Semideliquescence

Semideliquescence is a phenomenon observed in certain salts where they absorb moisture from the atmosphere to form a saturated solution, but this process does not continue indefinitely. Unlike highly deliquescent salts that dissolve completely into their absorbed water, semideliquescent salts reach an equilibrium. At this point, the concentration of the salt in the absorbed water is equal to the saturation concentration of that salt at the ambient temperature and humidity.

This means that the salt crystals will appear wet or even form a visible layer of liquid on their surface, but the bulk of the solid material will remain. The absorbed water is not sufficient to dissolve the entire salt mass. The extent of semideliquescence depends on the relative humidity of the surrounding air. If the humidity drops below the level required for saturation, the salt may re-crystallize, and the absorbed water will evaporate.

Examples of semideliquescent salts include sodium sulfate decahydrate (Glauber's salt) under certain conditions and some hydrated calcium chlorides. Understanding semideliquescence is important in various applications, such as hygroscopy, material science, and the storage and handling of chemicals, to prevent unwanted changes in their physical state.