AmmoniaOxford AQA IGCSE Chemistry: Revision notes
Section 1
How can you recognise ammonia gas by smell?
Ammonia gas has a characteristic sharp, choking smell. This smell alone can give a strong clue when identifying an unknown gas, but it should always be confirmed using the litmus paper or hydrogen chloride test, since ammonia is irritating and should not be smelled directly in large amounts.
Section 2
What is the litmus paper test for ammonia?
The test for ammonia gas uses damp red litmus paper. If ammonia is present, the damp red litmus paper turns blue.
This happens because ammonia is an alkaline gas — it dissolves in the water on the litmus paper to form an alkaline solution, which turns red litmus blue.
Section 3
What is the hydrogen chloride test for ammonia?
Ammonia can also be identified by holding a container of hydrogen chloride gas near it. If ammonia is present, the two gases react to form a white smoke of solid ammonium chloride.
This white smoke test is a distinctive, visible confirmation of ammonia, separate from the litmus paper colour change.
For full marks, describe the observation precisely as 'white smoke', not just 'white gas' or 'white solid' — the reaction produces fine solid particles that look like smoke.
Must Know
- Ammonia gas has a characteristic sharp, choking smell
- Ammonia turns damp red litmus paper blue, because it is alkaline
- Ammonia forms a white smoke of ammonium chloride when hydrogen chloride gas is held near it
- These are two separate, standard tests used to confirm ammonia's identity
- Ammonium chloride is the white solid product of ammonia reacting with hydrogen chloride
That's the notes covered.
Carry on to the next subtopic.