OCR GCSE Chemistry A: Types of chemical reactions (J248) – Revision Notes

These notes condense sub-topic C3.3 Types of chemical reactions for quick recall. Source: the OCR GCSE (9–1) Chemistry A (Gateway Science) J248 specification (Version 4.0, August 2026), covering learning outcomes C3.3a–C3.3k, mathematical learning outcome CM3.3i, and the PAG C6 (titration) and PAG C7 (production of salts) practical suggestions. C3.3b, C3.3g, C3.3i and C3.3j are Higher Tier only; the rest are assessed at both tiers. For full explanations and longer worked examples, read the Types of chemical reactions study guide first.

For exam-style questions with worked answers, go to the Types of chemical reactions practice questions. The OCR GCSE Chemistry hub links every J248 topic, and the printable checklist lets you track what you have revised. A free 10-minute diagnostic is a quick way to decide what to revise next.

Core definitions

Term	Meaning
Oxidation (C3.3a)	Gain of oxygen
Reduction (C3.3a)	Loss of oxygen
Oxidation (C3.3b, Higher Tier only)	Loss of electrons
Reduction (C3.3b, Higher Tier only)	Gain of electrons
Redox	A reaction in which oxidation and reduction both happen
Oxidising agent	Oxidises something else; is itself reduced
Reducing agent	Reduces something else; is itself oxidised
Acid	Forms H⁺ ions when it dissolves in water
Alkali	A soluble base; its solution contains OH⁻ ions
Base	Neutralises an acid, e.g. a metal oxide or hydroxide
Neutralisation	Acid + alkali or base → salt + water
pH	Measures relative acidity and alkalinity; 7 is neutral

C3.3a: redox in terms of oxygen

Method in steps

1. Find the substance that loses oxygen: it is reduced and is the oxidising agent.
2. Find the substance that gains oxygen: it is oxidised and is the reducing agent.
3. Name whole substances (nickel(II) oxide, not nickel).

Worked reminder. NiO + H₂ → Ni + H₂O. Nickel(II) oxide loses oxygen: reduced, oxidising agent. Hydrogen gains oxygen to become water: oxidised, reducing agent.

C3.3b: redox in terms of electrons (Higher Tier only)

OIL RIG, for electrons: Oxidation Is Loss, Reduction Is Gain.

Method in steps

1. Split the ionic equation into two half equations.
2. Electrons appear on the right of the oxidation half equation (lost).
3. Electrons appear on the left of the reduction half equation (gained).
4. Electrons lost must equal electrons gained.

Worked reminder. Mg + Zn²⁺ → Mg²⁺ + Zn

• Mg → Mg²⁺ + 2e⁻: magnesium is oxidised (reducing agent).
• Zn²⁺ + 2e⁻ → Zn: zinc ions are reduced (oxidising agent).

Metal + acid is also redox: Fe + 2H⁺ → Fe²⁺ + H₂. Iron atoms lose electrons; hydrogen ions gain them.

C3.3c: ions in acids and alkalis

• Acid in water → H⁺ ions: HCl → H⁺ + Cl⁻; H₂SO₄ → 2H⁺ + SO₄²⁻
• Alkali solution → OH⁻ ions: LiOH → Li⁺ + OH⁻; Ca(OH)₂ → Ca²⁺ + 2OH⁻
• The H⁺ ions are free in solution, not still attached to the acid molecule (a misconception the specification names).
• Alkalis can be just as corrosive as acids (another named misconception).

C3.3d and C3.3e: neutralisation

Reaction type	General equation	Example
Acid + alkali	salt + water	LiOH + HCl → LiCl + H₂O
Acid + base (metal oxide)	salt + water	CuO + 2HCl → CuCl₂ + H₂O
Acid + base (insoluble metal hydroxide)	salt + water	Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

Ionic equation for every aqueous acid–alkali neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l). Spectator ions (the metal ion and the acid’s negative ion) do not change, so they are omitted.

Salt names: hydrochloric acid → chloride; nitric acid → nitrate; sulfuric acid → sulfate.

C3.3f: carbonates and metals with acids

• acid + metal carbonate → salt + water + carbon dioxide
• acid + (some) metal → salt + hydrogen

Method in steps for a balanced equation

1. Name the products from the patterns above.
2. Write correct formulae (charges: Ca²⁺, Al³⁺, NO₃⁻, SO₄²⁻).
3. Balance metal atoms, then the acid’s ion group, then H and O.
4. Count every atom on both sides.

Worked reminders

• CaCO₃ + 2HNO₃ → Ca(NO₃)₂ + H₂O + CO₂
• 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂

Copper, silver and gold do not react with dilute hydrochloric or sulfuric acid, which is why the specification says “some metals”.

PAG C7: making a pure, dry salt from an insoluble base

1. Warm the dilute acid.
2. Add the solid base or carbonate in excess, stirring, until no more reacts.
3. Filter: the unreacted solid stays in the paper.
4. Heat the filtrate to boil away part of the water, then let it crystallise as it cools.
5. Filter off the crystals and dry them on filter paper or in a warm oven.

Why excess solid? It guarantees no acid remains, and unreacted solid is simple to remove by filtering.

C3.3g: strength and concentration (Higher Tier only)

Pair of words	Describes	Example
Concentrated / dilute	Amount of acid in a given volume (ratio of amount to volume)	2 mol/dm³ is more concentrated than 0.1 mol/dm³
Strong / weak	Degree of ionisation	Strong: hydrochloric, nitric, sulfuric (fully ionised). Weak: ethanoic, citric, carbonic (partly ionised)

Must-know distinction: “weak” is not “dilute”. A concentrated weak acid and a dilute strong acid both exist.

Concentration as a ratio (CM3.3i). Divide amount (mol) by volume (dm³). Convert cm³ to dm³ by dividing by 1000.

C3.3h, C3.3i and C3.3j: pH

• C3.3h (both tiers): pH measures relative acidity and alkalinity. Below 7 acidic, 7 neutral, above 7 alkaline.
• C3.3i (Higher Tier only): more H⁺ → lower pH. A neutral solution has equal H⁺ and OH⁻. Whole-number pH only.
• C3.3j (Higher Tier only): H⁺ concentration ×10 → pH falls by 1.

pH change	Change in H⁺ concentration
Down 1	10 times more
Down 2	100 times more
Down 3	1000 times more
Up 1	10 times less

Worked reminder (Higher Tier only). pH 6 → pH 3 is a fall of 3, so the H⁺ concentration is 10 × 10 × 10 = 1000 times greater.

Titration curve (Higher Tier only). Alkali added to acid: pH starts low, rises slowly, then jumps steeply through 7 near exact neutralisation, then levels off high. Acid added to alkali gives the mirror image: starts high, falls slowly, drops steeply, then levels off low.

C3.3k and PAG C6: measuring pH

Method	How	Strengths and limits
Universal indicator	Add drops or use paper; compare with colour chart	Quick and cheap; approximate whole-number pH; colour judgement is subjective; hard to use in coloured solutions
pH meter / probe	Calibrate in buffer solutions; rinse with distilled water between samples; read display	More precise; works in coloured solutions; can log pH continuously during a titration

Universal indicator colours run roughly red (strongly acidic), orange, yellow, green (neutral), blue, purple (strongly alkaline). Always use the chart supplied with your indicator.

Quick self-test

1. In CuO + H₂ → Cu + H₂O, name the substance oxidised and the oxidising agent.
2. Define reduction in terms of oxygen.
3. (Higher Tier only) Write the half equation for a calcium atom forming a calcium ion, and say whether it is oxidation or reduction.
4. (Higher Tier only) In Br₂ + 2e⁻ → 2Br⁻, is bromine oxidised or reduced?
5. Which ion do all acids form in water?
6. Write the ionic equation for neutralisation, with state symbols.
7. Name the salt formed from potassium hydroxide and sulfuric acid, and write the balanced equation.
8. Give all three products of lithium carbonate reacting with sulfuric acid.
9. (Higher Tier only) Solution C has 0.30 mol of acid in 250 cm³. Solution D has 0.090 mol in 50 cm³. Which is more concentrated?
10. (Higher Tier only) By what factor does the H⁺ concentration change from pH 2 to pH 3?
11. (Higher Tier only) Explain why ethanoic acid can be concentrated and weak at the same time.
12. Give one advantage of a pH probe over universal indicator paper.

Answers

1. Hydrogen is oxidised (gains oxygen). The oxidising agent is copper(II) oxide.
2. Reduction is the loss of oxygen.
3. Ca → Ca²⁺ + 2e⁻; electrons are lost, so this is oxidation.
4. Reduced: bromine gains electrons.
5. Hydrogen ions, H⁺.
6. H⁺(aq) + OH⁻(aq) → H₂O(l)
7. Potassium sulfate: 2KOH + H₂SO₄ → K₂SO₄ + 2H₂O
8. Lithium sulfate, water and carbon dioxide: Li₂CO₃ + H₂SO₄ → Li₂SO₄ + H₂O + CO₂
9. C: 0.30 ÷ 0.250 = 1.2 mol/dm³. D: 0.090 ÷ 0.050 = 1.8 mol/dm³. D is more concentrated.
10. pH rises by 1, so the H⁺ concentration is 10 times smaller.
11. Concentrated means a large amount of acid per dm³; weak means only partly ionised. The two describe different things.
12. Any one: gives a more precise reading; not affected by the colour of the solution; can record continuously; no subjective colour matching.

Where marks are usually lost

• Calling the oxidising agent “oxidised”; it gets reduced.
• Giving the metal, not its oxide, when asked what is reduced.
• (Higher Tier only) Writing the electrons for an oxidation on the left-hand side.
• Leaving water out of acid + carbonate products, or forgetting carbon dioxide.
• Writing Al₂SO₄₃ or AlSO₄ instead of Al₂(SO₄)₃: brackets around a group that appears more than once.
• Writing H⁺ + OH⁻ → H₂O without state symbols when they are asked for.
• (Higher Tier only) Using “weak” when the question means “dilute”, or the reverse.
• (Higher Tier only) Saying a pH drop of 2 means twice as much H⁺, instead of 100 times.
• Forgetting to convert cm³ to dm³ before dividing.
• In PAG C7, adding excess acid rather than excess solid, so the salt is contaminated.

Official syllabus

OCR GCSE (9–1) Chemistry A (Gateway Science) J248 specification, Version 4.0 (August 2026), for first assessment in 2018, published by OCR. Sub-topic C3.3 Types of chemical reactions, learning outcomes C3.3a–C3.3k, mathematical learning outcome CM3.3i and practical suggestions (PAG C6, PAG C7).