Ammonia, NH3 / The test for ammonia gas
2 marks · HardA colourless liquid is suspected to contain water, but may also have dissolved salts. You are given a thermometer and a Bunsen burner. State how you would use these to test for the purity of the water and describe what result confirms that it is pure.
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Also accept: Measure the boiling point; pure water boils at exactly 100°C (at 1 atmosphere).; Measure the freezing point; pure water freezes at exactly 0°C (at 1 atmosphere).
Common mistakes: Only stating temperature without method.; Incorrect pure water boiling/freezing points.
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Marking points
M1 · 1 mark · States to measure the boiling or freezing point of the liquid.
A1 · 1 mark · Correctly states that pure water boils at exactly 100°C or freezes at exactly 0°C (at 1 atmosphere).
Worked solution
Heat the liquid and measure its boiling point using the thermometer. If it boils at exactly 100°C (at 1 atmosphere), it is pure water.