Hydrated salts have water molecules
inside their crystal structure called water of crystallization.
When a hydrated salt is heated, its water of crystallization
are removed and the anhydrous
salt is formed.
For example:
Na2CO3.10H2O
(s)----->Na2CO3
(s) + 10H2O
(g)
Note that the steam formed condenses on the cooler
sides of the test tube to form water droplets.
