Purity and Separating Mixtures

This guide covers subtopic C2.1, Purity and Separating Mixtures, from Topic C2, Elements, Compounds and Mixtures, of OCR GCSE (9–1) Chemistry A (Gateway Science) J248. It is assessed on Paper 1 at Foundation Tier and on Paper 3 at Higher Tier, alongside Topics C1 (Particles) and C3 (Chemical Reactions).

Where this fits in J248

C2.1 builds directly on the particle model and atomic structure content of Topic C1 – specifically the Atomic Structure guide – by asking what happens when atoms and compounds combine into the pure and impure substances met in everyday chemistry. It also lays the groundwork for C2.2 Bonding, since distinguishing elements, compounds and mixtures is a prerequisite for understanding how and why atoms bond in the first place.

Syllabus coverage

OCR GCSE (9-1) CHEMISTRY A (GATEWAY SCIENCE, J248) — C2.1 PURITY AND SEPARATING MIXTURES

• Explaining what is meant by the purity of a substance, and distinguishing chemical purity from the everyday, non-scientific sense of “pure” (such as “pure” fruit juice, which is not a single chemical substance)
• Using melting point data to distinguish pure substances from impure ones, since a genuinely pure substance melts and boils at a single, sharp temperature, while impurities widen and lower the melting point range
• Calculating relative formula mass (Mr) of a compound from the relative atomic masses of its constituent elements
• Deducing the empirical formula of a compound from given data
• Explaining that many useful materials are mixtures, and that mixtures can often be separated using physical, rather than chemical, methods
• Describing, explaining and applying the separation techniques of filtration, crystallisation, distillation (both simple and fractional) and chromatography
• Describing the techniques of paper and thin-layer chromatography, and recalling that chromatography separates substances based on their differing solubility and affinity for the mobile and stationary phases
• Interpreting chromatograms, including calculating and using Rf values to identify substances
• Suggesting suitable purification techniques and suitable chromatographic methods for separating and identifying the components of a given mixture

How to approach it

Because this subtopic mixes conceptual understanding (what purity actually means chemically) with calculation (relative formula mass, empirical formula, Rf values) and practical technique knowledge (four named separation methods), revise it in three passes rather than one: first the concept of purity and how melting point evidences it, then the calculation skills, then the practical techniques and when each applies. For the separation techniques specifically, build a simple decision framework – filtration separates an insoluble solid from a liquid; crystallisation separates a dissolved solid from its solvent by evaporation; simple distillation separates a liquid from dissolved solids or from a liquid with a very different boiling point; fractional distillation separates two or more miscible liquids with similar boiling points; chromatography separates substances in a mixture based on differing solubility. Being able to select the right technique for a described mixture, rather than only defining each technique in isolation, is what most exam questions actually test.

Worked example: Rf value

A chromatogram shows a spot that has travelled 3.2 cm from the baseline, while the solvent front has travelled 8.0 cm. The Rf value is calculated as distance travelled by the spot divided by distance travelled by the solvent: 3.2 ÷ 8.0 = 0.4. Because Rf values are consistent for a given substance under the same experimental conditions (same solvent, same paper, same temperature), this figure can then be compared against known reference values, or against other spots on the same chromatogram run under identical conditions, to help identify an unknown substance or check the purity of a sample – a pure substance produces a single spot, while an impure sample produces multiple spots at different Rf values.

Worked example: relative formula mass and empirical formula

To find the relative formula mass of calcium carbonate, CaCO3, add the relative atomic masses of every atom present: calcium (40) + carbon (12) + three oxygens (3 × 16 = 48) = 100. This single figure is used throughout GCSE chemistry calculations, from percentage composition to reacting masses, so accuracy here matters well beyond C2.1 itself. For empirical formula, suppose an analysis shows a compound contains 2.4 g of carbon and 0.6 g of hydrogen. Divide each mass by its atomic mass to find the mole ratio: carbon 2.4 ÷ 12 = 0.2, hydrogen 0.6 ÷ 1 = 0.6. Dividing both by the smaller value (0.2) gives a ratio of 1:3, producing the empirical formula CH3. Working systematically through these two steps – moles of each element, then simplifying to the smallest whole-number ratio – avoids the most common error, which is attempting to compare raw masses directly without first converting to moles.

Common mistakes

Assuming “pure” always means a single element or compound in the everyday sense, rather than applying the chemical definition (a single substance, tested by a sharp melting or boiling point). Confusing simple distillation (separating a liquid from a dissolved solid, or liquids with very different boiling points) with fractional distillation (separating liquids with similar boiling points, requiring a fractionating column). Forgetting that Rf values must be calculated using measurements from the same baseline and solvent front, not mixed between different chromatograms run under different conditions. Choosing crystallisation for a mixture where the solid is insoluble, when filtration is the appropriate first step.

Quick revision checklist

• Explain purity in the chemical sense and describe how melting point data evidences it.
• Practise relative formula mass and empirical formula calculations from given data.
• Match each of the four separation techniques (filtration, crystallisation, distillation, chromatography) to the type of mixture it is suited to.
• Calculate Rf values correctly and explain how they are used to identify or check the purity of a substance.
• Be able to suggest an appropriate purification or chromatographic method for an unfamiliar mixture described in an exam question.

Related resources

• Purity and Separating Mixtures revision notes
• Purity and Separating Mixtures practice questions

Official syllabus

OCR GCSE (9-1) Chemistry A (Gateway Science) J248 specification, Version 4.0, August 2026 — ocr.org.uk.