Ammonia, NH3 / The test for ammonia gas
3 marks · EasyDescribe how anhydrous copper(II) sulfate can be used to test an unknown liquid for water.
State the colour change you would see if water is present.
Explain if this test indicates whether the water is pure.
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(a) Add a few drops of the unknown liquid to white anhydrous copper(II) sulfate.
(b) The copper(II) sulfate turns blue.
(c) No, it only shows the presence of water, not its purity.
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Marking points
B1 (a) · 1 mark · Describes correct method: Add the liquid to white anhydrous copper(II) sulfate.
B1 (b) · 1 mark · States colour change: white to blue.
B1 (c) · 1 mark · States that the test does not show the water is pure.
Worked solution
(a) Add a few drops of the unknown liquid to white anhydrous copper(II) sulfate.
(b) The copper(II) sulfate will turn blue if water is present.
(c) This test only shows water is present. It does not prove the water is pure (it could contain impurities).