OxfordAQA International GCSE Chemistry
OxfordAQA International GCSE Chemistry 3.4 Chemical Analysis Exam Questions
Free OxfordAQA International GCSE Chemistry revision for 3.4 Chemical Analysis: what you need to know, common mistakes and exam-style questions with answers. Covers purity, separating mixtures, paper chromatography and Rf values, gas tests, flame tests and tests for positive and negative ions.
Practise 3.4 Chemical Analysis with past papers
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About OxfordAQA International GCSE Chemistry 3.4 Chemical Analysis
Chemical Analysis in OxfordAQA International GCSE Chemistry is about identifying what a substance is and how pure it is. You use melting and boiling points to judge purity, separate mixtures by filtration, crystallisation and distillation, and use paper chromatography with Rf values. The separate Chemistry course adds tests for hydrogen, oxygen, carbon dioxide, ammonia and chlorine. Everyone learns flame tests, sodium hydroxide precipitate tests for metal ions, and tests for carbonate, halide and sulfate ions. This topic draws on bonding and salts and is ideal for short recall marks, so learn every test as method, observation and conclusion. Examiners penalise vague colours.
What you need to know
- Purity A pure substance contains only one element or compound and melts and boils at a fixed temperature; impurities change and widen these values. Purity is important in food and drugs.
- Separation Mixtures are not chemically combined and can be separated by filtration (insoluble solid from liquid), crystallisation (soluble solid from solution) and distillation (liquid from solution or liquids with different boiling points).
- Chromatography Paper chromatography has a stationary phase (paper) and a mobile phase (solvent); components separate by relative solubility. Rf = distance moved by spot / distance moved by solvent front. A pure substance gives one spot.
- Gas tests (separate Chemistry) Hydrogen: lighted splint gives a squeaky pop. Oxygen: relights a glowing splint. Carbon dioxide: turns limewater cloudy. Ammonia: turns damp red litmus blue and forms white smoke with hydrogen chloride. Chlorine: turns damp blue litmus red then bleaches it white.
- Flame tests Lithium crimson, sodium yellow, potassium lilac, calcium red, barium green. This is a required practical: identify the metal ion in an unknown compound.
- Sodium hydroxide tests Aluminium, calcium and magnesium ions give white precipitates, and only aluminium hydroxide dissolves in excess. Copper(II) gives a blue precipitate, iron(II) green and iron(III) brown.
- Negative ions Carbonate: add dilute acid, carbon dioxide turns limewater cloudy. Halides: add dilute nitric acid then silver nitrate; chloride white, bromide cream, iodide yellow precipitate. Sulfate: add dilute hydrochloric acid then barium chloride; white precipitate.
Common mistakes in 3.4 Chemical Analysis
- Writing that limewater goes cloudy as the test for carbonates without adding acid. You must add dilute acid first to release carbon dioxide.
- Drawing the chromatography start line in ink. Use pencil so it does not dissolve and run.
- Giving iron(III) hydroxide as orange or red. The expected answer is a brown precipitate.
- Saying a glowing splint pops with hydrogen. A lighted splint gives a squeaky pop with hydrogen; a glowing splint relights in oxygen.
- Forgetting the acid in halide and sulfate tests. Nitric acid (halides) and hydrochloric acid (sulfates) remove carbonate ions that would also form precipitates.
Try three 3.4 Chemical Analysis exam-style questions
1. A spot of dye moved 3.6 cm and the solvent front moved 8.0 cm. Calculate the Rf value. (2 marks)
Show the mark scheme answer
Rf = 3.6 / 8.0; = 0.45.
2. Describe a test to show that a solution contains sulfate ions. Give the result. (3 marks)
Show the mark scheme answer
Add dilute hydrochloric acid; then add barium chloride solution; white precipitate forms.
3. A solid gives a lilac flame and its solution gives a cream precipitate with acidified silver nitrate. Identify the solid. (2 marks)
Show the mark scheme answer
Potassium (lilac flame); bromide (cream precipitate); so potassium bromide.
3.4 Chemical Analysis FAQs
What does OxfordAQA International GCSE Chemistry 3.4 Chemical Analysis cover?
It covers purity and melting points, separating mixtures, paper chromatography and Rf values, tests for common gases, flame tests and chemical tests for metal ions, carbonates, halides and sulfates.
Which paper is 3.4 Chemical Analysis examined in?
OxfordAQA International GCSE Chemistry has two equally weighted papers, Paper 1 and Paper 2, each 1 hour 30 minutes and 90 marks. Content from any part of the specification can be assessed in either paper, so revise this topic for both.
What is the best way to learn the chemical tests?
Make a table with three columns: test, positive result and ion or gas identified, then self-quiz until you can recall exact colours. Include the acid added first in the carbonate, halide and sulfate tests.
