hemihydraatti

Hemihydraatti is a chemical term referring to a hydrate compound that contains one molecule of water for every two formula units of the compound. This means that in the crystal structure of a hemihydrate, half a water molecule is associated with each molecule of the anhydrous salt. It is a type of solvate, specifically a hydrate because the solvent is water.

The term "hemi" originates from the Greek word "hēmi," meaning half. Thus, "hemihydrate" literally translates to "half hydrate." This notation is often used in chemical formulas, for example, calcium sulfate hemihydrate is written as CaSO4·½H2O. Despite the fractional notation, it signifies a fixed stoichiometric ratio of water to the compound.

A common example of a hemihydrate is calcium sulfate hemihydrate, also known as plaster of Paris. When heated, gypsum (calcium sulfate dihydrate, CaSO4·2H2O) loses 1.5 molecules of water, forming calcium sulfate hemihydrate. This compound readily absorbs water and hardens, making it useful in construction and casting.

The properties of a hemihydrate can differ significantly from its anhydrous form and other hydrates of the same compound. These differences can include solubility, melting point, and crystal structure. Understanding the hydration state is crucial in many chemical and industrial applications where precise control over material properties is required.