1. Removal of water from a substance.
2. A chemical reaction in which a compound loses hydrogen and oxygen in the ratio 2:1. For instance, ethanol passed over hot pumice undergoes dehydration to ethene:
Substances such as concentrated sulphuric acid, which can remove H2O in this way, are known as dehydrating agents. For example, with sulphuric acid, methanoic acid gives carbon monoxide: