Edexcel GCSE Chemistry Topic 3 Chemical changes: a complete overview

What Topic 3 actually demands

Chemical changes is a core Paper 1 topic that brings together acids, salts and electricity. Edexcel tests precise recall of the reactions and salts, confident equation writing including half-equations, and the practical methods for preparing salts and carrying out electrolysis. The pH and electrolysis content rewards clear reasoning at the particle level.

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

Acids, bases and the pH scale

Acids are sources of hydrogen ions and alkalis are sources of hydroxide ions. The pH scale (0 to 14) measures acidity: the lower the pH, the higher the hydrogen ion concentration, and a fall of one pH unit means a ten-fold rise in hydrogen ion concentration. Strong acids are fully dissociated and weak acids only partially dissociated; concentrated and dilute describe the amount dissolved, a separate idea. Neutralisation is $H^+ + OH^- \rightarrow H_2O$.

Reactions of acids

The salt is named from the acid (chlorides, sulfates, nitrates). Acids react with:

• metals to give a salt and hydrogen (squeaky pop with a lighted splint),
• metal oxides and hydroxides (bases) to give a salt and water (neutralisation),
• metal carbonates to give a salt, water and carbon dioxide (turns limewater cloudy).

Preparing salts

The method depends on the solubility of the salt and the base:

• Soluble salt from an insoluble base: add base in excess, filter off the excess, crystallise.
• Soluble salt from a soluble alkali: titrate to find the exact acid volume, repeat without indicator, crystallise.
• Insoluble salt: mix two soluble solutions to precipitate the salt, then filter, wash and dry.

Solubility rules predict which salts are insoluble (most carbonates and hydroxides; barium and lead sulfate; silver and lead chloride).

Electrolysis

Electrolysis breaks down a molten or dissolved ionic compound. Cations go to the cathode (reduction, electrons gained), anions to the anode (oxidation, electrons lost). Molten compounds give the metal and the non-metal. In solution, the cathode gives hydrogen unless the metal is less reactive than hydrogen; the anode gives oxygen unless a concentrated halide gives the halogen. Half-equations balance the electrons. The copper sulfate core practical shows copper deposited at the cathode and the dependence on the type of electrode.

How Topic 3 is examined

A typical Edexcel profile for this topic:

• Acids and pH. Strong versus weak, the pH and hydrogen-ion-concentration relationship.
• Reactions and salts. Naming products, writing equations, and the gas tests.
• Salt preparation. Six-mark method questions for soluble and insoluble salts.
• Electrolysis. Predicting electrode products and writing half-equations.

Check your knowledge

A mix of recall and application questions covering Topic 3. Attempt them under timed conditions, then check against the solutions.

1. State the ion released by all acids and the ion released by all alkalis. (2 marks)
2. Explain the difference between a strong acid and a weak acid. (2 marks)
3. Name the products of magnesium reacting with hydrochloric acid. (2 marks)
4. Describe the test for carbon dioxide. (1 mark)
5. Why is the insoluble base added in excess when preparing a soluble salt? (1 mark)
6. State what is produced at the cathode and anode when molten sodium chloride is electrolysed. (2 marks)
7. Write the half-equation for hydrogen forming at a cathode. (2 marks)
8. As pH falls from 5 to 2, by what factor does the hydrogen ion concentration change? (2 marks)