Defining a Chemical Hydrate
In chemistry, a hydrate is an inorganic compound that contains water molecules loosely bonded within its crystal structure. These water molecules are not chemically integrated into the main compound's formula but are associated with it in a definite molar ratio. Hydrates are often crystalline solids, and the water molecules, referred to as 'water of crystallization' or 'water of hydration,' contribute to the compound's overall structure and properties.
Composition and Naming Conventions
The water molecules in a hydrate are typically incorporated into the crystal lattice, either by being coordinated to a metal ion or by being present as interstitial molecules. The definite ratio of water to the anhydrous compound is indicated in its chemical formula using a dot, such as CuSO₄·5H₂O for copper(II) sulfate pentahydrate. The naming convention involves stating the name of the anhydrous compound followed by a Greek prefix indicating the number of water molecules, then 'hydrate' (e.g., mono-, di-, tri-, tetra-, penta-, hexa-).
Practical Examples of Hydrates
A common example is plaster of Paris, which is calcium sulfate hemihydrate (CaSO₄·½H₂O). Another well-known hydrate is Epsom salt, chemically known as magnesium sulfate heptahydrate (MgSO₄·7H₂O), widely used in baths. Copper(II) sulfate, when hydrated, is bright blue (CuSO₄·5H₂O), but upon heating, it loses its water of hydration to become anhydrous copper(II) sulfate, a white powder (CuSO₄), demonstrating a reversible physical change.
Importance and Applications
Hydrates are significant in various scientific and industrial applications. They are crucial in understanding mineral formation and crystallography, where the presence and arrangement of water molecules influence crystal structure. In industry, hydrates are used as drying agents (desiccants) due to their ability to absorb water, and in laboratories, they can be used to prepare solutions of known concentrations by accurately weighing the hydrated form.