Water in cristals & Hydratation

Hygroscopic substances like to absorb water(vapor) from the atmosphere. That's why they are applied as drying substance, for example in packages with electronic apparatus. When they absorb water, the water molecules are built in in the salt cristals. In the meantime, these cristals remain solid and dry for a long time.
The water molecules are built in in the ionic lattices.
Only after absorbing very much water(vapor), the substance may become wet.

Examples: Copper)II)sulfate (light blue); Calcium Chloride; Sodium carbonate; and many other salts. But also phorphorpentoxyde (P2O5) is extremely hygroscopic; only: this substance does not only absorb the water, but also reacti with it.

Dehydrated (dry) Copper(II)sulfate can be used to prove the presence of small amounts of water in a mixture.