Anhydrous – IGCSE Chemistry Definition

Anhydrous means “without water.” In chemistry, an anhydrous substance is one that does not contain water of crystallisation in its crystal structure. It is the opposite of hydrated. For example, anhydrous copper(II) sulfate has the formula CuSO₄, while hydrated copper(II) sulfate is CuSO₄·5H₂O.

How anhydrous forms are made

Anhydrous salts are produced by heating the hydrated form to drive off the water of crystallisation. For example, heating blue CuSO₄·5H₂O produces white CuSO₄. The process is reversible: adding water to the anhydrous salt restores the hydrated form and the original colour.

Anhydrous calcium chloride is produced similarly and is used as a drying agent in the laboratory because it absorbs water from gases or liquids that pass over it.

As a test for water

Anhydrous copper(II) sulfate is used as a chemical test for the presence of water. The white powder turns blue when water is added. This confirms the liquid is water (or contains water), but it does not confirm the water is pure. To test for pure water, check the boiling point (100 degC at standard pressure) or melting point (0 degC).

As a drying agent

Anhydrous calcium chloride and anhydrous sodium sulfate are commonly used as drying agents. When a gas is passed through a U-tube or drying tube containing the anhydrous substance, water vapour is absorbed, leaving a dry gas. In the rusting experiment, anhydrous calcium chloride placed in a sealed test tube with an iron nail absorbs all moisture, preventing rusting and demonstrating that water is needed for corrosion.

Exam context

Paper 4 questions test the distinction between hydrated and anhydrous forms, particularly in the context of calculations involving water of crystallisation and in the test for water using anhydrous copper(II) sulfate.

Worked exam question

White powder X turns blue when water is added to it. (a) Suggest the identity of powder X. (b) This test confirms water is present. Explain why a further test is needed to confirm the water is pure. (2 marks)

(a) Anhydrous copper(II) sulfate [1]. (b) The test shows water is present but does not distinguish between pure water and a solution containing water. To confirm purity, check the boiling point is exactly 100 degC (at 1 atm) [1].

Common mistakes

Candidates confuse “anhydrous” with “dehydrated.” While similar in meaning, the term used in IGCSE chemistry is anhydrous. Another error is thinking that the colour change from white to blue is a test for pure water. It tests only for the presence of water, not its purity.