OxfordAQA International GCSE Chemistry: Purity, chromatography and identification of common gases (9202) – 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 from your board or school.

The questions follow the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards. It covers section 3.4.1 Purity and chromatography and section 3.4.2 Identification of common gases, statements a to e in each. The qualification is untiered, so every question suits every candidate. All melting points and Rf values you need are given in the questions.

Links: study guide for this unit · revision notes for this unit · course hub · checklist · free diagnostics

Questions

1. Define these terms.

(a) State what is meant by a pure substance in chemistry. [1] (b) State what is meant by a mixture. [1]

2. Use these data: pure compound T melts at 151 °C. Three white solids, X, Y and Z, are tested.

Solid	Observation on heating
X	Melts sharply at 118 °C
Y	Starts to melt at 140 °C, fully melted at 146 °C
Z	Melts sharply at 151 °C

Identify which solid is pure T, which is impure T, and which is a different pure substance. Give a reason for each. [3]

3. Explain why it is important to measure the purity of a substance used in a medicine. [2]

4. This question is about separating mixtures.

(a) Name the method used to remove powdered chalk from water. [1] (b) Name the method used to collect pure water from sea water. [1] (c) Describe how to obtain dry crystals of zinc sulfate from zinc sulfate solution. [3]

5. A student sets up a paper chromatography experiment to analyse a permanent marker ink.

(a) Explain why the start line must be above the level of the solvent. [1] (b) Explain why the start line is drawn in pencil. [1] (c) Explain why a lid is placed on the beaker. [1] (d) Suggest why the student uses ethanol rather than water as the solvent. [1]

6. A brown food colouring is analysed by paper chromatography. The solvent front is 9.0 cm above the baseline. The colouring gives three spots, with centres 2.7 cm, 5.4 cm and 7.2 cm above the baseline. Reference dyes run in the same solvent have these Rf values: A 0.30, B 0.45, C 0.80.

(a) Calculate Rf for the lowest, middle and highest spots. [2] (b) Which reference dyes are in the brown colouring? What can you conclude about the remaining spot? [2]

7. In a different solvent a dye has an Rf value of 0.55. The solvent front moves 6.0 cm from the baseline. Calculate how far the dye spot moves. [2]

8. This question is about how paper chromatography works.

(a) Name the stationary phase and the mobile phase. [1] (b) Explain why two dyes in the same ink travel different distances up the paper. [2]

9. The pencil baseline on a chromatogram sits 1.2 cm up from the lower edge of the paper. Measured from that same edge, the solvent front is at 8.7 cm and the centre of a spot is at 3.6 cm. A student writes Rf = 3.6 ÷ 8.7 = 0.41.

(a) Identify the student’s error. [1] (b) Calculate the correct Rf value. [2]

10. Four test tubes each contain one gas: hydrogen, oxygen, carbon dioxide or ammonia. The tubes are not labelled.

(a) Describe the test for hydrogen and give the result. [2] (b) Describe the test for oxygen and give the result. [2] (c) Describe the test for carbon dioxide and give the result. [2] (d) Write a balanced equation for the reaction that causes the result in part (c). [1]

11. Two gas jars, J and K, each contain a gas with a sharp, choking smell. One contains ammonia and the other contains chlorine.

(a) Explain why smelling the gases is not a suitable way to identify them. [2] (b) Describe one test, using litmus paper, that identifies each gas. Give the result for each gas. [3] (c) Describe a second test for ammonia, other than litmus, and give the result. [1]

12. A manufacturer claims that a sports drink is coloured with a single permitted dye, D. An analyst runs the drink and a pure sample of D side by side on paper, using water as the solvent. The solvent front moves 6.8 cm. The drink gives two spots, 1.7 cm and 5.1 cm above the baseline. Pure D gives one spot, 5.1 cm above the baseline.

(a) Calculate the Rf values of the two spots from the drink. [2] (b) Evaluate the manufacturer’s claim. [2] (c) A later batch of the drink gives a single spot level with D in water. Suggest how the analyst could get stronger evidence that this batch contains only dye D. [2] (d) Give one reason why food manufacturers measure the purity of food colourings. [1]

Answers

1. (a) A single element or compound with no other substance mixed in [1]. (b) Two or more elements or compounds mixed together without being chemically combined [1]. Examiner insight: For the mixture definition, “not chemically combined” is the phrase that does the work; “two substances together” on its own is too vague to credit.

2. Z is pure T: it melts sharply at the data value, 151 °C [1]. Y is impure T: it melts below 151 °C and over a range [1]. X is a different pure substance: it melts sharply, but at 118 °C, not 151 °C [1]. Examiner insight: Each identification needs its reason; for Y, name both features (lower temperature and a range), because either alone is a weaker argument.

3. A medicine must contain the correct amount of the active compound so that the dose is right [1]. Impurities could be harmful or could stop the medicine working properly [1]. Examiner insight: An explain question on two marks expects two separate points; “so it is safe” repeated in different words counts as one idea.

4. (a) Filtration [1]. (b) Distillation [1]. (c) Heat the solution to evaporate some of the water until it is saturated [1]. Set it aside to cool; crystals grow as it cools [1]. Separate the crystals by filtering, then leave them somewhere warm to dry [1]. Examiner insight: Heating the solution until all the water has gone is evaporation to dryness, not crystallisation; describe stopping part-way and cooling.

5. (a) If the solvent covered the line, the ink would dissolve into the solvent in the beaker instead of moving up the paper [1]. (b) Pencil does not dissolve in the solvent, so the line does not run and mix with the spots [1]. (c) To stop the solvent evaporating from the paper [1]. (d) Permanent marker ink does not dissolve in water, but does dissolve in ethanol [1]. Examiner insight: In part (b), “pencil does not smudge” misses the chemistry; the point is that graphite is insoluble in the solvent while ink is not.

6. (a) Rf = spot distance ÷ solvent distance: 2.7 ÷ 9.0, 5.4 ÷ 9.0 and 7.2 ÷ 9.0 [1]. Rf values 0.30, 0.60 and 0.80 [1]. (b) The colouring contains A and C [1]. No reference has Rf 0.60, so the middle spot is some other substance; B is absent [1]. Examiner insight: Show at least one division in full so your method is clear, and give each Rf to two decimal places to match the reference values.

7. Distance = Rf × distance moved by solvent = 0.55 × 6.0 [1] = 3.3 cm [1]. Examiner insight: Write the rearranged formula before substituting, and give the unit, cm, with the answer.

8. (a) Stationary phase: the paper. Mobile phase: the solvent [1]. (b) The dyes have different solubilities in the solvent [1]. The more soluble dye spends more time in the moving solvent, so it travels further up the paper [1]. Examiner insight: Link the difference in solubility to the distance travelled; saying only “they have different Rf values” restates the observation without explaining it.

9. (a) The student used the paper’s lower edge as the zero, not the baseline [1]. (b) Spot distance = 3.6 − 1.2 = 2.4 cm and solvent distance = 8.7 − 1.2 = 7.5 cm [1]. Rf = 2.4 ÷ 7.5 = 0.32 [1]. Examiner insight: Both distances must be corrected; subtracting 1.2 cm from only one of them is a common slip that gives a wrong value.

10. (a) Hold a lighted splint near the mouth of the tube [1]. A pop is heard [1]. (b) Put a glowing splint into the tube [1]. The splint relights [1]. (c) Bubble the gas through limewater (calcium hydroxide solution) [1]. The limewater turns cloudy white [1]. (d) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O [1] Examiner insight: Lighted and glowing splints are not interchangeable, and “the splint goes out” is not a positive result for any gas in this unit, so it does not identify carbon dioxide.

11. (a) Both gases have the same kind of sharp, choking smell, so smell cannot tell them apart [1]. Both gases are toxic, so deliberately breathing them in is unsafe [1]. (b) Hold damp litmus paper in each gas [1]. Ammonia turns damp red litmus blue [1]. Chlorine turns damp blue litmus red and then bleaches it white [1]. (c) Hold the stopper from a bottle of concentrated hydrochloric acid near the gas; ammonia gives white smoke (of ammonium chloride) [1]. Examiner insight: For chlorine, give the full sequence, red then white; “bleaches litmus” without the colour change, or the wrong starting colour of litmus, weakens the answer.

12. (a) 1.7 ÷ 6.8 = 0.25 [1]; 5.1 ÷ 6.8 = 0.75 [1]. (b) The drink gives two spots, so it contains at least two substances [1]. One spot matches D (same distance, Rf 0.75), but the other (Rf 0.25) is a second substance, so the claim of a single dye is not supported [1]. (c) Run the drink and pure D again using a second, different solvent [1]. If the drink still gives one spot level with D, that is stronger evidence it contains only D [1]. (d) To check there are no harmful substances in the colouring / that the food contains only what the label states [1]. Examiner insight: An evaluate question needs a judgement backed by the evidence; quote the number of spots and the matching Rf value rather than just saying “the claim is wrong”.

Where marks are usually lost

• Stating that impurities raise a melting point; they lower it and make it melt over a range.
• Treating a sharp melting point at the wrong temperature as impure, rather than as a different pure substance.
• Describing heating to dryness when the question asks for crystallisation.
• Taking chromatogram measurements from the lower edge of the paper rather than the start line.
• Rf values written with units or greater than 1.
• Claiming one spot in one solvent proves purity, without mentioning a second solvent or a reference.
• Swapping the lighted and glowing splints, or giving a test without the result.
• Leaving out “damp” before litmus, or the bleaching step for chlorine.
• Using smell to tell ammonia from chlorine.

Next steps

• Recap fast with the revision notes for this unit.
• Re-learn anything you missed in the study guide for this unit.
• See the whole course on the OxfordAQA IGCSE Chemistry hub and tick off topics on the printable checklist.
• Try one of the free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, published by OxfordAQA. This practice set covers section 3.4.1 Purity and chromatography and section 3.4.2 Identification of common gases.