Desiccant

A desiccant, drying agent^[1] or dehydrating agent^[2] is a substance that is (good at or) effective at removing water; "desiccate" means "make very dry".^[3] Many types of desiccants exist, and are used for different purposes.

Silica is a common desiccant.

Desiccants are packaged with many goods that could be damaged by water and humidity, like dried food and medical drugs. These desiccants are usually chemically inert materials. Silica is a very common food-safe desiccant. Some dehumidifiers use desiccants to remove water from the air.

Chemistry

(Inert) desiccants like silica work by hydration: they attract water molecules into their structures without changing themselves or the water. Reactive desiccants like phosphorus pentoxide undergo a chemical reaction with water, usually hydrolysis, and produce a different chemical.

Desiccants are used in the chemical industry to safely handle chemicals that are sensitive to water. Calcium chloride is a common desiccant for laboratory drying tubes, which remove water from vapors; oil refineries use zeolite molecular sieves to dry petroleum products and prevent ice from damaging pipes. Solvents for water-sensitive chemicals need to be dried with a desiccant before they can be used.

The chemical industry also uses reactive desiccants in dehydration reactions. Phosphorus pentoxide^[4] and thionyl chloride^[5] are powerful reactive desiccants that can both desiccate solutions and prepare many compounds by reversing a hydrolysis reaction.