AQA GCSE Chemistry
AQA GCSE Chemistry 4.8.1 Purity, Formulations and Chromatography Exam Questions
Free AQA GCSE Chemistry exam questions for 4.8.1 Purity, Formulations and Chromatography, with a mark scheme for every pack. Covers pure substances in chemistry, melting and boiling points, formulations, paper chromatography, Rf values and the chromatography required practical.
4.8.1 Purity, Formulations and Chromatography exam questions
2 question packs of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About AQA GCSE Chemistry 4.8.1 Purity, Formulations and Chromatography
AQA GCSE Chemistry 4.8.1 starts chemical analysis by asking how we know whether a substance is pure. You learn the chemist's definition of a pure substance, how melting and boiling points reveal purity, what formulations are and how paper chromatography separates and identifies substances. It is assessed on Paper 2 and links to separating mixtures in 4.1.1 and identification tests in 4.8.2 and 4.8.3. Examiners frequently ask you to calculate Rf values, explain why the start line is drawn in pencil, and interpret chromatograms to decide which substances are in a mixture.
What you need to know
- Pure substances In chemistry a pure substance is a single element or compound, not mixed with any other substance. This differs from everyday use, such as 'pure' orange juice.
- Melting and boiling points Pure substances melt and boil at specific, fixed temperatures. Impure substances melt and boil over a range of temperatures, and impurities usually lower the melting point.
- Formulations A formulation is a mixture designed as a useful product, with each component in a measured quantity for a particular purpose. Examples: fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
- Chromatography Paper chromatography has a stationary phase (the paper) and a mobile phase (the solvent). Substances separate because they have different attractions to the paper and the solvent. A pure substance produces a single spot in all solvents.
- Rf values Rf = distance moved by substance / distance moved by solvent. Rf is always less than 1 and depends on the solvent; compounds can be identified by comparing Rf values with known substances.
- Required practical Draw a pencil start line, add spots of samples, stand the paper in solvent with the level below the line, mark the solvent front when it nearly reaches the top, then measure distances and calculate Rf values.
Common mistakes in 4.8.1 Purity, Formulations and Chromatography
- Drawing the start line in pen. Ink would dissolve and separate; use pencil, which is insoluble.
- Putting the solvent level above the start line. The spots would dissolve into the solvent instead of moving up the paper.
- Measuring the spot distance from the bottom of the paper. Measure from the start line to the centre of the spot, and to the solvent front.
- Saying a mixture always produces more spots than a pure substance in one solvent. Two substances can travel the same distance, so a pure substance should give one spot in every solvent.
- Calling any natural product 'pure'. In chemistry pure means one substance only.
Try three 4.8.1 Purity, Formulations and Chromatography exam-style questions
1. A spot moves 3.6 cm and the solvent front moves 9.0 cm. Calculate the Rf value. (2 marks)
Show the mark scheme answer
3.6 / 9.0; = 0.40.
2. A sample of aspirin melts between 128 °C and 134 °C. Pure aspirin melts at 136 °C. What does this show? Explain. (2 marks)
Show the mark scheme answer
The sample is impure; because it melts over a range of temperatures and below the melting point of pure aspirin.
3. Explain how paper chromatography separates the dyes in an ink. (3 marks)
Show the mark scheme answer
The solvent (mobile phase) moves up the paper (stationary phase); dyes have different solubilities in the solvent or different attractions to the paper; so they move different distances.
4.8.1 Purity, Formulations and Chromatography FAQs
What does AQA GCSE Chemistry 4.8.1 cover?
It covers the meaning of a pure substance, using melting and boiling points to test purity, formulations, paper chromatography and Rf values, including the chromatography required practical.
Which paper is 4.8.1 on?
Section 4.8 Chemical analysis is assessed on AQA GCSE Chemistry Paper 2.
What are the key points to learn for the chromatography practical?
Learn why you use a pencil line, why the solvent must be below the start line, why a lid is used to stop evaporation, how to mark the solvent front and how to calculate Rf.
