How to Study Acids and Bases for IGCSE Chemistry

Acids and bases is one of the highest-scoring topics in 0620 because it appears on every paper, every session, and the question types repeat predictably. It is also one of the broadest topics, touching the pH scale, indicators, neutralisation, salt preparation, oxide classification, strong versus weak acids, and the reactions of acids with metals, bases, and carbonates. The sheer volume of content can feel overwhelming, but it organises neatly into three blocks.

Block 1: The pH scale, indicators, and definitions

This is the foundation. Start here and make sure it is solid before moving to reactions.

Key content:

• The pH scale: 0-14. Below 7 is acidic, 7 is neutral, above 7 is alkaline.
• Acids produce H+ ions in aqueous solution. Bases neutralise acids. Alkalis are soluble bases that produce OH- ions in aqueous solution.
• Universal indicator gives a range of colours; litmus gives only red (acid) or blue (alkali). Know when to use each.
• Strong acids (HCl, H2SO4, HNO3) are fully ionised in solution. Weak acids (ethanoic acid, citric acid) are partially ionised. This is different from concentrated/dilute, which refers to the amount of solute per volume.

Study method: Use flashcards for definitions and indicator colours. Test yourself by classifying a list of substances as acidic, neutral, or alkaline. Practise the distinction between strong/weak and concentrated/dilute — it is a favourite exam question and many students confuse the terms.

Block 2: Reactions of acids

Acids react with four types of substance. Each produces predictable products. Learn the pattern, then practise writing balanced equations.

Acid + metal → salt + hydrogen

Only metals above hydrogen in the reactivity series react with dilute acids. Magnesium reacts vigorously; copper does not react at all. Example: Mg + H2SO4 → MgSO4 + H2

Acid + metal oxide (base) → salt + water

Metal oxides are basic. The reaction is a neutralisation. Example: CuO + 2HCl → CuCl2 + H2O

Acid + metal hydroxide (base or alkali) → salt + water

Another neutralisation. If the hydroxide is soluble (e.g., NaOH), it is an alkali. Example: NaOH + HNO3 → NaNO3 + H2O

Acid + metal carbonate → salt + water + carbon dioxide

The extra product is CO2, which can be tested with limewater. Example: CaCO3 + 2HCl → CaCl2 + H2O + CO2

Study method: Write all four reaction types from memory, with one example equation each. Check against the textbook. Repeat daily for a week. The pattern is the key: once you know that acid + carbonate always gives salt + water + CO2, you can predict the products for any acid-carbonate combination.

Naming salts: The acid determines the anion. HCl gives chlorides. H2SO4 gives sulfates. HNO3 gives nitrates. The metal (or metal in the base/carbonate) gives the cation. This rule lets you name any salt from any reaction. Practise by working backwards: “What acid and what base would you use to make zinc sulfate?” → Sulfuric acid + zinc oxide.

Block 3: Salt preparation methods

This block carries the most marks on Paper 4 and is the most commonly failed structured question. Three methods, each suited to a different situation:

Method 1: Excess insoluble base/metal/carbonate + acid

Use when the base, metal, or carbonate is insoluble or in excess.

1. Add acid to a beaker and warm.
2. Add the insoluble base in portions until in excess (unreacted solid remains).
3. Filter to remove excess solid.
4. Evaporate the filtrate gently to the point of crystallisation (not to dryness).
5. Cool and allow crystals to form. Pat dry with filter paper.

Suits: CuSO4 (from CuO + H2SO4), ZnCl2 (from Zn + HCl).

Method 2: Titration (soluble base/alkali + acid)

Use when both the acid and the base are soluble and there is no visible sign of reaction completion.

1. Titrate the alkali against the acid using an indicator to find the exact volume needed.
2. Repeat the titration without the indicator, adding the exact volume of alkali.
3. Evaporate to crystallisation point, cool, and collect crystals.

Suits: NaCl (from NaOH + HCl), KNO3 (from KOH + HNO3).

Method 3: Precipitation (mixing two soluble salts to form an insoluble salt)

Use when the desired salt is insoluble.

1. Mix solutions of two soluble salts that will produce the insoluble salt as a precipitate.
2. Filter to collect the precipitate.
3. Wash with distilled water and dry.

Suits: BaSO4 (from BaCl2 + Na2SO4), AgCl (from AgNO3 + NaCl).

Study method: For each method, know when to use it, what the key steps are, and what observations you would make. Draw a decision flowchart: Is the salt soluble or insoluble? If insoluble → precipitation. If soluble → is the base soluble or insoluble? If insoluble → excess method. If soluble → titration.

Practise with specific salts: “How would you prepare pure, dry crystals of lead(II) iodide?” (precipitation: mix lead(II) nitrate with potassium iodide). “How would you prepare pure, dry crystals of magnesium sulfate?” (excess method: MgO + H2SO4). See the acids and bases exam guide for full worked examples of each method.

Oxides

Metal oxides are basic (most) or amphoteric (aluminium oxide, zinc oxide). Non-metal oxides are acidic. An amphoteric oxide reacts with both acids and alkalis.

This is a small part of the topic but worth a few marks. Card the classifications and know one example reaction for an amphoteric oxide:

• Al2O3 + 6HCl → 2AlCl3 + 3H2O (reacting with acid)
• Al2O3 + 2NaOH + 3H2O → 2NaAl(OH)4 (reacting with alkali — the exact equation varies by scheme, but the concept is what matters)

Study sequence

1. Definitions, pH scale, indicators — 2 days. Flashcard the definitions.
2. Four acid reactions with balanced equations — 3 days. Write equations from memory daily.
3. Salt preparation methods — 4 days. Learn the decision flowchart and practise with named salts.
4. Strong vs weak, oxide classification — 2 days.
5. Past paper questions on acids and bases — ongoing.

Total study time: roughly two weeks of focused work, with daily flashcard review continuing after that. The topic is large but highly structured, and the exam questions are predictable. Master the patterns and you will pick up marks consistently.