spontaneous loss of water by a hydrated salt, which occurs when the aqueous vapor pressure of the hydrate is greater than the partial pressure of the water vapour in the air. For example, because the vapour pressures of washing soda (Na₂CO₃·10H₂O) and Glauber's salt (Na₂SO₄·10H₂O) normally exceed that of the water vapour in the atmosphere, these salts effloresce (i.e., lose all or part of their water of hydration), and their surfaces assume a powdery appearance. Hydrated cupric sulfate, or blue vitriol (CuSO₄·5H₂O), the aqueous vapour pressure of which is lower, undergoes efflorescence only if the air in contact with it is relatively dry. See also deliquescence.