Anhydrous Definition

What Anhydrous Means in Chemistry

These are crystals of sodium chloride or table salt. Most salts are anhydrous, yet readily attract water to form hydrated substances.
These are crystals of sodium chloride or table salt. Most salts are anhydrous, yet readily attract water to form hydrated substances. Sheri Neva / Getty Images

Definition of Anhydrous

Anhydrous literally means 'no water'. Substances without water are anhydrous. The term is most often applied to crystalline substances when the water of crystallization is removed.

Anhydrous can also refer to the gaseous form of some concentrated solutions or pure compounds. For example, gaseous ammonia is called anhydrous ammonia to distinguish it from the aqueous solution. Gaseous hydrogen chloride is called anhydrous hydrogen chloride, to distinguish it from hydrochloric acid.

Anhydrous solvents are used to perform certain chemical reactions that either cannot proceed in the presence of water or that yield unwanted products. Examples of reactions with anhydrous solvents include the Wurtz reaction and Grignard reaction.

Examples of Anhydrous Substances

  • Table salt is anhydrous sodium chloride (NaCl).
  • Gaseous HCl is anhydrous, to differentiate it from hydrochloric acid, which is a solution of 37% HCl in water (w/w).
  • Heating copper(II) sulfate pentahydrate (CuSO4·5H2O) yields anhydrous copper(II) sulfate (CuSO4).

How Anhydrous Chemicals Are Prepared

The method of preparation depends on the chemical. In some cases, simply applying heat can drive off water. Storage in a desiccator can slow rehydration. Solvents may be boiled in the presence of a hygroscopic material, to prevent water from returning to solution.