What is the term for a hydrate losing water when exposed to air?

The term for a hydrate losing water when exposed to air is efflorescence. This phenomenon occurs when a hydrate, which is a compound that includes water molecules in its crystal structure, loses its water of hydration to the atmosphere. When the pressure of water vapor in the air is lower than that of the water vapor held in the hydrate, the water evaporates, leading to the formation of a powdery substance or a desiccated solid.

Efflorescence is particularly common in certain salts, which can change visually and physically upon loss of water. For example, a common hydrate like copper(II) sulfate pentahydrate can transition into anhydrous copper(II) sulfate and often appears as a blue solid that becomes white upon losing water.

In contrast, hydration refers to the process of a compound combining with water, while dehydration typically involves the removal of water but not specifically due to exposure to air. Crystallization is the process of forming crystals from a solution or a solid, which is different from the loss of water. Understanding these distinctions helps clarify the unique process of efflorescence in chemistry.