Acids and Bases: IGCSE Chemistry Exam Guide

Acids and bases questions appear on every 0620 paper, every series. They span the pH scale, indicators, neutralisation reactions, salt preparation methods, and the behaviour of oxides. The good news is that the question types are highly repetitive: learn the patterns below and you can predict what the examiner wants before you finish reading the stem. The full topic content is covered under acids, bases and salts; this guide is about turning that knowledge into exam marks.

The five question types you will meet

Almost every acids-and-bases question on Papers 2 and 4 falls into one of five categories. Recognising the type tells you what format the answer needs.

1. pH and indicators. Given a substance, state whether it is acidic, neutral or alkaline; give the pH range; name a suitable indicator and state the colour change. Paper 2 loves this as a one-mark MCQ; Paper 4 sets it as a short-answer row.
2. Neutralisation equations. Write a word or balanced symbol equation for an acid reacting with a base, carbonate or metal. The mark scheme awards one mark for correct formulae and one for balancing.
3. Salt preparation method. Given a named salt, describe how to prepare a pure, dry sample. This is a 4-6 mark structured question on Paper 4 and appears in some form almost every series.
4. Strong vs weak acids. Compare the behaviour of hydrochloric acid (strong) with ethanoic acid (weak) of the same concentration. The question tests whether you can distinguish ionisation from concentration.
5. Oxide classification. Classify an oxide as acidic, basic or amphoteric and predict its reaction with acid or alkali.

Salt preparation: the marks and the method

Salt preparation is the highest-value structured question in the acids-and-bases section. The method depends on whether the base is soluble or insoluble.

Insoluble base or excess metal/carbonate method

Use this when the base (or metal or carbonate) is insoluble, or when you can add an excess of it:

1. Add the acid to a beaker and warm gently.
2. Add the insoluble base (or metal or carbonate) in small portions until it is in excess — you see unreacted solid remaining.
3. Filter to remove the excess solid.
4. Evaporate the filtrate gently to the point of crystallisation (not to dryness).
5. Allow to cool and crystallise. Pat dry with filter paper.

The marks that students drop: forgetting to mention “excess” (the examiner needs to know how you know the acid has all reacted), writing “boil” instead of “evaporate gently” (boiling spatters and gives poor crystals), and omitting “filter” when an excess solid was added.

Titration method (soluble base)

Use this when both the acid and the base are soluble and there is no visible sign of excess (sodium hydroxide dissolving in acid looks the same whether you have added enough or too much):

1. Titrate the acid against the alkali using an indicator to find the exact volume needed.
2. Repeat without the indicator, using the volume you found.
3. Evaporate gently to crystallise.

The key mark here is the repeat without indicator — the examiner awards a mark for recognising that the indicator would contaminate the salt.

Writing equations that score full marks

The mark scheme splits equation marks into two: correct formulae and correct balancing. Earn both by following a fixed sequence:

1. Write the word equation first (mentally or in the margin).
2. Convert each name to its correct formula. Common errors: writing HCl2 instead of HCl, NaOH2 instead of NaOH, or forgetting that sulfuric acid is H2SO4 (two hydrogens).
3. Balance by adjusting coefficients only.
4. Include state symbols if the question asks for them (aq, l, s, g).

Example: Magnesium carbonate reacts with dilute hydrochloric acid.

MgCO3 + 2HCl -> MgCl2 + H2O + CO2

The three products of an acid-carbonate reaction are the salt, water and carbon dioxide. Forgetting carbon dioxide loses one mark; writing MgCl instead of MgCl2 loses another.

Strong vs weak: how to phrase it

The exam tests this distinction with a compare question, usually asking you to explain differences in pH, rate of reaction or conductivity between two acids of the same concentration.

The phrasing that scores:

• “HCl is a strong acid; it fully ionises in water, so the concentration of H+ ions is high.”
• “Ethanoic acid is a weak acid; it partially ionises in water, so the concentration of H+ ions is lower at the same concentration.”

The phrasing that does not score: “HCl is stronger”, “ethanoic acid is weaker” without the ionisation explanation. The mark is for linking the acid strength to the degree of ionisation, not just labelling it.

Do not confuse this with concentrated/dilute, which refers to the amount of solute dissolved. A dilute strong acid (dilute HCl, pH around 1-2) is still fully ionised; a concentrated weak acid (concentrated ethanoic acid) is still only partially ionised.

Oxide classification: a two-minute topic worth easy marks

Metal oxides are basic (they react with acids to form a salt and water). Non-metal oxides are acidic (they react with alkalis or dissolve in water to form acids). A few metal oxides are amphoteric (they react with both acids and alkalis); the two the syllabus names are aluminium oxide and zinc oxide.

The MCQ often tests this with a table: given four oxides, identify the acidic one, the basic one or the amphoteric one. The theory paper may ask you to write the equation for an amphoteric oxide reacting with both HCl and NaOH.

Common mark-losing errors

1. Writing “calcium” as the product instead of “calcium chloride”. The salt must be named fully.
2. Omitting water from neutralisation products. Acid + base -> salt + water, always.
3. Confusing “add excess” with “add a lot”. Excess means more than enough to react with all the acid; it is the visible sign that the reaction is complete (unreacted solid settles).
4. Using “amount” when you mean “concentration” or “volume”. Amount in chemistry means moles. The examiner will not credit ambiguity.
5. Forgetting CO2 in acid + carbonate reactions. Three products, not two.

These errors are catalogued more broadly in common exam mistakes.

Worked Paper 4 question

Q: Describe how you would prepare a pure, dry sample of copper(II) sulfate crystals starting from copper(II) oxide and dilute sulfuric acid. (5 marks)

Model answer:

1. Warm the dilute sulfuric acid in a beaker (1).
2. Add copper(II) oxide powder in small portions, stirring, until it is in excess (unreacted black solid remains) (1).
3. Filter the mixture to remove the excess copper(II) oxide (1).
4. Heat the filtrate gently to evaporate some water / to the point of crystallisation (1).
5. Allow to cool so that crystals form. Pat dry with filter paper (1).

Each numbered point earns one mark. The words “excess”, “filter”, “gently” and “crystallise” are each load-bearing. Drop any one and you lose a mark, even if the rest of the method is perfect.

If acids and bases is one of the topics costing you marks, a focused session on the salt-preparation method and equation writing usually closes the gap quickly. Our trial lesson can diagnose exactly where in the method your marks are being lost.