1) Take few crystals of copper sulphate in a dry test tube.
2) Heat it for a few minutes.
3) The salt (crystals) turn white (in colour).
1) We notice water droplets on sides of the test tube.
2) This shows that crystals of salts are not really dry.
3) Add 2-3 drops of water on the sample of copper sulphate obtained after heating.
Result: The blue colour of copper sulphate is restored.