Some hydrated salts, when exposed to the atmosphere, give off part of all of their water of crystallization spontaneously to form a lower dehydrated or anhydrous salt. Such salts are known as efflorescent and the phenomenon is called Efflorescence.
Efflorescence occurs when the hydrate's aqueous vapor pressure is greater than the water vapor's partial pressure in the air. For example, because the vapor pressures of washing soda (Na2CO3·10H2O) and Glauber's salt (Na2SO4·10H2O) normally exceed that of the water vapor in the atmosphere, these salts effloresce (i.e., lose all or part of their water of hydration), and their surfaces assume a powdery appearance.
