Deliquescence

Leave a container of table salt open in a humid kitchen for long enough, and it will often clump together into a damp, solid mass, even though no liquid was ever added directly. This happens through a process called deliquescence, in which certain salts absorb water vapor directly from the surrounding air until they dissolve entirely into a solution, without ever needing to touch liquid water. The phenomenon depends on a specific property of the salt called its equilibrium relative humidity: the humidity level at which the water vapor pressure directly above a saturated solution of that salt equals the water vapor pressure of the surrounding air. If the air's humidity is higher than that threshold, the salt will continue absorbing moisture indefinitely, since the surrounding air still has more water vapor to give up than the salt's own solution does. If the humidity is lower than the threshold, the reverse happens instead, and the salt will actually release moisture back into the air until a dry equilibrium is reached. Calcium chloride, a compound widely used in industrial drying agents and de-icing products, has an unusually low equilibrium relative humidity, which is exactly what makes it so effective: it will keep absorbing moisture from the air even in conditions that most other salts would consider fairly dry. Manufacturers exploit this property deliberately, packaging deliquescent salts inside sealed containers specifically to prevent them from drawing moisture out of the surrounding air before they're ever used.