Acids, Bases and Indicators Exam Questions

(a) A substance that produces hydrogen ions (H+) in aqueous solution / a proton donor [1]

(b) A substance that neutralises an acid / a proton acceptor [1]

(c) A soluble base / a base that dissolves in water to produce hydroxide ions (OH-) [1]

Examiner note: All alkalis are bases, but not all bases are alkalis. Copper oxide is a base (it neutralises acids) but not an alkali (it is insoluble in water). Sodium hydroxide is both a base and an alkali.

(a) A = acidic, B = neutral, C = alkaline [1]

(b) Solution A [1]

(c) Solution C [1]

Examiner note: pH < 7 = acidic, pH = 7 = neutral, pH > 7 = alkaline. Lower pH means more H+ ions. Higher pH means more OH- ions. Universal indicator is red in strong acid, green when neutral, and purple in strong alkali.

(a) Magnesium + hydrochloric acid → magnesium chloride + hydrogen [1]

(b) Copper oxide + hydrochloric acid → copper chloride + water [1]

(c) Sodium hydroxide + hydrochloric acid → sodium chloride + water [1]

(d) Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide [1]

Examiner note: Learn the four reaction patterns: acid + metal → salt + hydrogen; acid + metal oxide → salt + water; acid + alkali → salt + water; acid + carbonate → salt + water + CO2. The salt name depends on which acid is used: HCl → chloride, H2SO4 → sulfate, HNO3 → nitrate.

(a) Bubbles / effervescence produced [1]; the zinc dissolves / gets smaller [1]

(b) Hold a burning splint near the gas — it burns with a squeaky pop if it is hydrogen [1]

Examiner note: The “squeaky pop” test is specific to hydrogen. Do not confuse it with the “relights a glowing splint” test (that is for oxygen) or the “limewater turns milky” test (that is for carbon dioxide).

• In water, HCl molecules dissociate / ionise to produce H+ ions and Cl- ions [1]
• It is the H+ ions (hydrogen ions) that give the solution its acidic properties [1]
• As a dry gas, HCl molecules are covalent and do not produce H+ ions, so no acidic properties are shown [1]

Examiner note: This is a key Supplement concept. Acids only behave as acids in water because water is needed for the ionisation: HCl(g) → H+(aq) + Cl-(aq). Dry HCl gas does not turn dry litmus paper red.

(a) Red [1]

(b) Blue [1]

Examiner note: Litmus is the simplest acid-base indicator: red in acid, blue in alkali. However, it cannot tell you the pH value — for that, use universal indicator or a pH meter.

• H+(aq) + OH-(aq) → H2O(l) [1]
• All acids produce H+ ions in solution and all alkalis produce OH- ions [1]
• The metal ions and acid anions are spectator ions that do not take part in the reaction, so the only reaction occurring is H+ + OH- → H2O in every case [1]

Examiner note: This ionic equation applies to all acid-alkali neutralisations. The spectator ions (e.g. Na+, Cl-, K+, NO3-) are present but unchanged. They appear in the full equation but cancel out in the ionic equation.

• Litmus paper can only show whether a solution is acidic (red), alkaline (blue), or neutral (no change / purple) [1]
• Universal indicator shows a range of colours that correspond to specific pH values [1]
• Universal indicator is more useful because it shows how strongly acidic or alkaline the solution is, not just whether it is acidic or alkaline [1]

Examiner note: For precise pH measurement, a pH meter is even better than universal indicator. However, universal indicator is the most common indicator used at IGCSE level for determining approximate pH.