Ammonia, NH3 / The test for ammonia gas
3 marks · NormalA sample of gas in a test-tube is suspected to be ammonia. A student tests this gas using damp red litmus paper. He repeats the test with chlorine gas for comparison.
State the result when ammonia gas is tested with damp red litmus paper.
State the result when chlorine gas is tested with damp red litmus paper.
Explain how these tests can be used to identify ammonia gas in a mixture of alkaline and acidic gases.
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(a) Red litmus paper turns blue. · must show blue result with ammonia
(b) Red litmus paper is bleached (turns white). · must show bleached or turns white with chlorine
(c) Ammonia can be identified because it turns red litmus blue, while acidic (like chlorine) gases bleach or turn the paper white. Comparing both results allows identification. · must show ammonia produces blue, chlorine bleaches
Common mistakes: Litmus turns red with ammonia; Litmus turns blue with chlorine
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Marking points
B1 (a) · 1 mark · Red litmus turns blue with ammonia gas.
B2 (b) · 1 mark · Red litmus is bleached or turns white with chlorine gas.
B3 (c) · 1 mark · Correct explanation of distinguishing ammonia from other gases by change to blue compared to bleaching by chlorine.
Worked solution
(a) Ammonia turns red litmus paper blue.
(b) Chlorine bleaches red litmus paper, turning it white.
(c) Ammonia is identified by its blue colour change, while acidic gases like chlorine bleach the paper. These opposite results allow identification in a mixture.