Ammonia, NH3 / The test for ammonia gas
2 marks · EasyExplain why measuring the boiling point of a liquid is used to test the purity of water. What result should you obtain if the water sample is pure?
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Pure water has a fixed boiling point of exactly 100°C at 1 atmosphere. Impurities cause the boiling point to rise. If the water is pure, it should boil at exactly 100°C.
Also accept: Pure water boils at exactly 100°C at 1 atmosphere.; Impure water boils at a higher temperature.; A pure sample boils at 100°C.
Common mistakes: Saying any boiling point is fine; Not stating atmospheric pressure
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Marking points
B1 · 1 mark · States that pure water boils at exactly 100°C (at 1 atm).
B1 · 1 mark · Explains that impurities cause the boiling point to increase (or change).
Worked solution
Pure water has a fixed boiling point of 100°C at 1 atmosphere. If impurities are present, the boiling point will be higher. Therefore, if the sample boils exactly at 100°C, the water is pure.