Some salts, while crystallizing out form their solutions, unite with definite quality of water which is known as water of crystallization.
Four substances which contain water of crystallization:
Na2CO3. 10H2O Washing soda
MgSO4. 7H2O Epsom salt
K2SO4.Al2(SO4)3 . 24H2O Potash alum
Na2SO4. 10H2O Glauber's salt