ScienceCBSE Class 10 · Acids, Bases and Salts

Chemical Properties of Acids and Bases

Updated 14 October 2026 · Fact-checked

Acids react with metals to give a salt and hydrogen, with metal carbonates and hydrogencarbonates to give a salt, carbon dioxide and water, and with bases or metal oxides to give a salt and water. Some bases, like sodium hydroxide, react with zinc to give hydrogen, and with non-metal oxides to give a salt and water.

Every question is one of five patterns: the flame marks hydrogen's pop test, the tube is CO₂ turning lime water milky, the drop is water.

Illustration drawn for QuestionBuddy.

Understand Chemical Properties of Acids and Bases

Acids share their chemical properties because in water they all give hydrogen ions, $H^{+} (a q)$ (shown also as hydronium, $H_{3} O^{+}$ ). Bases share theirs because in water they give hydroxide ions, $O H^{-} (a q)$ . Bases that dissolve in water are called alkalis. Learn the reactions as five general patterns; every board question is one of them with particular substances filled in.

1. Acid + metal → salt + hydrogen. Zinc granules in dilute sulphuric acid fizz: $Z n + H_{2} S O_{4} \to Z n S O_{4} + H_{2}$ . The metal pushes hydrogen out of the acid. To test the gas, pass it through soap solution and bring a burning candle near a bubble: it burns with a pop. Hydrochloric and acetic acids behave alike with suitable metals. Nitric acid is the exception you meet in the Metals chapter: it is a strong oxidising agent, so with most metals it does not release hydrogen.

2. Base + metal (some metals only). Warm zinc with sodium hydroxide solution and hydrogen again forms, with sodium zincate: $2 N a O H + Z n \to N a_{2} Z n O_{2} + H_{2}$ . Not every metal does this; note the salt has a negative ion made of the metal and oxygen.

3. Acid + metal carbonate or hydrogencarbonate → salt + carbon dioxide + water. $N a_{2} C O_{3} + 2 H C l \to 2 N a C l + H_{2} O + C O_{2}$ and $N a H C O_{3} + H C l \to N a C l + H_{2} O + C O_{2}$ . Test: the gas turns lime water milky as a white precipitate of calcium carbonate forms, $C a (O H)_{2} + C O_{2} \to C a C O_{3} + H_{2} O$ . Passing excess carbon dioxide makes the milkiness disappear because soluble calcium hydrogencarbonate forms. Limestone, chalk, marble and egg shells are calcium carbonate, so they too fizz with acid.

4. Acid + base → salt + water (neutralisation). Add dilute HCl drop by drop to sodium hydroxide coloured pink by phenolphthalein; the pink fades as the base is used up, and returns if more base is added. $N a O H + H C l \to N a C l + H_{2} O$ . At the ion level the change is just $H^{+} + O H^{-} \to H_{2} O$ .

5. Oxides. Black copper oxide dissolves in dilute HCl to give a blue-green solution of copper(II) chloride: metal oxide + acid → salt + water, the same pattern as a base, so metal oxides are basic. Carbon dioxide reacts with the base calcium hydroxide to give salt and water, the same pattern as an acid, so non-metal oxides are acidic.

Why are glucose and alcohol not acids though they contain hydrogen? Their solutions do not conduct electricity, so they produce no ions. Acid solutions conduct because they contain ions. Dry HCl gas does not change dry blue litmus: hydrogen ions separate from HCl only in water ( $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$ ).

One safety point is also tested: dissolving a concentrated acid or base in water is highly exothermic, so add acid slowly to water with stirring, never water to acid.

Chemical Properties of Acids and Bases: formulas and key facts

Acid + metal: $Z n (s) + H_{2} S O_{4} (a q) \to Z n S O_{4} (a q) + H_{2} (g)$; Pattern: acid + metal → salt + hydrogen. Hydrogen burns with a pop.
Base + metal (zinc): $2 N a O H (a q) + Z n (s) \to N a_{2} Z n O_{2} (s) + H_{2} (g)$; Sodium zincate forms; not all metals react with bases.
Acid + carbonate: $N a_{2} C O_{3} (s) + 2 H C l (a q) \to 2 N a C l (a q) + H_{2} O (l) + C O_{2} (g)$; Hydrogencarbonate: $N a H C O_{3} + H C l \to N a C l + H_{2} O + C O_{2}$ .
Lime water test: $C a (O H)_{2} (a q) + C O_{2} (g) \to C a C O_{3} (s) + H_{2} O (l)$; Milky; with excess $C O_{2}$ : $C a C O_{3} + H_{2} O + C O_{2} \to C a (H C O_{3})_{2}$ (soluble), milkiness goes.
Neutralisation: $N a O H (a q) + H C l (a q) \to N a C l (a q) + H_{2} O (l)$; Ionic form: $H^{+} (a q) + O H^{-} (a q) \to H_{2} O (l)$ .
Metal oxide + acid: $C u O (s) + 2 H C l (a q) \to C u C l_{2} (a q) + H_{2} O (l)$; Blue-green solution; metal oxides are basic.

How to solve Chemical Properties of Acids and Bases questions

For 'what happens when' and 'identify the compound' questions on acids and bases, work in this order.

1Decide which of the five patterns the reaction fits: acid + metal, base + metal, acid + carbonate or hydrogencarbonate, acid + base, or oxide reactions.
2Write the general word equation for that pattern (for example acid + carbonate → salt + carbon dioxide + water).
3Fill in formulae: the salt takes the metal from one reactant and the anion from the acid ( $C l^{-}$ chloride, $S O_{4}^{2 -}$ sulphate, $N O_{3}^{-}$ nitrate).
4Balance the equation with coefficients.
5Add the observation and gas test: pop with a burning splint or candle for hydrogen; lime water milky for carbon dioxide; colour changes for indicators.
6For 'identify X' riddles, work backwards from the clue: a gas that puts out a flame and turns lime water milky means a carbonate reacted with an acid.

Chemical Properties of Acids and Bases in real life

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 + metal carbonate

Suppose you leave a piece of chalk overnight in a bowl of vinegar while cleaning your classroom duster, and in the morning the chalk has shrunk and a few bubbles are still rising.

Explain what happened using the general reaction, name the gas, describe a test for it, and write the balanced equation for calcium carbonate reacting with hydrochloric acid as a model of this reaction.

Try it first, then show the solutionSolution

Chalk is calcium carbonate; vinegar contains acetic acid. An acid reacts with a metal carbonate to give a salt, carbon dioxide and water, so the chalk is slowly used up.
The bubbles are carbon dioxide.
Test: pass the gas through lime water; it turns milky because insoluble calcium carbonate forms.
Model equation with hydrochloric acid: $C a C O_{3} (s) + 2 H C l (a q) \to C a C l_{2} (a q) + H_{2} O (l) + C O_{2} (g)$ (check: Ca 1, C 1, O 3 = 1 + 2, H 2, Cl 2).

Answer: The acid in vinegar reacts with calcium carbonate in chalk, giving a salt, water and carbon dioxide bubbles; the gas turns lime water milky. Model: $C a C O_{3} + 2 H C l \to C a C l_{2} + H_{2} O + C O_{2}$ .

Memory hook: Chalk in vinegar fizzes: carbonate plus acid always gives carbon dioxide.

Tricks to remember Chemical Properties of Acids and Bases

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

MASH for acid + metal
Metal + Acid → Salt + Hydrogen.
Use it when: Writing products of acid-metal reactions.
Carbonates give SCW
Carbonate (or hydrogencarbonate) + acid → Salt + Carbon dioxide + Water.
Use it when: Completing carbonate equations and egg-shell or marble riddles.
Oxides copy their element
Metals make basic oxides; non-metals make acidic oxides.
Use it when: Predicting how an oxide reacts with an acid or a base.

Chemical Properties of Acids and Bases: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Word to balanced equations · 2 marks · Very short answerBoard level

Write the word equation and the balanced equation when (a) dilute hydrochloric acid reacts with magnesium ribbon, (b) dilute sulphuric acid reacts with aluminium powder.

Try it first, then show the solutionSolution

(a) Magnesium + hydrochloric acid → magnesium chloride + hydrogen; $M g (s) + 2 H C l (a q) \to M g C l_{2} (a q) + H_{2} (g)$ (1 mark).
(b) Aluminium + sulphuric acid → aluminium sulphate + hydrogen; $2 A l (s) + 3 H_{2} S O_{4} (a q) \to A l_{2} (S O_{4})_{3} (a q) + 3 H_{2} (g)$ (1 mark).

Answer: (a) $M g + 2 H C l \to M g C l_{2} + H_{2}$ . (b) $2 A l + 3 H_{2} S O_{4} \to A l_{2} (S O_{4})_{3} + 3 H_{2}$ .

Example 2: Identify compound A · 3 marks · Short answerHarder board level

Metal compound A reacts with dilute hydrochloric acid with effervescence. The gas evolved puts out a burning candle. One of the products is calcium chloride. (a) Identify A and the gas. (b) Write the balanced equation. (c) What would you see if this gas were passed through lime water for a long time? Write the equations.

Try it first, then show the solutionSolution

(a) A gas that puts out a flame and comes from a compound fizzing with acid is carbon dioxide; with calcium chloride formed, A is calcium carbonate, $C a C O_{3}$ (1 mark).
(b) $C a C O_{3} (s) + 2 H C l (a q) \to C a C l_{2} (a q) + H_{2} O (l) + C O_{2} (g)$ (1 mark).
(c) Lime water first turns milky, $C a (O H)_{2} + C O_{2} \to C a C O_{3} + H_{2} O$ ; with excess gas the milkiness disappears as soluble calcium hydrogencarbonate forms, $C a C O_{3} + H_{2} O + C O_{2} \to C a (H C O_{3})_{2}$ (1 mark).

Answer: (a) A = $C a C O_{3}$ , gas = $C O_{2}$ . (b) $C a C O_{3} + 2 H C l \to C a C l_{2} + H_{2} O + C O_{2}$ . (c) Milky, then clear again with excess $C O_{2}$ .

Example 3: Neutralisation with an indicator · 4 marks · Short answerHarder board level

Two drops of phenolphthalein are added to 2 mL of dilute NaOH, and dilute HCl is then added drop by drop. (a) Describe the colour changes. (b) Write the equation and the ionic equation. (c) What happens if a few drops of NaOH are added again? Explain. (d) 10 mL of the NaOH needs 8 mL of the HCl for complete neutralisation. How much of the same HCl will 20 mL of the NaOH need?

Try it first, then show the solutionSolution

(a) Pink in the base; the pink fades and the solution becomes colourless when all the base is neutralised (1 mark).
(b) $N a O H + H C l \to N a C l + H_{2} O$ ; $H^{+} (a q) + O H^{-} (a q) \to H_{2} O (l)$ (1 mark).
(c) The pink returns because the solution again contains unreacted base (1 mark).
(d) Double the base needs double the acid: $2 \times 8 = 16$ mL (1 mark).

Answer: (a) Pink to colourless. (b) $N a O H + H C l \to N a C l + H_{2} O$ . (c) Pink reappears. (d) 16 mL.

Example 4: Why curd is not kept in copper · 3 marks · Short answerHarder board level

(a) Why should curd and other sour substances not be stored in brass or copper vessels? (b) Copper oxide dissolves in dilute hydrochloric acid. State the colour of the solution, write the balanced equation, and use it to explain why metal oxides are called basic oxides.

Try it first, then show the solutionSolution

(a) Sour substances contain acids, which react with metals such as copper (and the copper and zinc in brass) over time, forming compounds that spoil the food and may be harmful (1 mark).
(b) The solution becomes blue-green due to copper(II) chloride: $C u O (s) + 2 H C l (a q) \to C u C l_{2} (a q) + H_{2} O (l)$ (1 mark).
(b) The products are a salt and water, exactly as when a base reacts with an acid; so metal oxides behave as bases and are called basic oxides (1 mark).

Answer: (a) Acids in curd react with the metal, contaminating the food. (b) Blue-green; $C u O + 2 H C l \to C u C l_{2} + H_{2} O$ ; salt and water form, as with a base.

Example 5: Hydrogen in glucose but no acid · 5 marks · Long answerHOTS / competency

Glucose, alcohol and hydrochloric acid all contain hydrogen. (a) Describe an activity to show that only HCl behaves as an acid. (b) Explain the result. (c) Dry HCl gas does not change the colour of dry blue litmus paper but turns moist blue litmus red. Explain with an equation.

Try it first, then show the solutionSolution

(a) Fix two nails in a cork in a beaker, connect them to a 6 V battery through a bulb and a switch. Pour in dilute HCl and switch on; then repeat with glucose solution and alcohol solution (2 marks).
(b) The bulb glows only with HCl. Current is carried by ions; HCl in water gives $H^{+}$ ions (and $C l^{-}$ ), while glucose and alcohol do not ionise. Acids are acids because they give $H^{+} (a q)$ ions, not merely because they contain hydrogen (2 marks).
(c) Hydrogen ions separate from HCl only in water: $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$ . Dry gas on dry paper gives no ions; moisture lets ions form, which turn blue litmus red (1 mark).

Answer: (a) Conductivity set-up with bulb. (b) Only acid solution gives ions, so only it conducts and is acidic. (c) $H^{+}$ ions form only in water.

Where students lose marks in Chemical Properties of Acids and Bases

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

Mistake: Wrong salt formula from an acid
Students forget the valency of the metal or the anion.
Wrong working
$A l + H_{2} S O_{4} \to A l S O_{4} + H_{2}$
Right working
$2 A l + 3 H_{2} S O_{4} \to A l_{2} (S O_{4})_{3} + 3 H_{2}$
Do this: Build the salt from ion charges ( $A l^{3 +}$ , $S O_{4}^{2 -}$ ) before balancing.
Usually costs: The equation mark.
Mistake: Forgetting water in the carbonate reaction
Students remember only the salt and the gas.
Wrong working
$N a_{2} C O_{3} + 2 H C l \to 2 N a C l + C O_{2}$
Right working
$N a_{2} C O_{3} + 2 H C l \to 2 N a C l + H_{2} O + C O_{2}$
Do this: Carbonate + acid gives three products: salt, carbon dioxide and water.
Usually costs: ½ to 1 mark.
Mistake: Incomplete gas tests
Students name the gas but not the test, or describe the wrong test.
Wrong working
Hydrogen turns lime water milky.
Right working
Hydrogen burns with a pop; carbon dioxide turns lime water milky.
Do this: Pair each gas with its test in your notes and in answers.
Usually costs: 1 mark in 2-3 mark questions.
Mistake: Saying all compounds with hydrogen are acids
Acids release hydrogen with metals, so hydrogen seems to be the cause.
Wrong working
Glucose is acidic because it contains hydrogen.
Right working
Glucose gives no $H^{+}$ ions in water, so it is not acidic.
Do this: Link acidity to $H^{+} (a q)$ ions, not to the presence of hydrogen.
Usually costs: 1-2 marks in explanation questions.

Exam tips

Start reaction answers with the general word pattern, then the balanced equation; examiners give credit for both.
Name observations: effervescence, pop sound, milkiness, blue-green solution, pink to colourless.
In 'identify X' riddles, write your reasoning in one line before the answer so partial marks are possible.
Remember the zinc and sodium hydroxide reaction as the exception that shows bases can also give hydrogen.
For dilution safety, write 'acid to water, slowly, with stirring' and give the reason (highly exothermic).

Practice questions on Chemical Properties of Acids and Bases

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
A student drops marble chips into dilute hydrochloric acid and bubbles the gas through lime water for a long time. What does she observe in the lime water?
(a) It stays clear throughout.
(b) It turns milky and stays milky.
(c) It turns milky and then becomes clear again.
(d) The gas burns with a pop above it.
Hint
Marble is calcium carbonate. Think about what happens when carbon dioxide is in excess.
Answer
(c). Marble (calcium carbonate) gives carbon dioxide with the acid. It first turns lime water milky (calcium carbonate forms), and excess carbon dioxide turns this into soluble calcium hydrogencarbonate, so the milkiness disappears.
Q2 · 2 marks · Very short answerBoard level
Which gas is usually given off when an acid reacts with a metal? Give one balanced example and describe a test for the gas.
Hint
Think of zinc and dilute sulphuric acid.
Answer
Hydrogen. $Z n + H_{2} S O_{4} \to Z n S O_{4} + H_{2}$ . Collect the gas in soap bubbles (or a test tube) and bring a burning candle near: it burns with a pop sound.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Carbon dioxide is an acidic oxide.
Reason (R): Carbon dioxide reacts with calcium hydroxide to give a salt and water.
(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
Which reaction pattern does salt + water from a base match?
Answer
(a). Reacting with a base to form a salt and water is how an acid behaves, so $C O_{2}$ , a non-metal oxide, is acidic.
Q4 · 3 marks · Short answerHarder board level
Zinc granules are warmed separately with (i) dilute sulphuric acid and (ii) sodium hydroxide solution. (a) Name the gas in each case. (b) Write both balanced equations. (c) What is unusual about the salt formed in (ii)?
Hint
Both give the same gas; compare the salts.
Answer
(a) Hydrogen in both. (b) (i) $Z n + H_{2} S O_{4} \to Z n S O_{4} + H_{2}$ ; (ii) $2 N a O H + Z n \to N a_{2} Z n O_{2} + H_{2}$ . (c) Sodium zincate has a negative ion made of the metal (zinc) and oxygen, $Z n O_{2}^{2 -}$ , instead of the metal being the positive ion.
Q5 · 3 marks · Short answerHOTS / competency
Suppose three unlabelled bottles hold dilute hydrochloric acid, dilute sodium hydroxide and distilled water. You have only phenolphthalein solution, test tubes and droppers. Explain step by step how you would identify all three, and give the reaction that makes your method work.
Hint
Phenolphthalein picks out only one liquid directly; then use that liquid to tell the other two apart.
Answer
Add two drops of phenolphthalein to a sample from each bottle: only the sodium hydroxide turns pink (1 mark). Put some of this pink solution into two test tubes and add the other two liquids drop by drop, one to each: the liquid that makes the pink fade to colourless is the acid, because it neutralises the base, $N a O H + H C l \to N a C l + H_{2} O$ (1 mark). The liquid that leaves the solution pink, only slightly paler from dilution, is distilled water (1 mark).
Q6 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Suppose a student builds a small soda-acid fire extinguisher: 20 mL of sodium hydrogencarbonate solution is placed in a wash bottle, and an ignition tube holding dilute sulphuric acid is hung inside. When the bottle is tilted, a stream comes out of the nozzle and puts out a burning candle.
(a) Name the gas produced and the type of compound that reacted with the acid. (1 mark)
(b) Write the balanced equation for the reaction. (1 mark)
(c) Explain why the stream puts out the flame and why the acid is kept in a separate tube until use. (2 marks)
Hint
Sulphuric acid has two replaceable hydrogens, so two hydrogencarbonates react with one acid molecule.
Answer
(a) Carbon dioxide; a metal hydrogencarbonate. (b) $2 N a H C O_{3} + H_{2} S O_{4} \to N a_{2} S O_{4} + 2 H_{2} O + 2 C O_{2}$ . (c) Carbon dioxide does not support burning and the stream cuts off the flame's air supply, so the candle goes out. The acid is kept apart so the reaction starts only when the bottle is tilted; mixed in advance, the gas would be used up before a fire.

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.Acid + metal gives?Show answer
A salt and hydrogen gas.
2.Acid + metal hydrogencarbonate gives?Show answer
A salt, carbon dioxide and water.
3.Why are metal oxides called basic oxides?Show answer
They react with acids to give salt and water, like bases.
4.What is an alkali?Show answer
A base that dissolves in water.
5.Product of zinc with sodium hydroxide?Show answer
Sodium zincate, $N a_{2} Z n O_{2}$ , and hydrogen.
6.Why does excess $C O_{2}$ clear milky lime water?Show answer
Calcium carbonate turns into soluble calcium hydrogencarbonate.

Chemical Properties of Acids and Bases: frequently asked questions

What are the main chemical properties of acids in Class 10?

Acids react with metals (salt + hydrogen), with metal carbonates and hydrogencarbonates (salt + carbon dioxide + water), with bases (salt + water) and with metal oxides (salt + water).

Do bases react with metals?

Some do. Zinc with warm sodium hydroxide gives sodium zincate and hydrogen, but such reactions are not possible with all metals.

What is neutralisation?

The reaction of an acid with a base to give a salt and water. At the ion level, H^+ and OH^- ions join to form water.

Why should acid be added to water and not water to acid?

Dissolving a concentrated acid is highly exothermic. Adding water to the acid can make the mixture splash out and cause burns, and the glass may crack from local heating.

How do you test for carbon dioxide?

Pass it through lime water. It turns milky as calcium carbonate forms.