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}(β€’)` Copper sulphate crystals which seem to be dry contain water of crystallisation.

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

`color{green}(β€’)` Upon moistening the crystals again with water, blue colour of the crystals reappears.

`color{green}(β€’)` 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}(β€’)` Gypsum contains two water molecules as water of cyrstallisation.

`color{green}(β€’)` It has the formula `color{red}(CaSO_4 .2H_2O)`.

`color{green}(β€’)` 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 𝐏π₯𝐚𝐬𝐭𝐞𝐫 𝐨𝐟 𝐏𝐚𝐫𝐒𝐬.

`color{green}(β€’)` 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}(β€’)` 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 β†’ 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 β€˜ππ₯𝐚𝐬𝐭𝐞𝐫 𝐨𝐟 ππšπ«π’π¬β€™