Hydrated and Anhydrous Salts

Some salt crystals contain water trapped inside their solid structure. Think of the crystal like a tidy Lego wall where some blocks are water molecules that help hold the shape.

Key ideas

• Hydrated substance: a compound that has water molecules chemically built into its crystals.
• Anhydrous substance: the same compound without any water in its crystals.
• Water of crystallisation: these are the water molecules present in hydrated crystals.

Formulas and the dot

We show water of crystallisation with a dot, e.g. $\text{CuSO}_4\cdot5\text{H}_2\text{O}$. This means each formula unit of copper(II) sulfate is joined with 5 water molecules inside the crystal. The water is part of the solid, not just wet on the surface.

Reversible change with heating

Heating drives off the water of crystallisation:

$$\text{Hydrated salt} \,\xrightarrow{\text{heat}}\, \text{Anhydrous salt} + \text{water}$$

Adding water can restore the hydrated crystal:

$$\text{Anhydrous salt} + \text{water} \,\to\, \text{Hydrated salt}$$

Important examples

• Copper(II) sulfate: $\text{CuSO}_4\cdot5\text{H}_2\text{O}$ is blue. Anhydrous $\text{CuSO}_4$ is white and turns blue when water is added. It is used to test for water.
• Cobalt(II) chloride: anhydrous is blue; hydrated (e.g. $\text{CoCl}_2\cdot6\text{H}_2\text{O}$) is pink. Blue cobalt chloride paper turns pink in water.

Common misconceptions

• Hydrated does not mean “wet.” The water is part of the crystal structure.
• Anhydrous salts can reabsorb water from air; keep them in sealed containers.
• Water of crystallisation changes the mass and sometimes the color of a salt.