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NAME SOME EFFLORESCENT SUBSTANCES.
In chemistry, efflorescence (which means "to flower out" in French) is the loss of water (or a solvent) of crystallization from a hydrated or solvated salt to the atmosphere on exposure to air.
Efflorescences can occur in natural and built environments. On porous construction materials it may present a cosmetic problem only, but can sometimes indicate serious structural weakness.
Example:
A 5 molar concentration aqueous droplet of NaCl will spontaneously crystallize at 45% relative humidity (298 K) to form an NaCl cube by the mechanism of homogeneous nucleation. The original water is released to the gas phase.
Gypsum (CaSO4.2H2O) is a hydrate solid that, in a sufficiently dry environment, will give up its water to the gas phase and form anhydrite (CaSO4).
Bluestone (CuSO4.5H2O) is a blue crystalline solid that when exposed to air, slowly loses water of crystallization from its surface to form a white layer of anhydrous copper(II) sulfate.


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