OCR GCSE Chemistry: Purity and Separating Mixtures — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Purity and Separating Mixtures study guide | Purity and Separating Mixtures revision notes

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Section A

1. Explain the chemical definition of a pure substance, and how melting point data can be used to test whether a sample is pure. [3]

2. Name the four separation techniques described in this sub-topic. [4]

Section B

3. Calculate the relative formula mass of magnesium sulfate, MgSO₄ (Mg = 24, S = 32, O = 16). [2]

4. A compound is found to contain 4.8 g of carbon and 1.2 g of hydrogen. Determine its empirical formula, showing your working. [4]

5. A student has a mixture of sand and salt dissolved in water and needs to recover both the sand and the salt as separate, dry solids. Describe the two-step process, naming the correct technique at each stage and explaining why it is suitable. [5]

6. Explain the difference between simple distillation and fractional distillation, and state when each is used. [4]

7. A chromatogram shows a spot that has travelled 4.5 cm from the baseline, while the solvent front has travelled 9.0 cm. (a) Calculate the Rf value. [2] (b) Explain why this Rf value can only be validly compared with other spots run on the same chromatogram under the same conditions. [2]

8. A chromatogram of an unknown sample is run alongside three reference substances. The unknown produces two spots, matching the Rf values of two of the three reference substances. Explain what this shows about the unknown sample. [3]

9. A student is given an unlabelled mixture containing two liquids with very similar boiling points. Suggest a suitable technique to separate them, and explain why a simpler method would not be appropriate. [4]

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Answers

1. A pure substance is a single chemical substance, not the everyday sense of “pure” (such as pure fruit juice, which is a mixture) [1] [1]. A genuinely pure substance melts and boils at a single, sharp temperature; the presence of impurities widens and lowers this melting point range, so melting point data can be used to test purity [1].

2. Filtration [1]; crystallisation [1]; distillation (simple and fractional) [1]; chromatography [1].

3. Mg (24) + S (32) + 4 × O (4 × 16 = 64) = 120 [1] [1].

4. Moles of carbon = 4.8 ÷ 12 = 0.4 [1]. Moles of hydrogen = 1.2 ÷ 1 = 1.2 [1]. Dividing both by the smaller value (0.4): carbon = 0.4 ÷ 0.4 = 1, hydrogen = 1.2 ÷ 0.4 = 3 [1]. Empirical formula: CH3 [1].

5. Step 1: separate the insoluble sand from the salt solution using filtration — the sand does not dissolve, so it is retained while the salt solution passes through as the filtrate [1] [1]. Step 2: separate the dissolved salt from the water using crystallisation — heating the filtrate evaporates the water until crystals begin to form, which are then allowed to cool and dry [1] [1] [1].

6. Simple distillation separates a liquid from dissolved solids, or liquids with a very different boiling point, using a single condensing step [1] [1]. Fractional distillation separates two or more miscible liquids with similar boiling points, and requires a fractionating column to achieve the finer separation needed [1] [1].

7. (a) Rf = distance travelled by spot ÷ distance travelled by solvent = 4.5 ÷ 9.0 = 0.5 [1] [1]. (b) Rf values are consistent for a given substance only under the same experimental conditions (same solvent, same paper, same temperature) [1], so comparing measurements taken from different chromatograms run under different conditions would not give a valid, meaningful comparison [1].

8. This shows the unknown sample is not a pure single substance, since it produces more than one spot [1] [1]. Matching the Rf values of two of the three reference substances indicates the unknown is a mixture containing (at least) those two identified substances [1].

9. A suitable technique is fractional distillation, using a fractionating column [1] [1]. A simpler method such as simple distillation would not be appropriate because it cannot achieve the fine separation needed when boiling points are very close together — the fractionating column allows repeated evaporation and condensation, giving a much sharper separation between the two liquids [1] [1].

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Where marks are usually lost

• Applying the everyday sense of “pure” rather than the chemical definition tested by a sharp melting or boiling point.
• Comparing raw masses directly in an empirical formula question, rather than first converting to moles.
• Confusing simple distillation (very different boiling points) with fractional distillation (similar boiling points, needs a fractionating column).
• Calculating or comparing Rf values using measurements taken from different chromatograms run under different conditions.
• Choosing crystallisation as the first step for a mixture containing an insoluble solid, when filtration is required first.

Approaching purity and separation questions

For any relative formula mass or empirical formula question, work systematically through moles first and simplify to a whole-number ratio only at the final step, since attempting to compare raw masses directly is the most common source of error. For a “suggest a suitable technique” question involving more than one separation step, use the decision framework as a flowchart rather than four isolated definitions: is there an insoluble solid to remove first (filtration), does a dissolved solid then need recovering (crystallisation), and if the mixture is entirely liquid, are the boiling points very different (simple distillation) or similar (fractional distillation)? For chromatography and Rf-value questions, always state explicitly that a valid comparison requires the same solvent, paper and temperature across all spots being compared, since this caveat is a specific, frequently tested point in its own right.