OCR GCSE Chemistry: Purity and Separating Mixtures — Revision Notes

Condensed for the final weeks. For the full explanation, use the Purity and Separating Mixtures study guide.

Purity — the chemical definition

A pure substance is a single chemical substance — not the everyday sense (e.g. “pure” fruit juice is NOT chemically pure, it’s a mixture). Melting point evidences purity: a genuinely pure substance melts/boils at a single, sharp temperature; impurities widen and lower the melting point range.

Relative formula mass (Mr)

Worked example: CaCO₃.

Ca (40) + C (12) + 3xO (3 x 16 = 48) = 100

This figure underpins calculations throughout GCSE chemistry (percentage composition, reacting masses) — accuracy here matters well beyond C2.1.

Empirical formula

Worked example: a compound contains 2.4 g carbon and 0.6 g hydrogen.

Step 1: moles of each element
        C: 2.4 / 12 = 0.2
        H: 0.6 / 1  = 0.6

Step 2: divide by the smaller value (0.2)
        C: 0.2/0.2 = 1
        H: 0.6/0.2 = 3

Empirical formula: CH3

Work systematically: moles first, THEN simplify to whole-number ratio — comparing raw masses directly, without converting to moles, is the most common error.

The five separation techniques — a decision framework

(The specification names filtration, crystallisation, distillation and chromatography as four separate outcomes, and treats distillation’s simple and fractional forms separately across those outcomes – so the table below lists five named techniques in total.)

Technique	Separates	Use when
Filtration	Insoluble solid from liquid	Solid doesn’t dissolve
Crystallisation	Dissolved solid from solvent	Evaporate solvent, solid remains
Simple distillation	Liquid from dissolved solids, OR very different boiling points	One clear boiling-point gap
Fractional distillation	Miscible liquids with SIMILAR boiling points	Needs a fractionating column
Chromatography	Substances by differing solubility/affinity	Identifying/separating components

Select the right technique for a described mixture — this is what most exam questions actually test, not defining each technique in isolation.

Chromatography and Rf values

Paper/thin-layer chromatography separates by differing solubility and affinity for the mobile/stationary phases.

Rf = distance travelled by spot / distance travelled by solvent

Worked example: spot travels 3.2 cm, solvent front travels 8.0 cm.

Rf = 3.2 / 8.0 = 0.4

Rf values are consistent for a given substance under the same conditions (same solvent, paper, temperature) — compare against known references or other spots on the same run. A pure substance produces a single spot; an impure sample produces multiple spots at different Rf values.

Worked example: choosing a technique for a described mixture

A student has a mixture of salt and sand dissolved/suspended in water, and needs to recover both the salt and the sand as separate, dry solids.

Step 1: separate the sand (insoluble) from the salt solution
        Technique: FILTRATION -- sand is insoluble, salt solution
        passes through as the filtrate

Step 2: separate the dissolved salt from the water
        Technique: CRYSTALLISATION -- heat the filtrate to evaporate
        water until crystals form, then allow to cool and dry

This two-step worked example is exactly the kind of “suggest a suitable purification technique” question OCR sets, and shows why the decision framework above is more useful memorised as a flowchart (insoluble solid? -> filtration; then dissolved solid to recover? -> crystallisation) than as four separate definitions with no connection between them.

How C2.1 connects to the rest of Topic C2

This subtopic builds directly on Topic C1’s particle model and atomic structure content by asking what happens once atoms and compounds combine into the pure and impure substances met in everyday chemistry. It also lays essential groundwork for C2.2 Bonding, since distinguishing elements, compounds and mixtures is a prerequisite for understanding how and why atoms bond together in the first place – treat purity and separation as the conceptual foundation the rest of Topic C2 depends on, not a self-contained block to revise in isolation.

Key terms

Purity (chemical sense) — a single chemical substance, tested by a sharp melting/boiling point. Empirical formula — the simplest whole-number ratio of atoms in a compound. Rf value — ratio of distance travelled by a spot to distance travelled by the solvent front in chromatography.

A note on chromatography as an identification tool

Beyond simply separating a mixture, chromatography’s real analytical power is comparing Rf values against known reference substances run on the SAME chromatogram, under identical conditions – since Rf values depend on the specific solvent, paper and temperature used, comparing across two different experimental runs is not valid. If a question shows a chromatogram with several reference spots alongside an unknown sample spot, matching the unknown’s Rf value (and its position relative to the reference spots) is the specific skill being tested, not just calculating a single Rf value in isolation.

Common mistakes

• Assuming “pure” always means the everyday sense, rather than the chemical definition tested by melting/boiling point.
• Confusing simple distillation (very different boiling points) with fractional distillation (similar boiling points, needs a column).
• Calculating Rf values by mixing measurements between different chromatograms run under different conditions.
• Choosing crystallisation for an insoluble solid, when filtration is the correct first step.

Quick self-test

• Calculate the relative formula mass of MgSO₄ (Mg=24, S=32, O=16).
• Find the empirical formula of a compound containing 4.8 g carbon and 1.2 g hydrogen.
• State which separation technique suits: (a) salt from seawater, (b) two liquids with similar boiling points, (c) sand from water.
• Calculate the Rf value for a spot travelling 4.5 cm when the solvent front travels 9.0 cm.
• Explain how melting point data can be used to judge a sample’s purity.

Related resources

• Purity and Separating Mixtures study guide
• 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.