deliquescere

Deliquescence, from the Latin deliquescere meaning to become liquid, is a chemical process in which certain solids absorb moisture from the surrounding air and dissolve into an aqueous solution. The term can refer to the process itself, the solid that undergoes it, or the resulting liquid phase. The related verb is deliquesce and the phenomenon is called deliquescence. It is distinct from simple hygroscopic moisture uptake, which may leave a material in solid form.

The phenomenon occurs when the solid’s lattice energy and hydration energy allow dissolution in the presence of ambient water vapor. Each substance has a characteristic deliquescence relative humidity (DRH); when ambient humidity reaches or exceeds this value at a given temperature, the solid will deliquesce. Temperature influences the DRH, and the exact humidity threshold is substance-specific.

Classic examples of deliquescent materials include calcium chloride, sodium hydroxide, potassium hydroxide, and magnesium chloride. These highly hygroscopic salts can absorb sufficient moisture from air to form a liquid brine. Not all hygroscopic substances deliquesce, however; deliquescence requires a combination of humidity, temperature, and the properties of the substance that favor dissolution upon moisture uptake.

Applications and implications: deliquescent compounds are used as drying agents and moisture buffers in laboratories and industry. They are typically stored in airtight containers to prevent moisture uptake. The dissolution of a deliquescent substance is often exothermic and can pose handling hazards, particularly with caustic materials such as NaOH or KOH, which necessitates appropriate safety precautions.