What is caustic soda?

The alkali part of ‘chlor-alkali’ often refers to caustic soda (usually sodium hydroxide but potassium hydroxide can also be formed).

It is the natural co-product of chlorine production; indeed, the laws of chemistry define that for every tonne of chlorine you produce, about 1.1 tonnes of caustic soda is also produced, together with 28 kg of hydrogen.

Caustic soda is also known as ‘lye’ and it is a white, solid ionic compound consisting of sodium (Na+) and hydroxide ions (OH). Due to its ionic nature, much like sodium chloride (common salt), it readily dissolves in water.

Find out more where caustic soda and caustic potash (potassium hydroxide) are used in our uses pages. You can also see some of the uses of caustic soda and caustic potash in our interactive trees, as well as our Chorine Things and Mr C videos..

Soda lye microcrystals