Chemical Changes: Exam Practice

This lesson provides a comprehensive revision and exam practice session covering all topics in the AQA GCSE Chemistry Chemical Changes unit (4.4). It brings together the reactivity series, extraction of metals, oxidation and reduction, acids and bases, neutralisation, salt preparation, strong and weak acids, and electrolysis. Use this lesson to consolidate your knowledge, identify gaps, and practise the types of questions you will encounter in the exam.

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Topic Overview

The Chemical Changes unit covers the following key areas:

graph TD
    A["AQA GCSE Chemistry<br/>Chemical Changes (4.4)"] --> B["4.4.1 Reactivity<br/>of Metals"]
    A --> C["4.4.2 Reactions of<br/>Acids"]
    A --> D["4.4.3 Electrolysis"]
    B --> E["Reactivity Series"]
    B --> F["Extraction of Metals"]
    B --> G["Oxidation & Reduction"]
    C --> H["Acids & Bases"]
    C --> I["Neutralisation &<br/>Salt Preparation"]
    C --> J["Strong & Weak<br/>Acids (H)"]
    D --> K["Electrolysis of<br/>Molten Compounds"]
    D --> L["Electrolysis of<br/>Aqueous Solutions"]

    style A fill:#4a90d9,color:#fff
    style B fill:#e74c3c,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#f39c12,color:#fff

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Key Definitions to Remember

Term	Definition
Reactivity series	An arrangement of metals in order of their reactivity, from most reactive to least reactive
Displacement reaction	A reaction where a more reactive element takes the place of a less reactive element in a compound
Oxidation	Gain of oxygen or loss of electrons (OIL)
Reduction	Loss of oxygen or gain of electrons (RIG)
Redox	A reaction involving both oxidation and reduction
Ore	A naturally occurring rock containing enough metal to make extraction economically worthwhile
Acid	A substance that produces H+ ions in aqueous solution
Base	A substance that neutralises an acid (metal oxide or hydroxide)
Alkali	A soluble base that produces OH- ions in aqueous solution
Neutralisation	The reaction of an acid with a base to produce a salt and water
pH scale	A scale from 0 to 14 measuring acidity/alkalinity
Strong acid [H]	An acid that is completely ionised in aqueous solution
Weak acid [H]	An acid that is only partially ionised in aqueous solution
Electrolysis	Decomposition of a molten or dissolved ionic compound using an electric current
Cathode	The negative electrode (attracts cations; reduction occurs here)
Anode	The positive electrode (attracts anions; oxidation occurs here)
Electrolyte	The molten or dissolved ionic compound that conducts electricity

Exam Tip: Before the exam, review all key definitions. Many 1-mark questions simply ask for a definition. Having precise definitions memorised is an easy way to pick up marks. Write each definition on a flashcard and test yourself until you can recall them perfectly.

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Essential Equations

Reactions of Metals

Reaction type	General equation
Metal + water	metal + water → metal hydroxide + hydrogen
Metal + acid	metal + acid → salt + hydrogen
Metal + metal salt	reactive metal + less reactive metal salt → new salt + displaced metal

Reactions of Acids

Reaction type	General equation
Acid + metal	acid + metal → salt + hydrogen
Acid + metal oxide	acid + metal oxide → salt + water
Acid + metal hydroxide	acid + metal hydroxide → salt + water
Acid + metal carbonate	acid + metal carbonate → salt + water + carbon dioxide

Neutralisation (Ionic)

H+(aq) + OH-(aq) → H2O(l)

Extraction

Method	General equation
Reduction with carbon	metal oxide + carbon → metal + carbon dioxide

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Common Question Types

Type 1: Reactivity and Displacement (3-4 marks)

Example: Iron is added to copper sulfate solution. Describe and explain what you would observe.

Model answer:

• The iron nail becomes coated with an orange-brown layer of copper.
• The blue colour of the solution fades (becomes paler/eventually colourless).
• Iron is more reactive than copper, so it displaces copper from the copper sulfate solution.
• Iron atoms lose electrons and form Fe2+ ions (oxidation). Cu2+ ions gain electrons and form copper atoms (reduction).

Type 2: Identifying Oxidation and Reduction (2-3 marks)

Example: In the reaction: ZnO + C → Zn + CO, identify which substance is oxidised and which is reduced.

Model answer:

• Carbon is oxidised — it gains oxygen (forms CO).
• Zinc oxide is reduced — it loses oxygen (zinc is released).
• Carbon is the reducing agent; zinc oxide is the oxidising agent.

Type 3: Salt Preparation (6 marks)

Example: Describe how to prepare a pure, dry sample of magnesium sulfate from magnesium oxide and dilute sulfuric acid.

Model answer:

1. Warm dilute sulfuric acid gently in a beaker.
2. Add magnesium oxide powder a spatula at a time, stirring continuously.
3. Continue adding magnesium oxide until it is in excess (unreacted white solid remains).
4. Filter the mixture through filter paper to remove excess magnesium oxide.
5. Pour the colourless filtrate (magnesium sulfate solution) into an evaporating basin.
6. Heat gently on a water bath until half the water has evaporated.
7. Leave to cool and crystallise slowly at room temperature.
8. Pat the crystals dry with filter paper.

Type 4: Electrolysis Predictions (4-6 marks)

Example: Predict the products of electrolysis of copper chloride solution using graphite electrodes. Explain your answer.

Model answer:

• At the cathode (-): Copper is deposited (orange-brown solid). Copper is less reactive than hydrogen, so Cu2+ ions are preferentially discharged. Cu2+ + 2e- → Cu (reduction).
• At the anode (+): Chlorine gas is produced (bubbles; bleaches damp litmus paper). A halide ion (Cl-) is present, so chlorine is produced. 2Cl- → Cl2 + 2e- (oxidation).

Exam Tip: For 6-mark questions, use the structure: Point, Evidence, Explanation (PEE). Make at least 3 distinct points, support each with evidence (an observation, equation, or fact), and explain using scientific reasoning. Write in full sentences and use correct scientific terminology.

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Practice Questions

Question 1 (2 marks)

State what is meant by the term "oxidation" in terms of: (a) Oxygen transfer (b) Electron transfer

Answer: (a) Oxidation is the gain of oxygen. (b) Oxidation is the loss of electrons.

Question 2 (3 marks)

Magnesium reacts with hydrochloric acid to produce magnesium chloride and hydrogen. Name the salt produced and state the test for the gas.

Answer:

• The salt is magnesium chloride.
• Test for hydrogen: apply a burning splint — hydrogen burns with a squeaky pop.

Question 3 (4 marks)

Explain why aluminium is extracted by electrolysis rather than by reduction with carbon.

Answer:

• Aluminium is more reactive than carbon in the reactivity series.
• Therefore, carbon is not reactive enough to displace aluminium from its oxide (carbon cannot reduce aluminium oxide).
• Electrolysis uses electrical energy to decompose the molten aluminium oxide.
• At the cathode, aluminium ions are reduced (gain electrons) to form aluminium metal: Al3+ + 3e- → Al.

Question 4 (3 marks) [H]

Explain the difference between a strong acid and a weak acid.

Answer:

• A strong acid is completely ionised in aqueous solution — all acid molecules dissociate into ions (forward arrow →).
• A weak acid is only partially ionised — an equilibrium is established between undissociated molecules and ions (reversible arrow ⇌).
• At the same concentration, a strong acid has a lower pH (more H+ ions) than a weak acid.

Question 5 (4 marks)

During the electrolysis of sodium chloride solution, hydrogen is produced at the cathode. Explain why hydrogen is produced instead of sodium.

Answer:

• Both Na+ ions and H+ ions (from water) are present in the solution and are attracted to the cathode.
• Sodium is more reactive than hydrogen (higher in the reactivity series).
• Because sodium is more reactive, it is harder to reduce — sodium ions do not gain electrons as easily as hydrogen ions.
• Therefore, H+ ions are preferentially discharged at the cathode, gaining electrons to form hydrogen gas: 2H+ + 2e- → H2.