Class 10 Are the crystals of salts really dry?, Plaster of paris

### Topics to be covered

=> Are the crystals of salts really dry?
=> Plaster of paris

### Are the Crystals of Salts really Dry?

color{green}("𝐎𝐁𝐒𝐄𝐑𝐕𝐀𝐓𝐈𝐎𝐍:")

color{green}(•) Copper sulphate crystals which seem to be dry contain water of crystallisation.

color{green}(•) When we heat the crystals, this water is removed and the salt turns white.

color{green}(•) Upon moistening the crystals again with water, blue colour of the crystals reappears.

color{green}(•) Water of crystallisation 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 color{red}(Cu SO_4 . 5 H_2O.)

### Plaster of Paris

color{green}(•) Gypsum contains two water molecules as water of cyrstallisation.

color{green}(•) It has the formula color{red}(CaSO_4 .2H_2O).

color{green}(•) On heating gypsum at color{red}(373 K), it loses water molecules and becomes calcium sulphate hemihydrate ( color{red}(CaSO_4 . 1/2H_2O ) ). This is called 𝐏𝐥𝐚𝐬𝐭𝐞𝐫 𝐨𝐟 𝐏𝐚𝐫𝐢𝐬.

color{green}(•) Plaster of paris is used for supporting fractured bones in the right position, for making toys, materials for decoration and for making surfaces smooth.

color{green}(•) Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass.

color{red}(underset("(Plaster of Paris)")( CaSO_4 . 1/2 H_2O ) +1 1/2 H_2O → underset("(Gypsum)")(CaSO_4 . 2 H_2O))

Two formula units of color{red}(CaSO_4) share one molecule of water therefore only half a water molecule is shown to be attached as water of crystallisation.

Calcium sulphate hemihydrates is called ‘𝐏𝐥𝐚𝐬𝐭𝐞𝐫 𝐨𝐟 𝐏𝐚𝐫𝐢𝐬’