Final answer:
Hygroscopic hydrates can be termed dessicates because they have the ability to spontaneously absorb water from the atmosphere. This occurs due to the presence of loosely bound water molecules in the hydrate compounds, known as waters of hydration.
Step-by-step explanation:
Hygroscopic hydrates can be termed dessicates because they have the ability to spontaneously absorb water from the atmosphere. This process is due to the presence of loosely bound water molecules in the hydrate compounds, known as waters of hydration. When the hydrate is exposed to a humid environment, the water molecules are attracted to the compound and become incorporated, effectively drying out the surrounding environment.
For example, a common hygroscopic hydrate is anhydrous copper sulfate (CuSO4). In its hydrated form, CuSO4·5H2O, there are five water molecules bound to each copper sulfate molecule. When this compound is exposed to air, the water molecules can be gradually absorbed, resulting in the formation of anhydrous CuSO4, which appears as a white powder.
In summary, hygroscopic hydrates can be termed dessicates because they have the ability to absorb water from the atmosphere, effectively drying out the surrounding environment.