Efflorescence: A whitish, powdery deposit on the surface of rocks or soil in dry regions. It is formed as mineral-rich water rises to the surface through capillary action and then evaporates. Efflorescence usually consists of gypsum, salt, or calcite.
Deliquescence, the process by which a substance absorbs moisture from the atmosphere until it dissolves in the absorbed water and forms a solution. Deliquescence occurs when the vapour pressure of the solution that is formed is less than the partial pressure of water vapour in the air.