Ammonia, NH3 / The test for ammonia gas
2 marks · HardA student performs a test for water using anhydrous copper(II) sulfate. The white powder turns blue when a sample liquid is added. State what this test does and does not show about the sample.
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Also accept: Test shows presence of water; does not prove the water is pure; Confirms that water is present, but not its purity
Common mistakes: Saying it proves purity; Omitting that other substances with water also give positive result
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Marking points
B1 · 1 mark · States clearly that the test shows the presence of water.
B1 · 1 mark · States that the test does not show the water is pure (other substances could be present).
Worked solution
The test with anhydrous copper(II) sulfate shows that water is present in the sample, but it does not prove that the water is pure. Other liquids containing water would also give the same result.