Ammonia, NH3 / The test for ammonia gas
5 marks · HardA student is given three unknown gases: Gas A, Gas B, and Gas C. She is told that one may be ammonia, another is hydrogen, and the third is chlorine. She uses damp red litmus paper to test each gas.
State what is observed when damp red litmus paper is placed in ammonia gas.
Describe what happens to damp red litmus paper when it is exposed to chlorine gas.
Explain why ammonia is the only alkaline gas among the three.
Suggest an additional chemical test to identify hydrogen, apart from using litmus paper. State what result confirms hydrogen is present.
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(a) Turns blue
(b) The red litmus paper is bleached and turns white
(c) Ammonia dissolves in water to produce hydroxide ions, making the solution alkaline; hydrogen and chlorine do not.
(d) Use a burning splint: hydrogen burns with a squeaky pop.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States that red litmus paper turns blue in ammonia gas.
B1 (b) · 1 mark · States that red litmus paper is bleached or turned white by chlorine.
M1 (c) · 1 mark · Explains ammonia makes solution alkaline by producing hydroxide ions.
A1 (c) · 1 mark · Distinguishes that hydrogen and chlorine do not make an alkaline solution.
B1 (d) · 1 mark · Mentions using a burning splint to test for hydrogen, and a squeaky pop confirms its presence.
Worked solution
(a) In ammonia gas, the damp red litmus paper turns blue.
(b) In chlorine gas, the litmus paper is bleached, turning white.
(c) Ammonia is an alkaline gas because when it dissolves in water, it produces hydroxide ions (OH⁻), making the solution alkaline. Hydrogen and chlorine do not produce hydroxide ions under these conditions.
(d) To test for hydrogen, insert a burning splint into the gas; hydrogen will burn with a ‘squeaky pop’ sound, confirming its presence.