Anhydrous Definition 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

Anhydrous literally means "no water." In chemistry, substances without water are labeled anhydrous. The term is most often applied to crystalline substances after 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 its aqueous form. Gaseous hydrogen chloride is called anhydrous hydrogen chloride to distinguish it from hydrochloric acid.

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

Examples

Anhydrous substances exist in solid, liquid, and gas forms.

  • Table salt is anhydrous sodium chloride (NaCl).
  • Gaseous HCl is anhydrous, which differentiates it from hydrochloric acid, a solution of 37 percent 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 the solution.