When copper sulphate crystals are heated in a test tube, we observe:
A. a colourless gas with no smell is evolved
B. blue colour of salt fades & no gas is evolved
C. blue colour of the salt fades & green gas is evolved
D. a brown gas is evolved

Hint: Here we recall water of crystallization of copper sulphate. Four water molecules present in hydrated copper sulphate molecules are bonded by coordinate bonds while one water molecule is bonded by hydrogen bond.

 Complete step by step answer:
Water of crystallization: Water of crystallization is the number of water molecules present in one formula unit of a salt. Formula units of copper sulphate contain five water molecules ($5{H_2}O$). When hydrated salt of copper sulphate is heated strongly, blue copper sulphate crystals turn white (due to the loss of water of crystallization). When $CuS{O_4}.5{H_2}O$ is heated it forms $CuS{O_4}$ and $5{H_2}O$ .
$CuS{O_4}.5{H_2}O$= (hydrated copper sulphate)
$CuS{O_4}$ = (anhydrous copper sulphate)
When anhydrous copper sulphate reacts again with water it will form Hydrated copper sulphate.

Correct answer is (B) visible.

Additional Information: On heating hydrated salt molecules become anhydrous as water molecules are evaporated. There may be equilibrium between hydrated salt molecules and anhydrous salt molecules and shifting of equilibrium in a particular side (forward direction or backward direction) depends on humidity in the environment. If humidity is more then more amount of hydrated salt molecules will form and if humidity is less then a certain value at a given temperature, then more amount of anhydrous salt molecules will form.

Note:
We can use water of crystallization also for other crystalline salts containing water molecules. For example: $MgS{O_4}.7{H_2}O$ and $N{a_2}C{O_3}.10{H_2}O$ .