In the above activity copper sulphate crystals which seem to be dry contain the water of crystallization, when these crystals are heated, water present in crystals is evaporated and the salt turns white.
When the crystals are moistened with water, the blue colour reappears.Water of crystallization is the fixed number of water molecules present in one formula unit of a salt. Five water molecules are present in one formula unit of copper sulphate. Chemical formula for hydrated copper sulphate is CuSO4 • 5H2 O.
Now you would be able to answer the question whether the molecule of Na2 CO3 • 10 H2 O is wet or not.
Another salt which possesses water of crystallisation is gypsum. It has two water molecules in its crystals and the formula is CaSO4 • 2H2 O.
Plaster of paris (CaSO4 • ½ H2 O)On careful heating of gypsum (CaSO4 2 H2 O) at 373 K it loses water molecules partially to become calcium sulphate hemihydrate (CaSO4 • ½ H2 O). This is called plaster of paris, the substance which doctors use as plaster for supporting fractured bones in the right position.½ Plaster of paris is a white powder and on mixing with water, it sets into hard solid mass due to the formation of gypsum.
CaSo4 • ½ H2O (plaster of paris)+1½H2O -> CaSo 4 • 2H2O(Gypsum)Note:You might have noticed that only half a water molecule is shown to be attached as water of crystallisation.
It is written in this form because two formula units of CaSO4 share one molecule of water.
Plaster of paris is used for making toys, materials for decoration and for making surfaces smooth.