The questions are on the acids and bases test. Take it under real conditions before reading the mark scheme.
Quick answer: Acids are substances that release hydrogen ions, H⁺, in water. This mark scheme gives the model answers and the marks for the 8 questions of the Chemistry 9-10 test on acids and bases, from acid, base or alkali to antacid tablet, with the equivalent answers to accept.
Model answers and mark schemes for the test. Credit equivalent wording, apply error carried forward in calculations, and award marks for each distinct chemical idea.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE C4, IGCSE 7
Question 1 Acid, base or alkali (2 marks)
- An alkali is a base that dissolves in water, forming OH⁻ ions.
- pH 7; the H⁺ (hydrogen) ion.
Marking: 1 mark for each part; both pieces of information are needed in part b.
Question 2 Carbonate and acid (3 marks)
- Sodium carbonate + nitric acid → sodium nitrate + water + carbon dioxide.
- Na₂CO₃ + 2HNO₃ → 2NaNO₃ + H₂O + CO₂.
Marking: 1 mark for the word equation; 1 mark for correct formulae; 1 mark for balancing.
Question 3 Neutralisation ions (2 marks)
- H⁺(aq) + OH⁻(aq) → H₂O(l).
- K⁺ and NO₃⁻.
Marking: 1 mark each; state symbols are not required for the mark.
Question 4 Copper chloride crystals (3 marks)
- The fizzing stops and some solid carbonate remains undissolved.
- To remove the excess copper carbonate that did not react.
- Heating to dryness would destroy the crystals and may decompose the salt; heating only until crystals appear on a glass rod and then cooling gives pure crystals.
Marking: 1 mark each.
Question 5 Strong and weak acids (3 marks)
- Hydrochloric acid is a strong acid that ionises completely, whereas ethanoic acid ionises only partly, so hydrochloric acid has a higher H⁺ concentration and therefore a lower pH.
- The reaction is reversible.
Marking: 1 mark for complete versus partial ionisation; 1 mark linking H⁺ concentration to pH; 1 mark for reversible.
Question 6 pH and hydrogen ions (2 marks)
- Two units apart, so 10 × 10 = 100 times.
- pH 3.
Marking: 1 mark each.
Question 7 Reading the pH scale (2 marks)
- An ant sting is acidic, so a mild alkali is best: soap (pH 10) or seawater at pH 8.1. Oven cleaner (pH 13) is too corrosive.
- 6.5 − 2.5 = 4 units, so 10⁴ = 10 000 times.
Marking: Accept seawater or soap with a reason; 1 mark for the factor.
Question 8 Antacid tablet (3 marks)
- M = 58; n = 2.9 ÷ 58 = 0.050 mol.
- Ratio 1 : 2, so n(HCl) = 0.10 mol.
- M(MgCl₂) = 95; m = 0.050 × 95 = 4.75 g.
Marking: 1 mark each; allow error carried forward.
