Past Paper Exam

10.1.2 — Tests for Water

Cambridge IGCSE Chemistry 0620 · Papers 2 & 4 · 15 Questions · 30 Marks

Instructions: Read each question carefully. Write your answer before revealing the mark scheme. Use the self-mark buttons to track your score.

CHEM-REALISM V3 · Positive water test results visualised

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Section A

Copper(II) Sulfate — The Blue Ghost

State the full chemical name of the white substance used to test for the presence of water.

[1 mark]

💡 Hint: Think: the substance must be ANHYDROUS — it contains no water of crystallisation.

State the colour change observed when anhydrous copper(II) sulfate is used to test for water.

[1 mark]

💡 Hint: Remember the direction: start colour → end colour.

Write the word equation for the reaction that occurs when anhydrous copper(II) sulfate reacts with water.

[2 marks]

💡 Hint: Water is the reactant. Think about what 'anhydrous' becomes when it absorbs water.

A student adds anhydrous copper(II) sulfate to a colourless liquid. The powder turns blue. What conclusion can the student make? Give a reason.

[2 marks]

💡 Hint: Conclusion = what the result tells you. Reason = WHY the test works.

Explain the difference between anhydrous copper(II) sulfate and hydrated copper(II) sulfate in terms of their composition and colour.

[2 marks]

💡 Hint: Think about what 'hydrated' means at the molecular level.

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Section B

Cobalt(II) Chloride — The Pink Revealer

State the name of the substance used on test paper to detect the presence of water.

[1 mark]

💡 Hint: It starts blue and ends pink — it is a cobalt compound.

State the colour change observed when cobalt(II) chloride paper is used to test for water.

[1 mark]

💡 Hint: Direction matters. Blue → ?

A student has two unlabelled liquids: liquid A turns cobalt(II) chloride paper from blue to pink. Liquid B has no effect on the paper. What can the student conclude about each liquid?

[2 marks]

💡 Hint: A positive result tells you water IS present. A negative result tells you water is NOT present (or it is below the detection threshold).

Compare the two water tests — anhydrous copper(II) sulfate and cobalt(II) chloride paper — in terms of: (i) colour change and (ii) one advantage of using cobalt(II) chloride paper.

[2 marks]

💡 Hint: Cobalt paper is prepared in advance. Think about convenience in the lab.

Q10

A student breathes onto a piece of cobalt(II) chloride paper. The paper turns pink. Explain what this result shows about the student's breath.

[2 marks]

💡 Hint: Human breath contains water vapour — the gaseous form of water.

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Section C

The Examiner's Trap — Don't Fall In!

Q11

⚠️ TRAP QUESTION: A student uses HYDRATED copper(II) sulfate (CuSO₄·5H₂O) to test for water instead of the anhydrous form. Explain fully why this test will NOT work.

[3 marks]

💡 Hint: Think: what colour is the hydrated form? Does it contain water already? Can it absorb more water and change colour?

Q12

⚠️ TRAP QUESTION: A positive test for water using anhydrous copper(II) sulfate proves that the liquid tested is pure water. Do you agree? Explain your answer.

[2 marks]

💡 Hint: What does the test actually detect? Does it distinguish between pure water and a solution containing water?

Q13

⚠️ TRAP QUESTION: A student claims that if cobalt(II) chloride paper turns from pink to blue, this is a positive test for water. Is the student correct? Explain.

[2 marks]

💡 Hint: Check the direction of the colour change. Pink → Blue or Blue → Pink?

Q14

⚠️ TRAP QUESTION: A student is given an unknown white powder and is told it is either anhydrous copper(II) sulfate or anhydrous cobalt(II) chloride. Describe a test the student could carry out to identify which substance it is, including the expected result.

[3 marks]

💡 Hint: You know both tests for water — use them cleverly. Add water and observe.

Q15

⚠️ TRAP QUESTION: Describe a two-stage procedure to test whether a colourless liquid is PURE water. Include the name of reagents used, expected observations, and what each stage tests for.

[3 marks]

💡 Hint: Stage 1 tests for the PRESENCE of water. Stage 2 tests for PURITY. What physical property confirms purity?

📊 Final Score

⚠️ Top 3 Examiner Traps — Memorise These!

1. Always write ANHYDROUS — 'copper(II) sulfate' alone scores ZERO.
2. State BOTH colours in the change — start AND end colour required.
3. A positive test proves water is present — NOT that the liquid is pure water.

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