Ammonia, NH3 / The test for ammonia gas

3 marks · Normal

Water found in a river is tested using anhydrous copper(II) sulfate. The solid turns blue. Explain why this test does not prove the water is pure, and suggest a further test to check its purity.

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

Show answer

The anhydrous copper(II) sulfate test only shows water is present, not that it is pure. Other substances such as dissolved salts or acids also turn it blue. To check purity, measure the boiling point: pure water boils at 100°C at 1 atmosphere.

Also accept: Measuring melting/freezing point is also an acceptable test for purity.

Common mistakes: Assuming any positive result proves purity.; Omitting 'other substances also turn it blue.'

Show mark scheme and worked solution

Marking points

B1 · 1 mark · States test only proves water present, not purity.

B2 · 1 mark · Explains that other liquids/solutions can also give positive result (impurities might be present).

B3 · 1 mark · Suggests measuring boiling point (100°C at 1 atm) or melting point (0°C at 1 atm) as further test.

Worked solution

The anhydrous copper(II) sulfate test only shows that water is present, not that it is pure. Dissolved substances such as salts or acids in water also turn anhydrous copper(II) sulfate blue.

To check if the water is pure, measure its boiling point. Pure water boils at exactly 100°C at 1 atmosphere pressure.