A dehydrating agent is a substance that dries or removes water from a material. In chemical reactions where dehydration occurs, the reacting molecule loses a molecule of water. Sulfuric acid, concentrated phosphoric acid, hot aluminum oxide, and hot ceramic are common dehydrating agents in these types of chemical reactions.
Dehydration reactions in organic synthesis often result in the conversion of the reacting molecule from one type of compound to another, such as the conversion of alcohols to alkenes. In medicine, dehydrating agents are used to clean and mount tissue slides to look at under a microscope. Common dehydrating agents for this use include ethanol, butanol and isopropanol.