Chemical Tests

5 marks · Easy

This question is about observations when testing for ammonia gas and chloride ions.

(a)

State what you would observe when ammonia gas is tested with damp universal indicator paper.

[1 mark]

(b)

A student adds dilute nitric acid and then silver nitrate solution to a solution of sodium chloride.

i. State the appearance of the precipitate formed.

ii. Give the name of the precipitate.

(i)

[1 mark]

(ii)

[1 mark]

(c)

Explain why nitric acid is added before silver nitrate when testing for chloride ions.

[2 marks]

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

(a) The paper turns blue. · must show blue colour on indicator paper

(b(i)) A white precipitate forms. · must show white precipitate

(b(ii)) Silver chloride. · must show silver chloride

(c) Nitric acid is added to remove ions (such as carbonate or hydroxide) that could also form precipitates with silver nitrate, preventing false positives for halide ions. · must show removes other ions like carbonate or hydroxide, prevents false positives for halide test

Show mark scheme and worked solution

Marking points

A1 (a) · 1 mark · Correct observation: paper turns blue with ammonia gas.

A1 (b(i)) · 1 mark · White precipitate is formed when silver nitrate is added to a solution containing chloride ions.

A1 (b(ii)) · 1 mark · The precipitate is named silver chloride.

E1 (c) · 2 marks · Explanation: Nitric acid removes/interfering ions (like carbonate or hydroxide), preventing other white precipitates/fake positives with silver nitrate.

Worked solution

a) The universal indicator paper turns blue when it is exposed to ammonia gas.

b)(i) The addition of silver nitrate to a solution containing chloride ions forms a white precipitate.

(ii) The precipitate is silver chloride.

c) Nitric acid is added before silver nitrate to remove other ions (such as carbonate or hydroxide) that might also form precipitates with silver nitrate, to ensure that only halide ions form a precipitate, so the test result is reliable.