AQA GCSE Chemistry 8462: Chemical analysis – 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.

These questions cover Topic 8, Chemical analysis (sections 4.8.1 to 4.8.3), of the AQA GCSE Chemistry (8462) specification, for teaching from September 2016 with exams from June 2018 (version 1.1). The topic is examined on Paper 2 at Foundation and Higher tier, and no part of it is Higher tier only, so every question here is for both tiers. Questions 5 and 7 to 9 draw on Required practicals 6 and 7.

Learn the content first in the study guide and the revision notes. The course hub is AQA GCSE Chemistry and the printable checklist lists every statement.

Questions

1. Name the gas that: (i) relights a glowing splint; (ii) bleaches damp litmus paper; (iii) burns with a pop when a lighted splint is held at the mouth of the tube. [3]

2. A carton is labelled “pure orange juice”. Explain why a chemist would not call orange juice a pure substance. [2]

3. Pure compound X melts at 135 °C. Sample A melts at exactly 135 °C. Sample B melts over the range 128–132 °C. Which sample is impure? Give two pieces of evidence from the data. [2]

4. A tin of paint has a mass of 250 g. It contains 45 g of pigment, plus a binder and a solvent.

(a) Calculate the percentage by mass of pigment in the paint. [2] (b) Explain why the paint is described as a formulation. [2]

5. A student uses paper chromatography to analyse a green ink. The origin line is 2.0 cm above the bottom of the paper. The solvent front is 10.0 cm above the bottom. The ink gives three spots, whose centres are 3.6 cm, 6.8 cm and 8.4 cm above the bottom.

(a) Explain why the origin line is drawn in pencil. [1] (b) Calculate the Rf value of each spot. [3] (c) Reference dyes in the same solvent have Rf values: E 0.20, F 0.45, G 0.80. State which reference dyes the ink contains, and what you can conclude about the remaining spot. [2] (d) Explain why the substances in the ink separate. [2]

6. This question is about flame tests.

(a) State the flame colours of a lithium compound and a calcium compound. [2] (b) A solid contains both sodium and potassium compounds. Explain why a flame test may fail to show that potassium is present. [2]

7. Sodium hydroxide solution is added to three solutions, P, Q and R.

Solution	Observation
P	green precipitate
Q	white precipitate that dissolves when excess NaOH is added
R	brown precipitate

(a) Identify the metal ion in each solution. [3] (b) Solution R is iron(III) chloride. Write a balanced equation for its reaction with sodium hydroxide. [2]

8. A student wants to find out whether a solution contains bromide ions.

(a) Describe the test and give the positive result. [3] (b) Explain why dilute hydrochloric acid must not be used in this test. [1]

9. A technician analyses an unknown white solid. It is a single ionic compound. The results are:

• flame test: orange-red flame
• sample dissolved in dilute nitric acid, then sodium hydroxide solution added: white precipitate that does not dissolve in excess
• solid + dilute hydrochloric acid: fizzing; the gas turns limewater milky.

(a) Identify the metal ion. Explain how the flame test and the hydroxide test together support your answer. [2] (b) Identify the negative ion, using the third result. [2] (c) Name the compound. [1] (d) Write a balanced equation for the reaction of the solid with hydrochloric acid. [2]

10. A laboratory uses flame emission spectroscopy to measure lithium in spring water. Standard solutions give these results:

Lithium concentration (mg/dm³)	0	2	4	6	8
Emission intensity (arbitrary units)	0	15	30	45	60

(a) Give two advantages of instrumental methods compared with chemical tests. [2] (b) A diluted sample gives an intensity of 39. Calculate its lithium concentration. [2] (c) The sample had been diluted by a factor of 10. Calculate the lithium concentration in the original spring water. [1] (d) Explain why flame emission spectroscopy can identify metal ions in a mixture when a flame test cannot. [1]

11. Two short analysis tasks.

(a) In a chromatogram the solvent moved 6.4 cm. A dye has an Rf value of 0.25 in this solvent. Calculate the distance the dye spot moved from the origin. [2] (b) Describe a test to show that a solution contains sulfate ions, and give the result. [2]

Answers

1. (i) Oxygen [1]; (ii) chlorine [1]; (iii) hydrogen [1]. [3] Examiner insight: Each gas scores separately, so one wrong name does not cost the other marks; chemical formulae (O2, Cl2, H2) are normally accepted in place of names.

2. A pure substance is a single element or compound, not mixed with anything else [1]. Orange juice is a mixture of many substances (such as water and sugars), even if nothing has been added to it [1]. [2] Examiner insight: The first mark needs the chemical definition; repeating the everyday meaning (“nothing added”) without contrasting it scores nothing.

3. Sample B is impure: it melts below the pure value of 135 °C [1] and over a range of temperatures rather than at one temperature [1]. [2] Examiner insight: Two separate pieces of evidence are needed for two marks; quoting the numbers (128–132 °C against 135 °C) makes each point creditworthy.

4. (a) 45 ÷ 250 × 100 [1] = 18% [1]. (b) It is a mixture designed as a useful product [1], with each component having a particular purpose and mixed in carefully measured quantities [1]. [4] Examiner insight: A bare 18 with no working still earns both marks in (a), but a wrong answer with no working earns none; show the division.

5. (a) Pencil does not dissolve in the solvent, so it will not move or separate and interfere with the spots [1]. (b) Solvent distance = 10.0 − 2.0 = 8.0 cm [1]. Spot distances from origin: 1.6, 4.8 and 6.4 cm [1]. Rf values: 0.20, 0.60 and 0.80 [1]. (c) The ink contains E and G [1]. The spot with Rf 0.60 matches none of the references, so it is a substance not in the reference set; F is absent [1]. (d) Each substance is distributed differently between the mobile phase (solvent) and the stationary phase (paper) [1]. Substances more attracted to the solvent travel further up the paper [1]. [8] Examiner insight: Measuring from the bottom of the paper (giving 0.36, 0.68, 0.84) loses the method mark, but the Rf mark can still be awarded as error carried forward if the division is done correctly.

6. (a) Lithium: crimson [1]. Calcium: orange-red [1]. (b) Sodium gives an intense yellow flame [1], which can mask the lilac colour of potassium [1]. [4] Examiner insight: “Red” alone is usually not enough for either lithium or calcium; the precise colour words crimson and orange-red are what distinguish them.

7. (a) P: iron(II), Fe²⁺ [1]. Q: aluminium, Al³⁺ [1]. R: iron(III), Fe³⁺ [1]. (b) Correct formulae Fe(OH)3 and NaCl [1]. FeCl3 + 3NaOH → Fe(OH)3 + 3NaCl [1]. [5] Examiner insight: “Iron” without the oxidation state is not credited, because iron(II) and iron(III) give different precipitate colours.

8. (a) Add dilute nitric acid [1], then silver nitrate solution [1]. A cream precipitate shows bromide ions [1]. (b) Hydrochloric acid adds chloride ions, which would form a white silver chloride precipitate and give a false result [1]. [4] Examiner insight: The colour must be cream for bromide; “white” or “yellow” describes a different halide and loses the result mark even if the reagents are right.

9. (a) Calcium, Ca²⁺ [1]. The white hydroxide that stays in excess rules out aluminium, and the orange-red flame shows calcium rather than magnesium [1]. (b) Carbonate, CO3²⁻ [1]: the acid produced carbon dioxide, shown by the limewater turning milky [1]. (c) Calcium carbonate [1]. (d) Correct formulae of products CaCl2, H2O and CO2 [1]. CaCO3 + 2HCl → CaCl2 + H2O + CO2 [1]. [7] Examiner insight: In a multi-test question the linking mark is only given when both tests are used together; naming calcium from the flame test alone scores the first mark only.

10. (a) Any two: more accurate [1]; more sensitive / works on small samples [1]; more rapid. (b) Intensity rises by 7.5 per mg/dm³ (15 ÷ 2) [1]; 39 ÷ 7.5 = 5.2 mg/dm³ [1]. (c) 5.2 × 10 = 52 mg/dm³ [1]. (d) It gives a line spectrum that can be compared with reference spectra, so each ion’s lines can be picked out even when colours would be masked [1]. [6] Examiner insight: Allow error carried forward in (c): a wrong value from (b) multiplied correctly by 10 earns the mark.

11. (a) Distance = Rf × solvent distance = 0.25 × 6.4 [1] = 1.6 cm [1]. (b) Add dilute hydrochloric acid, then barium chloride solution [1]; a white precipitate forms [1]. [4] Examiner insight: Both reagents are needed for the method mark in (b), and the result must include the colour “white” as well as “precipitate”.

Where marks are usually lost

• Measuring chromatography distances from the bottom of the paper, not the origin line.
• Giving Rf values with units or above 1.
• Confusing the glowing splint (oxygen) with the burning splint (hydrogen).
• Using “red” for lithium or calcium instead of crimson or orange-red.
• Writing “iron” without (II) or (III).
• Stating “precipitate forms” without its colour.
• Acidifying a halide test with hydrochloric acid.
• Identifying a compound from one test when two are needed to separate calcium, magnesium and aluminium.
• Forgetting to scale a diluted sample’s concentration back up.

Next steps

• Chemical analysis revision notes
• Chemical analysis study guide
• AQA GCSE Chemistry course hub
• Printable AQA GCSE Chemistry checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

AQA GCSE Chemistry (8462) specification, for teaching from September 2016, GCSE exams June 2018 onwards, version 1.1, published by AQA – section 4.8 Chemical analysis.