Ammonia, NH3 / The test for ammonia gas
4 marks · NormalA student is given a colourless liquid and asked to determine if it is pure water. The student has access to anhydrous copper(II) sulfate, a thermometer, a beaker, and a Bunsen burner.
Describe how the student could use anhydrous copper(II) sulfate to test whether the liquid contains water.
State the colour change that would be seen if water is present.
Describe how the student can use a thermometer and a Bunsen burner to test if the water is pure.
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(a) Add a few drops of the liquid to white anhydrous copper(II) sulfate. If water is present, the solid turns blue.
(b) It changes from white to blue.
(c) Heat the liquid and measure its boiling point with the thermometer. If it boils at 100°C at 1 atmosphere pressure, it is pure water.
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Marking points
M1 (a) · 1 mark · Describes adding liquid to anhydrous copper(II) sulfate.
A1 (a) · 1 mark · States blue colour if water present or idea of colour change.
B1 (b) · 1 mark · Colour change from white to blue stated.
B2 (c) · 1 mark · Describes measuring boiling point and compares to 100°C.
Worked solution
(a) The student should add a few drops of the liquid to white anhydrous copper(II) sulfate. If water is present, the solid turns blue.
(b) The colour changes from white to blue.
(c) The student heats the liquid and measures its boiling point with a thermometer. If it boils at 100°C at 1 atmosphere, the water is pure.