Anhydrous

Summary: Containing no water of crystallisation. Anhydrous salts are often used as drying agents or to test for water. Tags: igcse chemistry definition practical-skills Created: 2026-07-14 Last Updated: 2026-07-14

Anhydrous describes a substance that contains no water molecules, and in the IGCSE context it most frequently refers to a salt from which all water of crystallisation has been removed, usually by heating. Many ionic compounds crystallise from aqueous solution with a specific number of water molecules incorporated into their crystal lattice in a fixed stoichiometric ratio — these are called hydrated salts (e.g., CuSO₄·5H₂O contains five water molecules per formula unit) — and the anhydrous form is produced when this water is driven off. Two anhydrous compounds are of particular importance in the IGCSE laboratory: anhydrous copper(II) sulfate (CuSO₄), which is a white powder, and anhydrous cobalt(II) chloride (CoCl₂), which is blue; both undergo dramatic colour changes when water is added, turning blue (hydrated CuSO₄·5H₂O) and pink (hydrated CoCl₂·6H₂O) respectively. These distinct colour changes form the basis of the standard IGCSE chemical tests for water: anhydrous CuSO₄ turns from white to blue, confirming the presence of water, while CoCl₂ paper turns from blue to pink. In the exam, students should know that the term anhydrous also applies to substances used as drying agents (such as anhydrous calcium chloride) and that heating a hydrated salt “to constant mass” ensures complete dehydration has been achieved.

Key term — IGCSE Chemistry (0620/0971) glossary.