Tetrahydrate

Part of speech: noun

Definitions

1. A chemical compound whose crystal structure incorporates exactly four molecules of water for each formula unit of the substance
2. A hydrated salt or other compound that holds four water molecules bound within its crystalline form
3. Describing a crystalline substance in which four water molecules are chemically bonded into the solid lattice

Etymology: Tetrahydrate is a chemist's compound word, built in English from two Greek-derived parts. The first, "tetra-", comes from Ancient Greek "tetra-", the combining form of "téssares", meaning "four", and it appears in English as a prefix meaning four. The second part, "hydrate", is a French-derived chemical term: French "hydrate" joined the English vocabulary as "hydrate", and it names a compound in which water molecules are bound into a substance's structure. "Hydrate" itself is built on the Greek "hudōr", meaning "water", through the combining form "hydro-" and the French "-ate" suffix, which chemists use for salts and similar compounds. Put together, "tetrahydrate" means a compound that carries four molecules of water for each molecule of the parent substance. Chemists use the prefix system this way, so a "monohydrate" holds one water molecule, a "dihydrate" two, and "tetrahydrate" four. The word is a modern coinage from classical roots, not an ancient term, and it reflects the systematic naming that grew up in chemistry in the 19th and 20th centuries.