What does anhydrous mean?
Anhydrous is a technical adjective meaning 'without water'. It is applied chiefly in chemistry and related sciences to substances from which all water has been removed — whether moisture clinging to a sample or water molecules locked into a crystal's structure. Anhydrous copper sulfate, for example, is white, while its hydrated form is vividly blue, making the pair a classic classroom demonstration. The word appears constantly in industrial contexts: anhydrous ammonia serves as fertilizer, anhydrous ethanol as fuel and solvent, and many laboratory reactions demand strictly anhydrous conditions because water would ruin them. Its opposite is hydrous or hydrated. Though unfamiliar outside specialist circles, anhydrous is precise and unambiguous, and anyone reading product labels, safety data sheets, or agricultural guidance will encounter it regularly. It derives from Greek roots meaning 'without water'.
adjectiveAnhydrous describes a substance, especially a chemical compound or crystalline solid, that contains no water — either free water or water of crystallization.
- Containing no water, especially as a result of deliberate removal of water of crystallization or moisture from a compound or substance.
"The chemist stored the anhydrous copper sulfate in a sealed desiccator to keep it from absorbing moisture from the air."
"The reaction must be carried out under strictly anhydrous conditions, since even traces of moisture will destroy the reagent."
"Anhydrous ethanol, with its water content removed, is blended into fuel for vehicles."
Anhydrous ammonia fertilizer is so thirsty for water that it can pull moisture out of living tissue on contact — which is exactly why farmers handle it with extreme care.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of anhydrous
Anhydrous comes from Greek: the privative prefix 'an-' ('not, without') combined with 'hydor' ('water'), via the Greek adjective 'anydros'. The word entered English through New Latin usage in scientific writing, becoming established in English chemical vocabulary around the early seventeenth century as systematic chemistry developed. It shares its root with related terms such as anhydride, anhydrite, and hydrate, all built on the same Greek word for water.
How anhydrous is actually used
Anhydrous belongs firmly to scientific and technical register — chemistry, pharmacology, agriculture, and industry — and is rarely used in everyday conversation. It carries no emotional connotation; it is purely descriptive. In chemistry it often contrasts explicitly with 'hydrated', as in hydrated versus anhydrous copper sulfate.
Easily confused with anhydrous
An anhydrous substance simply contains no water, whereas an anhydride is a specific type of compound formed by removing water from another compound (typically an acid).
Anhydrite is a specific mineral (calcium sulfate), while anhydrous is the general adjective meaning 'without water' that can apply to any substance.