10.1.2 · Water Purity
Past Paper Practice · Papers 2, 4 & 6 · Data Analysis & Lab Skills
📋 EXAMINER'S BRIEFING
This paper tests your ability to distinguish between tests for the presence of water (CoCl₂ / CuSO₄ tests) and tests for purity of water (melting/boiling points). You must also explain the effect of dissolved impurities and justify the use of distilled water in practical analysis. These distinctions are tested repeatedly in Paper 4 and Paper 6.
Section A: Standard Values
Boiling & melting point tests for purity
Section B: Effect of Impurities
How dissolved solutes change physical properties
Section C: Lab Practices
Why distilled water? Practical analysis skills
Describe how you would test whether a sample of water is chemically pure using physical methods. State the expected results for a pure sample.
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A student records that a liquid boils at 100 °C and melts at 0 °C. What conclusion can the student draw about this liquid? Justify your answer.
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State what is meant by a 'fixed melting point' and explain why only pure substances have one.
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A chemist is given three unlabelled colourless liquids (A, B, C). She measures their melting and boiling points: A melts at –2 °C and boils at 102 °C. B melts at 0 °C and boils at 100 °C. C melts at –5 °C and boils at 98 °C. Which liquid is pure water? Identify what is unusual about liquid C.
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Anhydrous cobalt(II) chloride paper is placed in a sample of liquid. It turns pink. What does this test confirm, and what does it NOT confirm? What further test would you do?
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A sample of water contains dissolved sodium chloride (table salt). Predict whether its boiling point will be higher than, lower than, or equal to 100 °C. Explain your prediction.
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Using the same salt water sample from Q6, predict its melting point. State whether it will be higher than, lower than, or equal to 0 °C.
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Explain, in terms of particles, why dissolved impurities raise the boiling point of water.
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A student tests four water samples. The results are shown below. Identify which sample is pure water and which contains the highest concentration of dissolved solutes. Justify both answers. Sample | Boiling Point | Melting Point W | 100 °C | 0 °C X | 101.5 °C | –1.5 °C Y | 103 °C | –3 °C Z | 100.5 °C | –0.5 °C
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Seawater boils at approximately 100.5 °C. A student says: 'This means seawater is only slightly impure.' Comment on this statement.
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Explain why distilled water, rather than tap water, is used when preparing solutions for chemical analysis in the laboratory.
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A student is testing for the presence of sulfate ions (SO₄²⁻) in a water sample. She adds dilute nitric acid, then aqueous barium nitrate. She observes a white precipitate. She used tap water to rinse her equipment. Explain why this result may be unreliable.
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Describe how distilled water is produced from tap water. Include the name of the process and what happens to the impurities.
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A student wants to confirm that a bottle of liquid is pure distilled water. Plan a complete set of tests she should carry out, including the expected results for each test.
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[Extended Response] A local factory is accused of contaminating a river. A scientist collects a water sample from the river. Describe a full laboratory investigation the scientist could carry out to determine: (a) whether the water is pure, (b) whether it contains dissolved mineral salts, and (c) why distilled water must be used throughout the investigation.
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🎯 Master Cheat Sheet: Water Purity
Test for PRESENCE of water (not purity)
Anhydrous CoCl₂ paper: blue → pink | Anhydrous CuSO₄: white → blue
Test for PURITY of water
Boiling point = exactly 100 °C | Melting point = exactly 0 °C (sharp, fixed values)
Effect of dissolved impurities
Boiling point RISES above 100 °C | Melting point FALLS below 0 °C | More solute = greater effect
Why distilled water in lab?
Tap water contains dissolved ions that could INTERFERE with chemical tests, giving false positives or inaccurate results
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