Ammonia, NH3 / The test for ammonia gas
4 marks · NormalA teacher tests four gases (W, X, Y, Z) using both damp red litmus paper and universal indicator paper. The results are shown.
| Gas | Red litmus paper | Universal indicator paper |
|---|---|---|
| W | No change | No change |
| X | Turns blue | Turns blue |
| Y | No change | Turns red |
| Z | Bleaches | Bleaches |
Which gas (W, X, Y, or Z) is most likely to be ammonia? Explain your answer.
Suggest the identity of one other gas (Y or Z), giving a reason.
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.
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(a) X is ammonia; it turns both red litmus and universal indicator blue, showing it is an alkaline gas.
(b) Z is most likely chlorine; it bleaches both papers. Y could be hydrogen chloride, as it turns universal indicator red, but does not change red litmus (already red).
Common mistakes: Confusing which gas turns indicator blue.; Not explaining the reason.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies X as ammonia.
B2 (a) · 1 mark · Explains the choice: both papers turn blue, indicating an alkaline gas.
B3 (b) · 1 mark · Suggests a correct identity for Y or Z (e.g. Z = chlorine, Y = hydrogen chloride).
B4 (b) · 1 mark · Gives a correct reason based on colour change.
Worked solution
(a) Gas X is ammonia. Ammonia is an alkaline gas, so it turns both red litmus and universal indicator paper blue.
(b) Gas Z is chlorine because chlorine bleaches damp litmus and universal indicator papers. Gas Y may be hydrogen chloride, as it turns universal indicator red (acidic), but has no effect on already red litmus.