Ammonia, NH3 / The test for ammonia gas
4 marks · NormalA student is given an unknown liquid and asked to test for the presence and purity of water.
Describe how the student could use anhydrous copper(II) sulfate to test for the presence of water in the liquid.
Explain how the student could determine if the liquid is pure water.
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(a) Add a few drops of the liquid to white anhydrous copper(II) sulfate. The powder turns from white to blue if water is present. · must show white anhydrous CuSO4, blue hydrated CuSO4
(b) Measure the boiling and melting points. The liquid is pure water if it boils at 100°C and freezes at 0°C (at 1 atm). · must show boiling point 100°C, freezing/melting point 0°C, comparison to pure water values
Common mistakes: Boiling point imprecisely described; Does not mention white to blue
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Marking points
B1 (a) · 1 mark · Add or mix unknown with white anhydrous copper(II) sulfate.
B2 (a) · 1 mark · Turns blue if water is present (accept: colour change from white to blue).
B3 (b) · 1 mark · Measure boiling point or melting/freezing point.
B4 (b) · 1 mark · States pure water boils at 100°C and/or freezes at 0°C at 1 atm.
Worked solution
(a) The student adds a few drops of the liquid to white anhydrous copper(II) sulfate.
If water is present, the white powder turns blue.
(b) The student can measure the boiling point and/or melting point of the liquid.
If the liquid boils at 100°C and freezes at 0°C (at normal atmospheric pressure), it is pure water.