The process whereby a hydrated salt loses its water of crystallization, partially or completely to t

Explanation

Efflorescence occurs when hydrated crystals release their water molecules into dry air. The solid loses water and often becomes a powder as it dries out.

Washing soda left in open air will effloresce, losing its water and becoming white powder. The crystals give up water to the atmosphere when humidity is low.

This is the opposite of deliquescence, where substances absorb water from air. Efflorescence happens when the vapor pressure of water in the crystals exceeds the humidity in surrounding air.