionic equation for the reaction that gives relief. (3 marks) (3 marks)

Step	Marks
During indigestion the stomach produces too much hydrochloric acid, which causes pain and irritation.	1
An antacid is a mild base that neutralises the excess acid.	0.5
Example: magnesium hydroxide (milk of magnesia) or sodium hydrogencarbonate.	0.5
$H^{+} (a q) + O H^{-} (a q) \to H_{2} O (l)$	1

(a) What is a universal indicator?
(b) State the range of the pH scale and the pH of a neutral, an acidic and a basic solution.
(c) How is pH related to the concentration of hydronium ions?
(d) Distinguish between a strong acid and a weak acid.
(e) Solution X has pH 2 and solution Y has pH 5. Which has more hydrogen ions, and which is the stronger acid? (5 marks) (5 marks)

Step	Marks
A universal indicator is a mixture of several indicators that shows different colours at different hydrogen ion concentrations.	1
pH is generally measured from 0 (very acidic) to 14 (very alkaline).	0.5
Neutral solution: pH 7; acidic: below 7; basic: above 7.	1
Higher the hydronium ion concentration, lower the pH.	0.5
A strong acid gives more $H^{+}$ ions in solution; a weak acid gives fewer $H^{+}$ ions (e.g. HCl and acetic acid of the same concentration).	1
X (pH 2) has more hydrogen ions and is the stronger acid, because a lower pH means a higher $H^{+}$ concentration.	1

Where students lose marks in Acids, Bases and Salts

The slips examiners see most often, and what to write instead.

Mistake: Reversing the dilution rule
Students remember that 'water and acid are mixed carefully' but not which one goes into the other.
Wrong working
Water should be added slowly to concentrated sulphuric acid with stirring.
Right working
Concentrated acid should be added slowly to water with constant stirring, because the process is highly exothermic. Adding water to acid can make the mixture splash out and cause burns.
Do this: Use 'Always Add Acid' and always give the reason (highly exothermic).
Usually costs: The full mark in an assertion-reason item, or 1 mark in a give-reason question.
Mistake: Wrong electrodes in the chlor-alkali process
Students guess, or mix up which gas forms where.
Wrong working
Hydrogen at the anode, chlorine at the cathode, NaOH near the anode.
Right working
Chlorine at the anode, hydrogen at the cathode, sodium hydroxide formed near the cathode.
Do this: Anode gets the anion (chloride gives chlorine). Write the electrode beside each product.
Usually costs: ½ mark per wrong product in identify-type and 5-mark questions.
Mistake: Wrong formula for Plaster of Paris
The half water molecule looks odd, so it is dropped or rounded.
Wrong working
Plaster of Paris is $C a S O_{4} \cdot H_{2} O$ ; on heating, gypsum gives $C a S O_{4} + 2 H_{2} O$ .
Right working
Plaster of Paris is $C a S O_{4} \cdot \frac{1}{2} H_{2} O$ : $C a S O_{4} \cdot 2 H_{2} O 373 K C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O$
Do this: Remember that two formula units share one water molecule, and count the water on both sides.
Usually costs: ½ to 1 mark for the formula or equation.
Mistake: Thinking a higher pH means more acidic
Students link a 'bigger number' with 'stronger', without thinking about which ion is meant.
Wrong working
Solution with pH 6 has more H⁺ ions than solution with pH 2.
Right working
Lower pH means higher H⁺ concentration, so pH 2 has more H⁺ ions than pH 6 and is more strongly acidic.
Do this: Before ordering, write 'lower pH = more H⁺' at the top of your answer.
Usually costs: 1 mark in ordering questions and the MCQ mark.
Mistake: Unbalanced carbonate equations
Students copy the hydrogencarbonate equation, which needs one HCl, onto the carbonate equation, which needs two.
Wrong working
$N a_{2} C O_{3} + H C l \to N a C l + H_{2} O + C O_{2}$
Right working
$N a_{2} C O_{3} (s) + 2 H C l (a q) \to 2 N a C l (a q) + H_{2} O (l) + C O_{2} (g)$
Do this: Count Na and Cl on both sides before moving on. A carbonate takes 2HCl; a hydrogencarbonate takes 1HCl.
Usually costs: ½ mark for an unbalanced equation, more if the state symbols are also missing.
Mistake: Saying lime water stays milky with excess CO₂
Students learn only the first half of the lime water test.
Wrong working
On passing excess carbon dioxide, more white precipitate forms and the lime water becomes more milky.
Right working
Excess carbon dioxide makes the milkiness disappear, because soluble calcium hydrogencarbonate forms: $C a C O_{3} + H_{2} O + C O_{2} \to C a (H C O_{3})_{2}$
Do this: Learn the test as a pair of equations: milky, then clear.
Usually costs: 1 mark in give-reason and case-based questions.
Mistake: Mixing up baking soda and washing soda
Both are sodium salts with 'soda' in the name.
Wrong working
Washing soda is NaHCO₃ and is used in antacids; baking soda is Na₂CO₃·10H₂O.
Right working
Baking soda is NaHCO₃ (antacid, baking powder, fire extinguisher). Washing soda is Na₂CO₃·10H₂O (glass, soap and paper industries, cleaning, removing permanent hardness).
Do this: Baking soda has an H in it (hydrogencarbonate); washing soda carries ten waters.
Usually costs: ½ to 1 mark per wrong formula or use.

NCERT exercise guide: Acids, Bases and Salts

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

In-text questions after section 2.1

What it tests: Using litmus to identify acids, bases and water; why sour foods react with some metal vessels; the gas given off when an acid reacts with a metal and how to test it; identifying a metal compound from the gas it gives with an acid.

How to approach it: Start from the general word equations: acid + metal gives salt + hydrogen; carbonate or hydrogencarbonate + acid gives salt + carbon dioxide + water. Name the gas, give its test (pop sound for hydrogen, lime water turning milky for carbon dioxide), then write a balanced equation. Count every atom before you finish.

Identifying a metal compound from the gas it gives

A metal compound A reacts with dilute hydrochloric acid with effervescence. The gas given off puts out a burning candle. One of the products is calcium chloride.
(a) Identify A and the gas, giving reasons.
(b) Write the balanced chemical equation.
(c) State what is seen when the gas is passed through lime water for a short time and then for a long time.

Show the solutionSolution

A gas that puts out a burning candle and comes from effervescence with an acid is carbon dioxide, so A must be a carbonate.
The salt formed is calcium chloride, so the metal in A is calcium. A is calcium carbonate, $C a C O_{3}$ .
$C a C O_{3} (s) + 2 H C l (a q) \to C a C l_{2} (a q) + H_{2} O (l) + C O_{2} (g)$
Check: Ca 1 = 1; C 1 = 1; O 3 = 3; H 2 = 2; Cl 2 = 2.
For a short time the lime water turns milky, because insoluble calcium carbonate forms. With excess carbon dioxide the milkiness disappears, because soluble calcium hydrogencarbonate forms.

Answer: A is calcium carbonate and the gas is carbon dioxide. $C a C O_{3} (s) + 2 H C l (a q) \to C a C l_{2} (a q) + H_{2} O (l) + C O_{2} (g)$ . Lime water turns milky at first and clears with excess gas.

In-text questions after sections 2.2 and 2.3

What it tests: Why acids show acidic character only in water; why acid solutions conduct electricity; why dry HCl gas does not affect dry litmus; the safe way to dilute an acid; how dilution or adding more base changes ion concentration.

How to approach it: Every answer here comes back to ions. Acids give $H^{+} (a q)$ or $H_{3} O^{+}$ only in water, bases give $O H^{-}$ in water, and ions carry current. Dilution lowers the ion concentration per unit volume, while dissolving more base raises the $O H^{-}$ concentration.

Questions on ions in solution and dilution

(a) Why does a solution of sodium hydroxide in water conduct electricity?
(b) How does the concentration of hydroxide ions change when this solution is diluted with water?

Show the solutionSolution

In water, sodium hydroxide splits into ions: $N a O H (s) H_{2} O N a^{+} (a q) + O H^{-} (a q)$
These ions move and carry the electric current, so the solution conducts.
On dilution, the same number of ions is spread through a larger volume, so the $O H^{-}$ concentration per unit volume decreases.

Answer: (a) It conducts because NaOH gives $N a^{+}$ and $O H^{-}$ ions in water, which carry current. (b) The $O H^{-}$ concentration per unit volume decreases on dilution.

In-text questions after section 2.3.1

What it tests: Comparing hydrogen ion concentration from pH values; the effect of $H^{+}$ concentration on the nature of a solution; treating acidic soil with quick lime, slaked lime or chalk.

How to approach it: Remember the direction of the scale: lower pH means more $H^{+}$ and a more acidic solution. For soil questions, link acidic soil with adding a base so that the pH comes back to the range plants need.

Comparing pH values and treating soil

(a) Solution P has pH 2 and solution Q has pH 5. Which has the higher hydrogen ion concentration? Are both acidic?
(b) Soil from a field gives a filtrate of pH 5 with universal indicator paper. What can the farmer add, and why?

Show the solutionSolution

Lower pH means higher hydrogen ion concentration, so P (pH 2) has more $H^{+}$ ions than Q.
Both values are below 7, so both are acidic; P is more strongly acidic.
pH 5 shows the soil is acidic.
The farmer can add quick lime (calcium oxide), slaked lime (calcium hydroxide) or chalk (calcium carbonate). These are basic and neutralise the excess acid, because plants need a specific pH range to grow well.

Answer: (a) P has more $H^{+}$ ions; both are acidic. (b) Add quick lime, slaked lime or chalk to neutralise the acidity of the soil.

In-text questions after section 2.4

What it tests: Common names and raw materials of bleaching powder; the sodium compound that softens hard water; the effect of heat on sodium hydrogencarbonate; the reaction of Plaster of Paris with water.

How to approach it: Learn each compound as a set: common name, chemical name, formula, how it is made (with equation) and its uses. Many short questions simply ask one item from this set, so a clear summary in your notes pays off.

Questions on preparing a salt and writing its equation

Gypsum is heated to 373 K. Name the product, write the balanced equation and give one use of the product.

Show the solutionSolution

At 373 K gypsum loses water and forms calcium sulphate hemihydrate, called Plaster of Paris.
$C a S O_{4} \cdot 2 H_{2} O 373 K C a S O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O$
Check water: 2 = ½ + 1½.
Use: doctors use it as plaster to support fractured bones in the right position.

Answer: Plaster of Paris, $C a S O_{4} \cdot \frac{1}{2} H_{2} O$ ; used for supporting fractured bones (also for toys and decoration materials).

End-of-chapter exercises

What it tests: MCQs on pH, gases and neutralisation volumes; word and balanced equations for acids with metals; describing the activity that shows hydrogen compounds need not be acids; reasons based on ions; strong versus weak acids; pH change of milk; storage of Plaster of Paris; uses of washing soda and baking soda.

How to approach it: For numerical MCQs on neutralisation, use simple proportion: double the base needs double the acid. For equation questions, write the word equation first, then formulae, then balance. For reasons, always name the ion or the reaction responsible. The milk and curd questions are solved in the worked examples on this page.

Neutralisation volume MCQ

10 mL of a sodium hydroxide solution is completely neutralised by 5 mL of a hydrochloric acid solution. How much of the same acid will neutralise 30 mL of the same NaOH solution?
(a) 5 mL
(b) 10 mL
(c) 15 mL
(d) 30 mL

Show the solutionSolution

The acid and base react in a fixed proportion: 10 mL NaOH needs 5 mL HCl.
30 mL NaOH is three times 10 mL.
So the acid needed is $3 \times 5 = 15$ mL.

Answer: (c) 15 mL

Key activities: Activities 2.2, 2.3, 2.5, 2.8 and 2.10

What it tests: Olfactory indicators (onion, vanilla, clove oil); hydrogen from zinc and acids; carbon dioxide from carbonates and hydrogencarbonates with lime water; conduction by acid solutions but not glucose or alcohol; heat released when an acid or base dissolves in water.

How to approach it: For each activity, learn four things: what is done, what is seen, what it shows, and one precaution. Board questions often give the observation and ask what it shows, or give the set-up and ask what you will see.

Activity-based question on dissolving an acid in water

A student adds a few drops of concentrated sulphuric acid to 10 mL of water in a beaker and swirls it. She then touches the base of the beaker.
(a) What does she feel, and what kind of process is this?
(b) What precaution must be taken while mixing a concentrated acid with water, and why?

Show the solutionSolution

The base of the beaker feels hot, because heat is released.
Dissolving an acid in water is a highly exothermic process.
The acid must be added slowly to water with constant stirring.
If water is added to the concentrated acid, the heat produced may make the mixture splash out and cause burns, and the glass may break due to local heating.

Answer: (a) The beaker becomes hot; the process is exothermic. (b) Add acid slowly to water with constant stirring, never water to acid, to avoid splashing, burns and breaking of the container.

Acids, Bases and Salts: previous year questions

Asked in 21 of the 21 board papers we hold (2023 to 2026), on average 6.6 marks per paper; 85 questions in all, counting OR alternatives. In 4 of 4 sample papers.

Counted from the real papers on our previous year papers page, not estimated. Average marks are over every board paper we hold, with an OR pair counted once. Chapter tags are ours.

Solved board questions from Acids, Bases and Salts

Real questions from recent papers. Try each one first, then open our step-by-step solution with marks per step.

2026, set 31/1/1, Q18 · 1 mark · MCQCompetency-based

Which one of the following can be used as an acid-base indicator by a visually impaired (blind) student?

(A)Turmeric
(B)Vanilla essence
(C)Methyl orange
(D)Litmus

Try it first, then show our solutionSolution

A blind student cannot see a colour change, so the indicator must signal acids and bases through some other sense, such as smell.
Vanilla essence is an olfactory indicator. Its smell can be detected in acidic solution but is lost in basic solution. Turmeric, methyl orange and litmus all work only by colour change.
1 mark
Correct option: (B) Vanilla essence.

Answer: Vanilla essence, option (B)

Where the marks are

Chooses vanilla essence, an olfactory indicator (1 mark)

See it in the full 2026 board paper, set 31/1/1

2026, set 31/1/1, Q25 · 2 marks · Very short answer

Answer the following.

(a)
What are amphoteric oxides?
1 mark
(b)
Categorise the following based on their nature: ZnO, Na₂O, CO₂
1 mark

Try it first, then show our solutionSolution

(a) Amphoteric oxides are metal oxides that react with both acids and bases to give salt and water. Examples are $ZnO$ and $A l_{2} O_{3}$ .
1 mark
(b) $ZnO$ is an amphoteric oxide, $N a_{2} O$ is a basic oxide (metal oxide), and $C O_{2}$ is an acidic oxide (non-metal oxide).
1 mark

Answer: (a) Metal oxides reacting with both acids and bases to form salt and water; (b) ZnO amphoteric, Na₂O basic, CO₂ acidic

Where the marks are

Correct definition of amphoteric oxide (1 mark)
All three oxides classified correctly (1 mark)

See it in the full 2026 board paper, set 31/1/1

2026, set 31/1/1, Q27(a) · 3 marks · Short answer

Explain chlor-alkali process with chemical equation. Name the products formed at anode and cathode.

Try it first, then show our solutionSolution

When electricity is passed through brine (an aqueous solution of sodium chloride), it decomposes to give sodium hydroxide (an alkali) and chlorine. Because the products are chlorine and an alkali, the process is called the chlor-alkali process.
1 mark
Equation: $2 NaCl (aq) + 2 H_{2} O (l) electricity 2 NaOH (aq) + C l_{2} (g) + H_{2} (g)$
1 mark
At the anode: chlorine gas ( $C l_{2}$ ) is formed.
½ mark
At the cathode: hydrogen gas ( $H_{2}$ ) is formed. Sodium hydroxide solution collects near the cathode.
½ mark

Answer: Electrolysis of brine gives NaOH, Cl₂ and H₂; Cl₂ at anode, H₂ at cathode

Where the marks are

Explains the process and the reason for its name (1 mark)
Balanced chemical equation (1 mark)
Cl₂ at anode (½ mark)
H₂ at cathode (½ mark)

See it in the full 2026 board paper, set 31/1/1

Every Acids, Bases and Salts question, by year

For 2027

Sample paper 2026-27: Q18 (1 mark, solved), Q20 (1 mark, solved), Q24 (1 mark, solved), Q27 (3 marks, solved)

2026

Set 31/1/1: Q18 (1 mark, solved), Q22 (1 mark, solved), Q25 (2 marks, solved), Q27(a) (3 marks, solved), Q27(b) (3 marks, solved)

Set 31/2/1: Q18 (1 mark, solved), Q21 (1 mark, solved), Q22 (1 mark, solved), Q28 (4 marks, solved)

Set 31/3/1: Q18 (1 mark, solved), Q27(a) (3 marks, solved), Q27(b) (3 marks, solved)

Set 31/4/1: Q19 (1 mark, solved), Q23 (1 mark, solved), Q25 (2 marks, solved), Q28 (4 marks, solved)

Set 31/5/1: Q20 (1 mark, solved), Q22 (1 mark, solved), Q25 (2 marks, solved), Q27(A) (3 marks, solved), Q27(B) (3 marks, solved)

Sample paper 2025-26: Q18 (1 mark, solved), Q20 (1 mark, solved), Q21 (1 mark, solved), Q28 (4 marks, solved)

2025

Set 31/1/1: Q3 (1 mark, solved), Q4 (1 mark, solved), Q37 (4 marks, solved)

Set 31/2/1: Q2 (1 mark, solved), Q3 (1 mark, solved), Q37 (4 marks, solved)

Set 31/3/1: Q2 (1 mark, solved), Q22 (2 marks, solved), Q28(a) (3 marks, solved), Q28(b) (3 marks, solved), Q37 (4 marks, solved)

Set 31/4/1: Q2 (1 mark, solved), Q18 (1 mark, solved), Q37 (4 marks, solved)

Set 31/5/1: Q3 (1 mark, solved), Q21 (2 marks, solved), Q27 (3 marks, solved)

Set 31/6/1: Q2 (1 mark, solved), Q6 (1 mark, solved), Q7 (1 mark, solved), Q28(a) (3 marks, solved), Q28(b) (3 marks, solved)

Sample paper 2024-25: Q3 (1 mark, solved), Q4 (1 mark, solved), Q28(A) (3 marks, solved), Q28(B) (3 marks, solved), Q37 (4 marks, solved)

2024

Set 31/1/1: Q5 (1 mark, solved), Q28 (3 marks, solved)

Set 31/2/1: Q2 (1 mark, solved), Q4 (1 mark, solved), Q37(a) (4 marks, solved)

Set 31/3/1: Q2 (1 mark, solved), Q3 (1 mark, solved), Q34(a) (5 marks, solved), Q34(b) (5 marks, solved)

Set 31/4/1: Q2 (1 mark, solved), Q21(a) (2 marks, solved)

Set 31/5/1: Q3 (1 mark, solved), Q4 (1 mark, solved), Q34(a) (5 marks, solved), Q34(b) (5 marks, solved)

Sample paper 2023-24: Q3 (1 mark, solved)

2023

Set 31/1/1: Q1 (1 mark, solved), Q4 (1 mark, solved), Q6 (1 mark, solved), Q21(a) (2 marks, solved), Q28 (3 marks, solved)

Set 31/2/1: Q2 (1 mark, solved), Q17 (1 mark, solved), Q37 (4 marks, solved)

Set 31/4/1: Q1 (1 mark, solved), Q4 (1 mark, solved), Q5 (1 mark, solved), Q6 (1 mark, solved), Q21(A) (2 marks, solved), Q21(B) (2 marks, solved), Q28 (3 marks, solved)

Set 31/5/1: Q1 (1 mark, solved), Q4 (1 mark, solved), Q5 (1 mark, solved), Q6 (1 mark, solved), Q21(a) (2 marks, solved), Q21(b) (2 marks, solved), Q28 (3 marks, solved)

Set 31/6/1: Q1 (1 mark, solved), Q2 (1 mark, solved), Q4 (1 mark, solved), Q17 (1 mark, solved), Q28(a) (3 marks, solved)

Practice questions on Acids, Bases and Salts

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which statement about Plaster of Paris is correct?
(a) It is obtained by heating gypsum at 373 K.
(b) Its formula is $C a S O_{4} \cdot 2 H_{2} O$ .
(c) It is made of blue crystals.
(d) On mixing with water it forms slaked lime.
Hint
Think of what gypsum loses on heating.
Answer
(a) It is obtained by heating gypsum at 373 K. Its formula is $C a S O_{4} \cdot \frac{1}{2} H_{2} O$ ; it is a white powder and sets to gypsum with water.
Q2 · 2 marks · Very short answerBoard level
Why are hydrogen ions always written as $H^{+} (a q)$ or $H_{3} O^{+}$ ? Write the equation for the reaction of HCl with water. (2 marks)
Hint
Can a hydrogen ion exist on its own?
Answer
Hydrogen ions cannot exist alone; they exist only after combining with water molecules, so they are shown as $H^{+} (a q)$ or hydronium ions (1).
$H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$ (1)
Q3 · 1 mark · Assertion-reasonBoard level
Assertion (A): An aqueous solution of sodium chloride has a pH of 7.
Reason (R): Sodium chloride is the salt of a strong acid and a strong base.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Recall which type of salt is neutral.
Answer
(a) Sodium chloride is formed from hydrochloric acid and sodium hydroxide; salts of a strong acid and a strong base are neutral with pH 7.
Q4 · 3 marks · Short answerBoard level
Write balanced chemical equations for the following and state the type of acid-base reaction shown by each:
(a) Dilute sulphuric acid reacts with sodium hydroxide solution.
(b) Calcium carbonate reacts with dilute hydrochloric acid.
(c) Carbon dioxide is passed through lime water. (3 marks)
Hint
Two of these are acid with base (or basic compound); one is carbonate with acid.
Answer
(a) $H_{2} S O_{4} + 2 N a O H \to N a_{2} S O_{4} + 2 H_{2} O$ : neutralisation.
(b) $C a C O_{3} + 2 H C l \to C a C l_{2} + H_{2} O + C O_{2}$ : carbonate + acid gives salt, carbon dioxide and water.
(c) $C a (O H)_{2} + C O_{2} \to C a C O_{3} + H_{2} O$ : a base reacting with a non-metallic (acidic) oxide to give salt and water.
Q5 · 5 marks · Long answerHarder board level
Describe an activity to show that glucose and alcohol, though they contain hydrogen, are not acids. Include the set-up, observations and conclusion. What would you observe if the activity were repeated with sodium hydroxide solution? (5 marks)
Hint
Use a bulb in a circuit to test for ions.
Answer
Fix two nails on a cork in a 100 mL beaker and connect them to a 6 V battery through a bulb and a switch (1). Pour dilute HCl, switch on: the bulb glows; repeat with dilute $H_{2} S O_{4}$ : it glows (1). Repeat with glucose and alcohol solutions: the bulb does not glow (1). Conclusion: acid solutions contain ions ( $H^{+}$ ) that carry current; glucose and alcohol do not give $H^{+}$ ions, so they are not acids (1). With NaOH solution the bulb glows, because it gives $N a^{+}$ and $O H^{-}$ ions (1).
Q6 · 3 marks · Short answerHarder board level
(a) Why is the dissolving of concentrated sulphuric acid or sodium hydroxide pellets in water dangerous if done carelessly?
(b) Why must the acid be added to water and not the reverse?
(c) What happens to the concentration of $H_{3} O^{+}$ ions when an acid is diluted? (3 marks)
Hint
Think about the heat released.
Answer
(a) Dissolving an acid or base in water is highly exothermic and gives out a lot of heat; such containers carry a warning sign (1).
(b) Adding water to concentrated acid may make the mixture splash out and cause burns, and the glass may break from local heating; acid is added slowly to water with stirring (1).
(c) The $H_{3} O^{+}$ concentration per unit volume decreases (1).
Q7 · 3 marks · Short answerHarder board level
Compound X is used in the kitchen to make pakoras crispy. On heating, X gives compound Y, water and a gas Z that turns lime water milky. Recrystallisation of Y gives compound W, used for removing permanent hardness of water.
(a) Identify X, Y, Z and W.
(b) Write the equations for the formation of Y from X and of W from Y. (3 marks)
Hint
Start from baking soda.
Answer
(a) X: sodium hydrogencarbonate, $N a H C O_{3}$ (½); Y: sodium carbonate, $N a_{2} C O_{3}$ (½); Z: carbon dioxide, $C O_{2}$ (½); W: washing soda, $N a_{2} C O_{3} \cdot 10 H_{2} O$ (½). Total for (a): 2 marks.
(b) $2 N a H C O_{3} heat N a_{2} C O_{3} + H_{2} O + C O_{2}$ (½); $N a_{2} C O_{3} + 10 H_{2} O \to N a_{2} C O_{3} \cdot 10 H_{2} O$ (½). Total for (b): 1 mark.
Q8 · 5 marks · Long answerHarder board level
In a practical, you study dilute HCl and dilute NaOH.
(a) State the effect of each on blue litmus and red litmus solution.
(b) What is seen when zinc granules are added to dilute HCl? Write the equation and give the test for the gas.
(c) What is seen when solid sodium carbonate is added to dilute HCl? Write the equation and the test for the gas.
(d) Write the equation for the reaction of zinc with warm NaOH solution. (5 marks)
Hint
Two gases, two different tests.
Answer
(a) HCl turns blue litmus red, no change in red; NaOH turns red litmus blue, no change in blue (1).
(b) Bubbles on zinc; $Z n + 2 H C l \to Z n C l_{2} + H_{2}$ ; gas burns with a pop sound near a burning candle (1½).
(c) Brisk effervescence; $N a_{2} C O_{3} + 2 H C l \to 2 N a C l + H_{2} O + C O_{2}$ ; gas turns lime water milky (1½).
(d) $2 N a O H + Z n \to N a_{2} Z n O_{2} + H_{2}$ (1).
Q9 · 3 marks · Short answerHOTS / competency
A student adds universal indicator to some dilute NaOH solution, which shows pH 12. She then adds dilute HCl slowly until it is in excess. Describe how the pH changes, state the pH at exact neutralisation, and explain what happens to $O H^{-}$ and $H^{+}$ ions during the change. (3 marks)
Hint
Follow the pH from the alkaline end, through 7, into the acidic range.
Answer
The pH falls from 12 towards 7 as acid is added, because $H^{+}$ ions react with $O H^{-}$ ions: $H^{+} (a q) + O H^{-} (a q) \to H_{2} O (l)$ , so $O H^{-}$ concentration decreases (1). At exact neutralisation the solution contains only salt (NaCl) and water, pH 7 (1). With excess acid, $H^{+}$ ions are left over and the pH drops below 7 (1).
Q10 · 3 marks · Short answerHOTS / competency
Suppose a dental study tracks the pH in a student's mouth: 7.0 before a meal, 5.0 ten minutes after eating a sugary sweet, and back to about 7.0 soon after brushing with toothpaste.
(a) At which reading is the tooth enamel at risk, and why?
(b) What makes the pH fall after the sweet?
(c) Use the data to justify when the student should brush. (3 marks)
Hint
Compare each reading with 5.5.
Answer
(a) At pH 5.0: tooth decay starts when mouth pH is below 5.5, and enamel (calcium hydroxyapatite) corrodes (1).
(b) Bacteria in the mouth break down sugar and food particles and produce acids (1).
(c) Brushing raises the pH back to about 7 because basic toothpaste neutralises the acid, so the student should clean the mouth soon after eating, not wait till bedtime (1).
Q11 · 3 marks · Short answerHarder board level
In a laboratory, 10 mL of a sodium hydroxide solution is exactly neutralised by 8 mL of a hydrochloric acid solution. A student then mixes 25 mL of the same NaOH solution with 16 mL of the same HCl and adds two drops of phenolphthalein.
(a) Is the final mixture acidic, basic or neutral? Show your working.
(b) What colour will phenolphthalein show in it?
(c) How much more of the same HCl must be added to neutralise the mixture exactly? (3 marks)
Hint
First work out how much acid 25 mL of the base really needs.
Answer
(a) 10 mL NaOH needs 8 mL HCl, so 25 mL NaOH needs $2.5 \times 8 = 20$ mL HCl. Only 16 mL was added, so some base is left over and the mixture is basic, pH above 7 (1).
(b) Pink, because phenolphthalein is pink in a basic solution (1).
(c) $20 - 16 = 4$ mL more HCl (1).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What colour does phenolphthalein show in sodium hydroxide solution?Show answer
Pink. The pink disappears when enough acid is added.
2.Which gas burns with a pop sound, and which reaction in this chapter gives it?Show answer
Hydrogen, given off when an acid reacts with a metal, such as zinc with dilute sulphuric acid.
3.Why do metal oxides count as basic oxides?Show answer
They react with acids to give a salt and water, just as bases do.
4.What is an alkali?Show answer
A base that dissolves in water.
5.Below what pH is rain called acid rain?Show answer
Below 5.6.
6.At which electrodes are chlorine and hydrogen given off in the chlor-alkali process?Show answer
Chlorine at the anode, hydrogen at the cathode.
7.What is water of crystallisation?Show answer
The water built into a salt's crystal, with a set number of water molecules for each formula unit.
8.At what temperature is gypsum heated to make Plaster of Paris?Show answer
373 K.
9.For each chlor-alkali product, give where it forms and one use.Show answer
Chlorine: at the anode; used for water treatment or to make bleaching powder.
Hydrogen: at the cathode; used as a fuel.
Sodium hydroxide: near the cathode; used to make soap.
10.Name the natural acid in curd, in tamarind and in an ant sting.Show answer
Curd: lactic acid. Tamarind: tartaric acid. Ant sting: methanoic acid.

Best order to study it

Start with indicators and the idea that acids give H⁺ and bases give OH⁻ in water; everything else rests on this.
Learn the reaction patterns in turn (metal, carbonate, hydrogencarbonate, base, metal oxide, non-metal oxide) and practise balancing one equation of each.
Move to dilution, conduction and the dry versus moist HCl activity, then the pH scale, strong and weak acids, and ordering by H⁺ concentration.
Study pH in daily life (body, rain, soil, stomach, teeth, stings) as applications of neutralisation.
Finish with salts: families and salt pH, then the chlor-alkali process, bleaching powder, baking soda, washing soda and Plaster of Paris, with every equation and use.

Quick revision checklist

The night before the exam: can you do each of these without looking?

Can you write acid + metal, carbonate + acid and hydrogencarbonate + acid equations, balanced and with state symbols?
Can you write both lime water equations and explain why the milkiness disappears?
Can you explain why glucose solution does not conduct but dilute HCl does?
Can you explain why dry HCl gas does not turn dry blue litmus red, using the H₃O⁺ equation?
Can you sort a list of pH values into strongly or weakly acidic, neutral and weakly or strongly basic, and order them by H⁺ concentration?
Can you state the pH values for the body, acid rain and tooth decay?
Can you predict the pH of a salt from the strength of its parent acid and base?
Can you write the chlor-alkali equation and name the product at each electrode?
Can you write the formulae and preparation equations of bleaching powder, baking soda, washing soda and Plaster of Paris, with two uses of each?
Can you explain why Plaster of Paris must be kept in a moisture-proof container?

Acids, Bases and Salts: frequently asked questions

Do I need to learn the logarithm definition of pH?

No. The 2026-27 syllabus says the definition of pH relating to logarithm is not required. You need to know that pH is a number from about 0 to 14 showing acidic or basic nature, and that a lower pH means more hydrogen ions.

Are the Do You Know boxes in this chapter asked in the board exam?

Information in NCERT text boxes is not assessed in the board exam. You may read them for interest, but put your revision time into the main text, the activities and the equations.

What is the difference between baking soda and baking powder?

Baking soda is sodium hydrogencarbonate alone. Baking powder is a mixture of baking soda and tartaric acid, a weak edible acid. When baking powder is heated or mixed with water, carbon dioxide is released and makes cakes soft and spongy.

Why is copper sulphate blue but turns white on heating?

Blue copper sulphate crystals contain five molecules of water of crystallisation per formula unit. Heating drives this water off and the salt turns white. Adding a few drops of water brings the blue colour back.

Is every salt neutral?

No. A salt of a strong acid and a strong base, like sodium chloride, is neutral with pH 7. A salt of a strong acid and a weak base is acidic, and a salt of a strong base and a weak acid is basic, like sodium acetate.

Why does the chlor-alkali process have that name?

It is named after two of its products: chlor for chlorine gas, given off at the anode, and alkali for sodium hydroxide, formed near the cathode. Hydrogen gas, the third product, is given off at the cathode.

Based on: CBSE Science (086) syllabus 2026-27; NCERT Science, Textbook for Class X (Reprint 2026-27)