Ammonia, NH3 / The test for ammonia gas
3 marks · NormalA student tests two samples, A and B, for water. Both samples turn white anhydrous copper(II) sulfate blue. When heated, sample A boils at 102°C and sample B boils at 100°C (at 1 atm pressure).
Explain, with reasons, which sample is pure water and which is not.
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Sample B is pure water because it boils at 100°C. Sample A is not pure water because it has a higher boiling point (102°C), which suggests the presence of impurities.
Common mistakes: Saying both are pure; Not mentioning impurity's effect
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Marking points
B1 · 1 mark · Identifies sample B as pure water (boils at 100°C at 1 atm).
B2 · 1 mark · States sample A is not pure water (boiling point higher than 100°C).
B3 · 1 mark · Gives the reason: impurities raise the boiling point above 100°C.
Worked solution
Sample B is pure water because it boils exactly at 100°C at 1 atmosphere.
Sample A is not pure because its boiling point is above 100°C.
This higher boiling point indicates that impurities are present in sample A.