In chemistry, the term "hydrate" refers to a substance that contains water, while an anhydrous substance contains no water. Hydrates and anhydrates differ greatly in their reactions to water and their common uses.
In organic chemistry, a hydrate is formed by the addition of hydrogen to another molecule to form a compound. Examples of organic hydrates are chloral hydrate, created from the reaction of hydrogen with chloral, and ethanol, formed when hydrogen is added to ethylene. In inorganic chemistry, the hydrate compound is formed when the compound contains water of crystallization. This happens when the hydrogen molecules have crystallized within the metal complex of the original molecule, or are bound to its metal center without changing the structure of the molecule.
By contrast, an anydrate or anhydrous material does not contain any water molecules. The water is usually removed by suction or by heating the substance to a high temperature. Anhydrous materials are typically used as drying agents because they absorb water from their surroundings. One example is a silica gel packet often included inside clothing pockets or other consumer goods to keep the surrounding area dry and mold-free. Many common anhydrates are quite dangerous to handle. Anhydrous ammonia, for example, is a pungent gas used as a fertilizer. The pungent fumes from this gas can suffocate a human and are extremely flammable. If ingested, it can cause quick dehydration.