AQA GCSE Chemistry 4.8 Chemical analysis: a complete overview

What topic 4.8 actually demands

Chemical analysis is about identifying substances and judging their purity. Topic 4.8 is recall-heavy: the examiners reward exact tests, results and definitions, plus careful chromatography method and $R_f$ arithmetic. It links to atoms and compounds (pure substances) and to the reactions of acids (ion tests).

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.

Purity and formulations

In chemistry, a pure substance is a single element or compound, which differs from the everyday meaning. A pure substance has a sharp melting and boiling point, while a mixture melts over a range and at a lower value, so melting point tests purity. A formulation is a mixture designed as a useful product, with each component in a measured quantity for a purpose; examples include paints, fuels, fertilisers, medicines and alloys.

Chromatography

Paper chromatography separates substances by their attraction to the stationary phase (the paper) versus the mobile phase (the solvent). A pure substance gives a single spot; a mixture gives several. The $R_f$ value is the distance moved by the spot divided by the distance moved by the solvent, always between 0 and 1, and is used to identify substances.

Tests for gases and ions

The four gas tests are: hydrogen (squeaky pop), oxygen (relights a glowing splint), carbon dioxide (limewater milky) and chlorine (bleaches litmus). Flame tests identify metal ions by colour, and sodium hydroxide gives coloured precipitates for some. Carbonates fizz with acid, halides give coloured precipitates with silver nitrate, and sulfates give a white precipitate with barium chloride. Instrumental methods are faster, more sensitive and more accurate.

How topic 4.8 is examined

A typical AQA profile for this topic:

• Recall of tests. The four gas tests, flame colours and ion tests with their results.
• Chromatography. Method, identifying pure versus mixture, and $R_f$ calculations.
• Purity. Using melting points to judge purity, and defining formulations.
• Comparison. Advantages of instrumental methods over chemical tests.

Check your knowledge

A mix of recall and calculation questions covering topic 4.8. Attempt them under timed conditions, then check against the solutions.

1. State what pure means in chemistry. (1 mark)
2. Explain how melting point can show whether a substance is pure. (2 marks)
3. Define a formulation and give one example. (2 marks)
4. Name the stationary and mobile phases in paper chromatography. (2 marks)
5. A spot moves $2.4$ cm while the solvent moves $6$ cm. Calculate the $R_f$ value. (1 mark)
6. Describe the test and result for oxygen. (2 marks)
7. State the colours given by lithium, sodium and copper ions in a flame test. (3 marks)
8. Describe the test and result for a sulfate ion. (2 marks)