The pH Scale Exam Questions
Practice IGCSE Chemistry exam questions on the pH scale. Covers pH values, strong and weak acids, strong and weak alkalis, and the relationship between pH and H+ concentration with mark schemes and examiner notes.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
These questions cover the pH scale. Write full answers before checking.
Question 1 (3 marks, Core)
(a) State the pH range for acidic solutions. (1)
(b) State the pH of a neutral solution. (1)
(c) State the pH range for alkaline solutions. (1)
Mark scheme
(a) pH 0 to less than 7 / pH < 7 [1]
(b) pH 7 [1]
(c) pH greater than 7 to 14 / pH > 7 [1]
Examiner note: The pH scale runs from 0 (strongest acid) to 14 (strongest alkali) at IGCSE level. Pure water has pH 7. Every unit decrease in pH represents a tenfold increase in H+ concentration.
Question 2 (2 marks, Supplement)
Define the terms strong acid and weak acid.
Mark scheme
- Strong acid: an acid that completely dissociates / ionises in aqueous solution [1]
- Weak acid: an acid that only partially dissociates / ionises in aqueous solution [1]
Examiner note: “Strong” does not mean “concentrated.” A strong acid is one that fully ionises (like HCl, H2SO4, HNO3). A weak acid partially ionises (like CH3COOH, H2CO3). Concentration refers to the amount of acid per dm3, which is a separate concept.
Question 3 (4 marks, Supplement)
Two solutions, both with a concentration of 0.1 mol/dm3, have different pH values:
| Solution | Acid | pH |
|---|---|---|
| A | Hydrochloric acid (HCl) | 1 |
| B | Ethanoic acid (CH3COOH) | 3 |
(a) Which is the strong acid? Explain your reasoning. (2)
(b) Which solution has the higher concentration of H+ ions? Explain. (2)
Mark scheme
(a) HCl (solution A) is the strong acid [1] because it has a lower pH at the same concentration, meaning it has fully dissociated to produce more H+ ions [1]
(b) Solution A [1] because it has a lower pH, which means a higher concentration of H+ ions. Each pH unit represents a tenfold difference, so pH 1 has 100 times more H+ than pH 3 [1]
Examiner note: Same concentration but different pH values means different degrees of ionisation. HCl fully ionises: HCl → H+ + Cl-. Ethanoic acid partially ionises: CH3COOH ⇌ CH3COO- + H+ (an equilibrium). This is why ethanoic acid has a higher pH at the same concentration.
Question 4 (3 marks, Core)
During a neutralisation reaction, dilute sodium hydroxide is gradually added to dilute hydrochloric acid containing universal indicator.
(a) State the colour of the indicator at the start. (1)
(b) Describe how the colour changes as NaOH is added. (1)
(c) State the pH at the exact point of neutralisation. (1)
Mark scheme
(a) Red [1] (the HCl solution is strongly acidic)
(b) The colour changes from red through orange and yellow to green [1]
(c) pH 7 [1]
Examiner note: At the point of neutralisation, all the H+ ions from the acid have reacted with OH- ions from the alkali. The resulting solution is neutral (pH 7) and the indicator is green. If excess NaOH is added, the pH continues to rise above 7 and the indicator turns blue/purple.
Question 5 (3 marks, Supplement)
Explain why adding water to a strong acid increases its pH but cannot make it alkaline.
Mark scheme
- Adding water dilutes the acid, reducing the concentration of H+ ions [1]
- Fewer H+ ions per unit volume means the pH increases (moves towards 7) [1]
- But the H+ ions are never removed completely, and no OH- excess is created, so the pH can approach 7 but never exceed it / never become alkaline [1]
Examiner note: Dilution changes concentration, not the nature of the solution. An acid can be diluted to approach pH 7 but cannot become pH 8 or above — that would require adding a base.
Question 6 (3 marks, Supplement)
Give one example each of: a strong acid, a weak acid, a strong alkali, and a weak alkali.
Mark scheme
- Strong acid: hydrochloric acid / sulfuric acid / nitric acid [1]
- Weak acid: ethanoic acid / citric acid / carbonic acid [1]
- Strong alkali: sodium hydroxide / potassium hydroxide [1]
- Weak alkali: ammonia solution (NH3 in water) [1]
(Award 1 mark for any three correct)
Examiner note: Common strong acids are HCl, H2SO4, and HNO3. The most commonly tested weak acid is ethanoic acid (CH3COOH). Ammonia dissolved in water is a weak alkali: NH3 + H2O ⇌ NH4+ + OH-.
Question 7 (3 marks, Supplement)
A solution of sulfuric acid has pH 1. A solution of ethanoic acid has pH 3. Both react with magnesium ribbon.
(a) Which acid reacts faster with magnesium? Explain why. (2)
(b) If the same volume and concentration of each acid is used, would the total volume of hydrogen gas produced be the same or different? Explain. (1)
Mark scheme
(a) Sulfuric acid (pH 1) reacts faster [1] because it has a higher concentration of H+ ions, so collisions between H+ ions and magnesium are more frequent [1]
(b) The total volume of hydrogen would be the same [1] — both acids have the same concentration and volume, so the same total number of moles of acid is present. The weak acid continues to dissociate as H+ is used up, eventually providing the same total amount of H+.
Examiner note: This is a crucial distinction: strong acids react FASTER (more H+ available at any moment), but weak acids of the same concentration produce the SAME TOTAL amount of product because equilibrium shifts forward as H+ is consumed.
What to revise if you scored below 5
If you confused strong/concentrated and weak/dilute, write out the definitions side by side — strong/weak refers to ionisation; concentrated/dilute refers to amount per dm3. Revisit the pH scale notes.
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