deliquescence

Deliquescence is the process by which certain solids absorb moisture from the atmosphere to such an extent that they dissolve in the absorbed water, forming an aqueous solution. A material that undergoes deliquescence has a characteristic deliquescence relative humidity (DRH); when ambient relative humidity exceeds the DRH, the solid begins to absorb water and eventually dissolves into a liquid. If humidity is lowered, the liquid can evaporate and the material may re-form as a solid.

Deliquescent substances are typically highly hygroscopic salts or acids. Calcium chloride is a well-known example; other examples include certain metal chlorides and hydroxides, as well as some strong acids. The tendency to deliquesce depends on the material and the surrounding humidity, and not all hygroscopic substances are deliquescent.

In practical use, deliquescent materials are employed as desiccants and moisture-control agents, especially in packaging and storage of moisture-sensitive items. They can, however, pose handling hazards due to corrosivity, exothermic dissolution, or reactive interactions with water or air components. Safety considerations include containment, ventilation, and compatibility with surrounding materials.

Deliquescence is the opposite of efflorescence, in which a hydrated solid loses water to the atmosphere and becomes a powder. It is related to, but distinct from, general hygroscopicity, which describes a material’s tendency to absorb moisture without necessarily dissolving. DRH is a key concept in designing humidity controls and in understanding chemical processes influenced by ambient moisture.