AQA GCSE Chemistry 4.4 Chemical changes: a complete overview

What topic 4.4 actually demands

Chemical changes brings together reactivity, acids and electricity to explain how substances are made and broken down. Topic 4.4 is full of predictable reactions and rules, so the examiners reward precise products, correct equations and clear reasoning from the reactivity series. The redox and electrolysis ideas also connect to quantitative chemistry and the required practicals.

This guide walks through all three dot points of the topic in specification order, then sets out the exam patterns AQA repeats. Each dot point has a matching page with practice questions; this overview ties them together.

Reactivity, redox and extraction

The reactivity series ranks metals by how readily they lose electrons to form positive ions. More reactive metals react more vigorously with water and acids and displace less reactive metals from their compounds. Oxidation is gain of oxygen or loss of electrons, and reduction is loss of oxygen or gain of electrons (OIL RIG). A metal's reactivity decides its extraction: metals less reactive than carbon are extracted by reduction with carbon, while more reactive metals need electrolysis.

Acids, alkalis and salts

Acids release $H^+$ ions (pH below 7) and alkalis release $OH^-$ ions (pH above 7). Neutralisation combines them to make water. Acids react with metals (salt + hydrogen), bases and alkalis (salt + water) and carbonates (salt + water + carbon dioxide), with the acid setting the salt: chlorides, sulfates or nitrates. A soluble salt is made by adding excess base to acid, filtering and crystallising. Strong acids fully ionise; weak acids only partly ionise.

Electrolysis

Electrolysis breaks down a molten or dissolved ionic compound using electricity, because the ions are then free to move. The cathode reduces positive ions and the anode oxidises negative ions. For solutions, remember the rules: hydrogen at the cathode unless the metal is less reactive than hydrogen, and oxygen at the anode unless a halide is present. Aluminium is extracted by electrolysis of molten aluminium oxide, with cryolite added to lower the melting point.

How topic 4.4 is examined

A typical AQA profile for this topic:

• Predicting products. Naming the salt and other products of acid reactions and the products at each electrode.
• Equations. Writing balanced symbol equations, ionic equations for neutralisation, and half-equations for electrodes.
• Reasoning. Using the reactivity series to explain extraction, displacement and reaction vigour.
• Practical. The making-a-soluble-salt and electrolysis required practicals, including apparatus and method.

Check your knowledge

A mix of recall and explanation questions covering topic 4.4. Attempt them under timed conditions, then check against the solutions.

1. Define oxidation in terms of electrons. (1 mark)
2. State the products when magnesium reacts with sulfuric acid. (2 marks)
3. Explain why iron can be extracted with carbon but aluminium cannot. (2 marks)
4. Write the ionic equation for neutralisation. (1 mark)
5. Name the products at each electrode when molten lead bromide is electrolysed. (2 marks)
6. Explain the difference between a strong acid and a concentrated acid. (2 marks)
7. State the products when nitric acid reacts with copper carbonate. (3 marks)
8. In the electrolysis of sodium chloride solution, name the gas formed at the anode and explain why. (2 marks)