Tetrahydrate

Tetrahydrate refers to a crystalline solid that incorporates four molecules of water within its crystal structure for each formula unit of the compound. This water is known as water of hydration or crystallization. The number preceding "hydrate" in a chemical name indicates the number of water molecules associated with each molecule of the salt. For example, a tetrahydrate of a salt would have the chemical formula showing a ratio of four water molecules to one unit of the salt.

The water molecules in a tetrahydrate are not chemically bonded to the ions of the salt but are held in place by electrostatic forces and hydrogen bonding. This presence of water of hydration can significantly affect the physical properties of the compound, such as its melting point, solubility, and density, compared to its anhydrous (water-free) form. Tetrahydrates are often formed when a hydrated salt crystallizes from an aqueous solution under specific temperature and concentration conditions. The specific arrangement of water molecules within the crystal lattice is characteristic of the particular compound and its hydration state. When heated, a tetrahydrate will typically lose its water of hydration, converting into a lower hydrate or the anhydrous form.