ChemistryICSE Class 10 · Study of Acids, Bases and Salts

Acids, Alkalis, Indicators and the pH Scale (ICSE Class 10)

Updated 17 October 2026 · Fact-checked

An acid is a substance that gives hydronium ions (H₃O⁺) as the only positive ions when dissolved in water; an alkali is a base soluble in water that gives hydroxide ions (OH⁻) as the only negative ions. Acids turn blue litmus red and alkalis turn red litmus blue. The pH scale from 0 to 14 measures acidity: below 7 is acidic, 7 is neutral and above 7 is alkaline.

pH below 7 is acidic and turns blue litmus red; pH above 7 is alkaline and turns red litmus blue; pH 7 is neutral.

Illustration drawn for QuestionBuddy.

Understand Acids, Alkalis, Indicators and the pH Scale

1. Acids in terms of ions. In water an acid molecule transfers a proton to a water molecule and gives hydronium ions, H₃O⁺, and negative ions. The hydronium ion is the only positive ion present, and it is responsible for acidic properties.

Hydrochloric acid: $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$
Nitric acid: $H N O_{3} + H_{2} O \to H_{3} O^{+} + N O_{3}^{-}$
Dilute sulphuric acid (both H atoms): $H_{2} S O_{4} + 2 H_{2} O \to 2 H_{3} O^{+} + S O_{4}^{2 -}$
Acetic acid (weak, partial): $C H_{3} C O O H + H_{2} O ⇌ H_{3} O^{+} + C H_{3} C O O^{-}$

Basicity is the number of hydronium ions given by one molecule: HCl is monobasic, H₂SO₄ is dibasic, H₃PO₄ is tribasic.

2. Bases and alkalis. A base is a substance (usually a metal oxide or hydroxide) that reacts with an acid to give salt and water only. An alkali is a base that dissolves in water, giving hydroxide ions, OH⁻, as the only negative ions.

$N a O H \to N a^{+} + O H^{-}$ and $K O H \to K^{+} + O H^{-}$
Ammonium hydroxide (weak): $N H_{4} O H ⇌ N H_{4}^{+} + O H^{-}$

All alkalis are bases, but not all bases are alkalis: copper(II) oxide and zinc oxide are bases (they react with acids) that do not dissolve in water.

3. Strong and weak. A strong acid or alkali is almost completely ionised in water (HCl, HNO₃, H₂SO₄, NaOH, KOH). A weak one is only slightly ionised (CH₃COOH, H₂CO₃, NH₄OH). Strength is about the degree of ionisation, not concentration.

4. Salts. A salt is a compound formed when the hydrogen of an acid is partly or completely replaced by a metal (or ammonium ion). Salts such as NaCl dissociate into ions in solution: $N a C l \to N a^{+} + C l^{-}$ .

5. Indicators.

Indicator	In acid	In alkali
Litmus	red (blue litmus turns red)	blue (red litmus turns blue)
Methyl orange	red (pink)	yellow
Phenolphthalein	colourless	pink

Neutral solutions leave litmus unchanged. Methyl orange is orange in neutral solution.

6. The pH scale. pH is a number from 0 to 14 showing how acidic or alkaline a solution is.

pH less than 7: acidic (the lower, the stronger); pH equal to 7: neutral (pure water); pH more than 7: alkaline (the higher, the stronger).
pH paper and universal indicator give a colour matched against a chart: roughly red for strong acids, orange or yellow for weak acids, green for neutral, blue for weak alkalis and violet for strong alkalis. Use the colour chart given in the question.
Each unit on the scale is a tenfold change in hydronium ion concentration (a fact for understanding only; no calculation is needed).

Acids, Alkalis, Indicators and the pH Scale: formulas and key facts

Ionisation of acids: $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$ ; $H_{2} S O_{4} + 2 H_{2} O \to 2 H_{3} O^{+} + S O_{4}^{2 -}$
Dissociation of alkalis: $N a O H \to N a^{+} + O H^{-}$ ; $K O H \to K^{+} + O H^{-}$
Neutralisation: $H_{3} O^{+} + O H^{-} \to 2 H_{2} O$
pH scale: pH below 7 acidic
pH 7 neutral
pH above 7 alkaline
Indicators: Litmus red in acid, blue in alkali
Methyl orange red in acid, yellow in alkali
Phenolphthalein colourless in acid, pink in alkali

pH below 7	acidic
pH 7	neutral
pH above 7	alkaline

Litmus	red in acid, blue in alkali
Methyl orange	red in acid, yellow in alkali
Phenolphthalein	colourless in acid, pink in alkali

How to solve Acids, Alkalis, Indicators and the pH Scale questions

For ionisation and calculation questions use this routine.

1Write the formula of the acid or alkali, then split it into ions; use water as a reactant for acids so that H₃O⁺ appears.
2Balance the equation for charge and atoms: H₂SO₄ gives two H₃O⁺, so 2H2O is needed.
3For mole questions, convert the amount of acid to moles of H₃O⁺ or OH⁻ using the equation.
4For indicator questions, state the colour in acid and in alkali for the indicator named.
5For pH, compare with 7 and use the stronger/weaker wording.

Acids, Alkalis, Indicators and the pH Scale 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.

Testing a mystery liquid in the kitchen

Suppose a student has three clear liquids labelled X, Y and Z. A drop of phenolphthalein turns X pink, Y stays colourless and Z stays colourless. Universal indicator turns Y red and Z green.

Classify X, Y and Z.

Try it first, then show the solutionSolution

Phenolphthalein turns pink only in an alkali, so X is alkaline.
Y stays colourless with phenolphthalein and turns universal indicator red, so Y is a strong acid.
Z gives green (pH about 7), so Z is neutral.

Answer: X: alkali; Y: strong acid; Z: neutral.

Memory hook: Pink = alkali for phenolphthalein; green = neutral for universal.

Tricks to remember Acids, Alkalis, Indicators and the pH Scale

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

Hydronium for acids, hydroxide for alkalis
Acids give H₃O⁺ (positive only); alkalis give OH⁻ (negative only).
Use it when: Definitions in terms of ions.
Phenolphthalein "Pink in Plus alkali"
Phenolphthalein is Pink in alkali and colourless in acid.
Use it when: Indicator colours.
pH 7 in the middle
Below 7 sour (acid), above 7 soapy (alkali), 7 pure water.
Use it when: Classifying a solution from its pH.

Acids, Alkalis, Indicators and the pH Scale: solved examples

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

Example 1: Definition · 2 marks · Short answerBoard level

Name the only positive ion present in every acid solution and the only negative ion in every alkali solution, and write one equation to show how each forms (use HCl and NaOH).

Try it first, then show the solutionSolution

Acid solutions contain the hydronium ion: $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$ .
Alkali solutions contain the hydroxide ion: $N a O H \to N a^{+} + O H^{-}$ .

Answer: Hydronium ion (H₃O⁺) for acids and hydroxide ion (OH⁻) for alkalis, with the equations above.

Example 2: Indicator colours · 3 marks · Short answerBoard level

State the colour of (a) methyl orange in dilute hydrochloric acid, (b) phenolphthalein in sodium hydroxide solution, (c) red litmus in potassium hydroxide solution.

Try it first, then show the solutionSolution

(a) Red.
(b) Pink.
(c) Blue.

Answer: (a) red (b) pink (c) blue.

Example 3: Ionisation of sulphuric acid · 3 marks · Short answerHarder board level

Write the equation for the ionisation of dilute sulphuric acid in water. How many moles of hydronium ions are present in a solution made from 0.25 mol of H₂SO₄?

Try it first, then show the solutionSolution

$H_{2} S O_{4} + 2 H_{2} O \to 2 H_{3} O^{+} + S O_{4}^{2 -}$ .
Ratio 1 : 2, so moles of $H_{3} O^{+} = 2 \times 0.25 = 0.5$ mol.

Answer: 0.5 mol of hydronium ions.

Example 4: Mass of alkali needed · 3 marks · Short answerHarder board level

What mass of sodium hydroxide neutralises 0.1 mol of dilute sulphuric acid? $H_{2} S O_{4} + 2 N a O H \to N a_{2} S O_{4} + 2 H_{2} O$ (Na = 23, O = 16, H = 1)

Try it first, then show the solutionSolution

Mole ratio H₂SO₄ : NaOH = 1 : 2, so 0.1 mol acid needs 0.2 mol NaOH.
Molar mass of NaOH = 23 + 16 + 1 = 40 g/mol.
Mass = 0.2 x 40 = 8.0 g.

Answer: 8.0 g of NaOH.

Example 5: Strong versus weak · 4 marks · Long answerHOTS / competency

Two solutions of equal concentration have pH 1 (solution P) and pH 4 (solution Q). Both react with magnesium. (a) Which is the stronger acid? (b) Explain with the help of ionisation. (c) Would the pH of P become more than 7 on adding a lot of water? Explain.

Try it first, then show the solutionSolution

(a) P is stronger, since a lower pH means more hydronium ions.
(b) P is almost completely ionised; Q is only partly ionised, so it gives fewer H₃O⁺ ions at the same concentration.
(c) No: dilution lowers the hydronium ion concentration, so the pH rises, but an acid never becomes alkaline by water alone; its pH stays below 7.

Answer: (a) P (b) P is almost fully ionised, Q only partly. (c) No, it only moves closer to 7.

Where students lose marks in Acids, Alkalis, Indicators and the pH Scale

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

Mistake: Writing H⁺ instead of H₃O⁺ in the definition
The syllabus defines acids in terms of hydronium ions, and the equation should show water.
Wrong working
HCl -> H⁺ + Cl⁻
Right working
HCl + H₂O -> H₃O⁺ + Cl⁻
Do this: Include water in the equation.
Usually costs: 1 mark.
Mistake: Calling every base an alkali
Only water-soluble bases are alkalis.
Do this: Define both and give an example of an insoluble base.
Usually costs: 1 mark.
Mistake: Mixing up the indicator colours
Phenolphthalein is colourless in acid (not pink), and methyl orange is red (not yellow) in acid.
Do this: Learn the table row by row.
Usually costs: 1 mark per item.
Mistake: Equating pH with concentration
A student says that a more concentrated acid always has a lower pH, ignoring strength.
Do this: Say that pH shows hydronium ions present, which depends on both strength and concentration.
Usually costs: 1 mark.

Exam tips

Always write the water molecule in the ionisation equation of an acid so that H₃O⁺ shows up.
For titration or neutralisation numericals, balance the equation before using mole ratios.
Name the indicator and the colour in acid and alkali together.
Remember that dilution of an acid raises the pH toward 7 but never beyond it.

Practice questions on Acids, Alkalis, Indicators and the pH Scale

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
The pH of a solution is 10. The solution is:
(a) strongly acidic
(b) neutral
(c) weakly alkaline
(d) weakly acidic
Hint
Compare with 7.
Answer
(c) weakly alkaline.
Q2 · 1 mark · MCQBoard level
Which of these is a weak acid?
(a) HCl
(b) HNO₃
(c) CH₃COOH
(d) H₂SO₄
Hint
Partly ionised.
Answer
(c) CH₃COOH.
Q3 · 2 marks · Short answerBoard level
Write the equation to show the ionisation of (a) nitric acid and (b) potassium hydroxide in water.
Hint
Acid gives H₃O⁺, alkali gives OH⁻.
Answer
(a) $H N O_{3} + H_{2} O \to H_{3} O^{+} + N O_{3}^{-}$ (b) $K O H \to K^{+} + O H^{-}$ .
Q4 · 2 marks · Short answerHarder board level
Distinguish between a base and an alkali, giving one example of a base that is not an alkali.
Hint
Solubility.
Answer
A base reacts with an acid to give salt and water only; an alkali is a base that dissolves in water, giving OH⁻ ions. Example of a non-alkali base: copper(II) oxide (or zinc oxide).
Q5 · 3 marks · Short answerHarder board level
Calculate the mass of sodium hydroxide needed to neutralise 3.65 g of hydrogen chloride. $N a O H + H C l \to N a C l + H_{2} O$ (H = 1, Cl = 35.5, Na = 23, O = 16)
Hint
Convert to moles first.
Answer
Moles of HCl = 3.65 / 36.5 = 0.1; moles of NaOH = 0.1; mass = 0.1 x 40 = 4.0 g.
Q6 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The pH of pure water is 7.
Reason (R): Pure water is neither acidic nor alkaline.
(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
What does pH 7 mean?
Answer
(a).
Q7 · 4 marks · Long answerHarder board level
A solution turns phenolphthalein pink and methyl orange yellow. (a) What is the nature of the solution? (b) Name the ion responsible. (c) What colour will red litmus turn? (d) Suggest a possible solute and give its dissociation equation.
Hint
Both indicators respond to alkalis.
Answer
(a) Alkaline. (b) Hydroxide ion, OH⁻. (c) Blue. (d) Sodium hydroxide: $N a O H \to N a^{+} + O H^{-}$ .
Q8 · 4 marks · Long answerHOTS / competency
Two dilute acid solutions of the same concentration have pH 2 and pH 6. Both are acids. (a) Which has the higher hydronium ion concentration? (b) A student says that the pH 6 solution is a strong acid since its pH is closer to 7. Is he right? Explain. (c) What will universal indicator show in each?
Hint
At equal concentration, a lower pH means more H₃O⁺ ions, so a stronger acid.
Answer
(a) The pH 2 solution. (b) No. At equal concentration, a higher pH means fewer H₃O⁺ ions, so the pH 6 acid is the weaker one (less ionised). (c) pH 2: red; pH 6: yellow or orange-yellow (per the chart given).

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.Ion common to all acids in water?Show answer
The hydronium ion, H₃O⁺.
2.Ion common to all alkalis in water?Show answer
The hydroxide ion, OH⁻.
3.Colour of methyl orange in alkali?Show answer
Yellow.
4.pH of a neutral solution?Show answer
7.
5.Give one weak alkali.Show answer
Ammonium hydroxide.

Acids, Alkalis, Indicators and the pH Scale: frequently asked questions

What is the difference between a base and an alkali?

An alkali is a base that dissolves in water; all alkalis are bases but not all bases are alkalis.

Why is acetic acid a weak acid?

It ionises only slightly in water, so few hydronium ions are present.

What colour does universal indicator show for a neutral solution?

Green, at pH 7.

What happens when an acid is mixed with an alkali?

They neutralise each other to form a salt and water.