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This lesson brings together all the topics from the Chemical Analysis unit of the AQA GCSE Chemistry specification (8.1). It provides structured exam practice covering pure substances and mixtures, formulations, chromatography, gas tests, flame tests, metal hydroxide precipitates, anion tests, instrumental analysis, and flame emission spectroscopy. Use this lesson to consolidate your knowledge and practise applying it in exam-style contexts.
Before attempting exam questions, ensure you are confident with all the key topics in this unit:
| Topic | Key Points |
|---|---|
| Pure substances and mixtures | Chemistry definition of "pure"; sharp melting/boiling points; effect of impurities |
| Formulations | Definition; examples (medicines, paints, alloys); each component in a measured quantity |
| Chromatography | Stationary/mobile phases; Rf values; interpreting chromatograms; required practical method |
| Gas tests | Hydrogen (squeaky pop), Oxygen (relights glowing splint), CO\u2082 (limewater milky), Chlorine (bleaches damp litmus) |
| Flame tests | Li (crimson), Na (yellow), K (lilac), Ca (orange-red), Cu (green) |
| NaOH precipitates | Ca/Mg/Al (white), Cu (blue), Fe\u00b2\u207a (green), Fe\u00b3\u207a (brown); Al dissolves in excess |
| Anion tests | Carbonates (HCl + limewater), Halides (HNO\u2083 + AgNO\u2083), Sulfates (HCl + BaCl\u2082) |
| Instrumental analysis [H] | Advantages; sensitivity; speed; accuracy; small samples |
| Flame emission spectroscopy [H] | Line spectra; calibration curves; advantages over flame tests |
Exam Tip: Before the exam, create a single revision sheet listing every test, its reagent(s), and the positive result. Being able to recall this information quickly is essential because chemical analysis questions appear on almost every paper.
The AQA exam tests chemical analysis through several question types:
graph TD
A["Chemical Analysis Exam Questions"] --> B["Multiple Choice (1 mark)"]
A --> C["Short Answer (1-3 marks)"]
A --> D["Extended Writing (6 marks)"]
A --> E["Data Interpretation"]
A --> F["Required Practical Questions"]
B --> B1["Identify the correct test or result"]
C --> C1["Describe a test; state a result; calculate Rf"]
D --> D1["Plan a method to identify an unknown compound"]
E --> E1["Interpret chromatograms or calibration curves"]
F --> F1["Describe chromatography or ion identification method"]
style A fill:#2c3e50,color:#fff
style D fill:#e74c3c,color:#fff
style F fill:#27ae60,color:#fff
A student measures the melting point of a sample of aspirin. The sample melts between 128 \u00b0C and 133 \u00b0C. The data book value for pure aspirin is 135 \u00b0C.
(a) Is the sample pure? Explain your answer.
Model answer: The sample is not pure (impure). A pure substance has a sharp, fixed melting point that matches the data book value. This sample melts over a range (128-133 \u00b0C) and the melting point is lower than the known value (135 \u00b0C), which indicates the presence of impurities.
State what is meant by a "pure substance" in chemistry.
Model answer: A pure substance is one that contains only one type of element or one type of compound.
A moisturising cream contains the following components:
| Component | Percentage |
|---|---|
| Water | 65% |
| Emollient (oil) | 20% |
| Emulsifier | 8% |
| Fragrance | 4% |
| Preservative | 3% |
Explain why this moisturising cream is a formulation.
Model answer: The moisturising cream is a formulation because it is a mixture that has been designed as a useful product (1). Each component is present in a measured quantity (specific percentages) (1). Each component has a specific purpose — for example, the emollient moisturises the skin, the emulsifier keeps the oil and water mixed, and the preservative prevents bacterial growth (1).
Exam Tip: For formulation questions, always mention three things: (1) it is a mixture, (2) each component is in a measured quantity, and (3) each component has a specific purpose. Giving an example of a component and its purpose demonstrates application of knowledge and is often worth an additional mark.
A student carries out chromatography on a food colouring. The solvent front travels 9.6 cm from the baseline. One of the dyes in the food colouring travels 5.76 cm from the baseline.
(a) Calculate the Rf value of this dye. (1 mark)
Model answer: Rf = distance moved by substance / distance moved by solvent front = 5.76 / 9.6 = 0.60
(b) The student finds that a known yellow dye has an Rf value of 0.60 in the same solvent. What conclusion can the student draw? (1 mark)
Model answer: The dye in the food colouring is likely to be the same substance as the known yellow dye, because they have the same Rf value in the same solvent.
(c) Why is it important that the same solvent is used? (1 mark)
Model answer: Rf values are only valid for comparison if the same solvent and conditions are used, because a substance will have a different Rf value in a different solvent.
A student adds zinc to dilute sulfuric acid. A gas is produced.
(a) Name the gas produced. (1 mark)
Model answer: Hydrogen
(b) Describe the test for this gas and state the positive result. (2 marks)
Model answer: Hold a burning splint at the mouth of the test tube (1). The gas burns with a squeaky pop (1).
(c) Write a word equation for this reaction. (1 mark)
Model answer: Zinc + Sulfuric acid \u2192 Zinc sulfate + Hydrogen
An unknown solution is tested. The results are shown below:
| Test | Observation |
|---|---|
| Flame test | Green flame |
| NaOH solution added | Blue precipitate |
(a) Identify the metal ion present. (1 mark)
Model answer: Copper(II) ion, Cu\u00b2\u207a
(b) Write the ionic equation for the reaction with NaOH. (2 marks)
Model answer: Cu\u00b2\u207a(aq) + 2OH\u207b(aq) \u2192 Cu(OH)\u2082(s)
(c) Name the blue precipitate. (1 mark)
Model answer: Copper(II) hydroxide
A student is given an unknown white powder. When dilute nitric acid is added followed by silver nitrate solution, a cream-coloured precipitate forms.
(a) Identify the anion present. (1 mark)
Model answer: Bromide ion, Br\u207b
(b) Name the cream precipitate. (1 mark)
Model answer: Silver bromide, AgBr
(c) Why was dilute nitric acid added before the silver nitrate? (1 mark)
Model answer: To remove interfering ions (such as carbonate or hydroxide ions) that could react with silver nitrate and give a false positive result.
A student is given an unknown ionic compound. Describe how the student could carry out tests to identify both the cation and the anion in the compound.
Model answer (6-mark structure):
Cation identification: The student should first carry out a flame test by cleaning a nichrome wire loop in concentrated hydrochloric acid, dipping it into the sample, and holding it in a blue Bunsen flame. The colour of the flame identifies the metal ion (e.g., crimson = lithium, yellow = sodium, lilac = potassium, orange-red = calcium, green = copper).
If the flame test is inconclusive, the student should dissolve the sample in water and add a few drops of sodium hydroxide solution. The colour of the precipitate identifies the metal ion (e.g., blue = copper(II), green = iron(II), brown = iron(III), white = calcium/magnesium/aluminium). If a white precipitate forms, adding excess NaOH will determine if it is aluminium (dissolves) or calcium/magnesium (remains).
Anion identification: The student should test for carbonates by adding dilute hydrochloric acid — if it fizzes and the gas turns limewater milky, carbonate ions are present.
For halides, the student adds dilute nitric acid followed by silver nitrate solution — a white precipitate indicates chloride, cream indicates bromide, and yellow indicates iodide.
For sulfates, the student adds dilute hydrochloric acid followed by barium chloride solution — a white precipitate of barium sulfate confirms sulfate ions.
The student combines the cation and anion results to identify the compound.
Exam Tip: For 6-mark extended writing questions, organise your answer with a clear structure. Use paragraphs or sections for cation tests and anion tests separately. For each test, state: the reagent, the observation, and the conclusion. This structure ensures you cover all the points needed for full marks. Quality of written communication is also assessed, so write in full sentences with correct scientific terminology.
Give three advantages of using instrumental methods of analysis rather than traditional chemical tests.
Model answer:
A scientist uses flame emission spectroscopy to analyse a water sample. The calibration data for sodium is shown below:
| Concentration (mg/L) | Emission Intensity |
|---|---|
| 0 | 0 |
| 5 | 25 |
| 10 | 50 |
| 15 | 75 |
| 20 | 100 |
The water sample gives an emission intensity of 60.
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