OCR GCSE Chemistry: Purity and Separating Mixtures — Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — 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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OCR’s real J248 papers open with a whole-paper Section A of 15 multiple-choice questions (15 marks, four-option A-D) before a longer Section B of short-answer and extended questions. The set below now matches that real 15-mark weighting, but every question is drawn from this topic (Purity and Separating Mixtures) alone, rather than sampling across the whole specification as a genuine paper’s Section A would — so it is still not a reproduction of a full paper’s Section A.

Section A — Multiple choice (topic sample)

1. Which of the following is the correct chemical definition of a pure substance? A) A substance that occurs naturally rather than being manufactured B) A single substance, not mixed with any other substance, regardless of how it was made C) A mixture of substances present in fixed proportions D) A substance that is safe to eat or drink [1]

2. A student heats a solid sample and finds that it melts gradually over a range of temperatures, rather than sharply at a single value. What does this most strongly suggest? A) The thermometer used is under-reading the true temperature B) The sample is a pure element rather than a pure compound C) The sample contains an impurity, which widens and lowers the melting point range D) The sample was heated too quickly for an accurate reading [1]

3. What is the relative formula mass of calcium carbonate, CaCO₃ (Ca = 40, C = 12, O = 16)? A) 60 B) 116 C) 68 D) 100 [1]

4. A mixture consists of an insoluble solid suspended in a solution of a soluble salt dissolved in water. Which two techniques, applied in the correct order, would recover the insoluble solid and then the salt as a separate dry solid? A) Filtration first, to remove the insoluble solid, then crystallisation, to obtain the dissolved salt as a dry solid B) Crystallisation first, then filtration C) Simple distillation first, then filtration D) Chromatography first, then crystallisation [1]

5. A chromatogram shows a spot that has travelled 3.0 cm from the baseline, while the solvent front has travelled 6.0 cm. What is the Rf value of this spot? A) 2.0 B) 0.5 C) 3.0 D) 0.33 [1]

6. What is a formulation? A) A pure single substance B) A mixture designed and made to have specific, useful properties, in which the components are mixed in carefully controlled proportions C) Any substance with a fixed melting point D) A synonym for a chemical compound [1]

7. Which separation technique obtains a solvent from a solution by evaporating it and then re-condensing it? A) Crystallisation B) Filtration C) Simple distillation D) Paper chromatography [1]

8. Which separation technique is most suitable for separating two or more miscible liquids with similar boiling points, such as the fractions of crude oil? A) Filtration B) Fractional distillation C) Crystallisation D) Simple distillation [1]

9. What is the relative formula mass of sodium hydroxide, NaOH (Na = 23, O = 16, H = 1)? A) 24 B) 40 C) 39 D) 41 [1]

10. A student wants to use chromatography to test whether a coloured sample is a single pure substance or a mixture. Which outcome would show the sample is pure? A) The sample produces two or more spots in a single solvent B) The sample produces only one spot, no matter which solvent is used to run it C) The sample does not move at all in any solvent D) The sample produces a different number of spots depending on which solvent is used [1]

11. A mixture contains an insoluble solid, with no dissolved substances present in the liquid. Which single technique would separate the solid from the liquid? A) Crystallisation B) Filtration C) Fractional distillation D) Chromatography [1]

12. Which observation would suggest that a solid sample is impure, rather than a single pure substance? A) It melts sharply at a single, specific temperature B) It boils at a temperature that matches a known data book value exactly C) It melts gradually over a range of temperatures below the expected melting point of the pure substance D) It has a fixed, reproducible density [1]

13. Which piece of apparatus is essential in fractional distillation but not required in simple distillation? A) A condenser B) A thermometer C) A fractionating column D) A heat source [1]

14. A spot on a chromatogram has an Rf value of exactly 0. What does this mean? A) The spot travelled exactly as far as the solvent front B) The spot did not move from the baseline at all C) The spot is definitely a pure substance D) The experiment used the wrong type of paper [1]

15. Which of these is a correct example of a formulation and its purpose? A) Distilled water, used because it is a single pure substance B) Paint: a mixture of pigment, solvent, binder and other additives, formulated to give specific properties such as colour and even coverage C) Gold jewellery, alloyed for decoration, which is not a formulation because it contains no liquid component D) Filtered tap water, which becomes a formulation once solid particles are removed [1]

Section B — Short answer and extended questions

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

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

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

19. 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]

20. 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]

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

22. 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]

23. 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]

24. 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. B) A pure substance is a single substance not mixed with any other substance; how or where it originated is irrelevant, and “pure” in the everyday sense (e.g. pure fruit juice) is a mixture, not a chemically pure substance [1].

2. C) A genuinely pure substance melts sharply at a single temperature; an impurity widens and lowers the melting point range, which is what a gradual melt over a range indicates [1].

3. D) Ca (40) + C (12) + 3 × O (3 × 16 = 48) = 100 [1].

4. A) The insoluble solid must be removed first by filtration (it is retained while the salt solution passes through as filtrate); crystallisation is then used to recover the dissolved salt from the filtrate, since it can only act on what is already in solution [1].

5. B) Rf = distance travelled by spot ÷ distance travelled by solvent = 3.0 ÷ 6.0 = 0.5 [1].

6. B) A formulation is a mixture that has been designed as a useful product, made by mixing its components in carefully controlled, specific proportions to give the mixture particular properties — how it was made or its origin is not what defines it [1].

7. C) Simple distillation evaporates the solvent from a solution and then condenses it back to a liquid, recovering the solvent itself; crystallisation recovers a dissolved solute, not the solvent, and filtration and chromatography do not involve evaporation and condensation at all [1].

8. B) Fractional distillation, which uses a fractionating column to allow repeated evaporation and condensation, is needed to separate miscible liquids with similar boiling points, such as the different fractions of crude oil; simple distillation cannot achieve fine enough separation for liquids this close in boiling point [1].

9. B) Na (23) + O (16) + H (1) = 40 [1].

10. B) A pure substance always produces a single spot on a chromatogram, whatever solvent is used to run it, even though the exact Rf value of that spot may differ between solvents; a mixture, by contrast, would be expected to separate into more than one spot in at least one solvent [1].

11. B) Filtration alone separates an insoluble solid from a liquid, since the solid is retained on the filter paper while the liquid passes through as the filtrate; no dissolved substances are present here, so no second step (such as crystallisation) is needed [1].

12. C) A genuinely pure substance melts sharply at one specific temperature; melting gradually over a range below the expected value is the signature of an impurity, which widens and lowers the melting point range [1].

13. C) A fractionating column is what allows fractional distillation to achieve much finer separation between liquids of similar boiling point than simple distillation can; a condenser, thermometer and heat source are used in both techniques [1].

14. B) An Rf value of 0 means the spot has not moved from the baseline at all, i.e. it travelled 0 cm while the solvent front moved a non-zero distance, giving Rf = 0 ÷ (solvent distance) = 0 [1].

15. B) Paint is a formulation: a mixture of pigment, solvent, binder and other additives, combined in controlled proportions specifically to give useful properties such as colour, even coverage and a suitable drying time [1].

16. 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].

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

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

19. 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].

20. 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].

21. 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].

22. (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].

23. 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].

24. 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.