Desiccants are chemicals that readily absorb moisture from the surrounding atmosphere or dry it out; these are also called hygroscopic compounds. Many of them, though not all, are salts. They enjoy a variety of applications both in the lab and in commerce, where reducing moisture inside packaging can help slow degradation of food or other goods.
Calcium chloride, calcium sulfate, activated carbon, zeolites and silica gel are all common desiccants. Silica gel is probably familiar to you as the pre-packaged desiccant inside many commercial products like vitamin bottles. Calcium chloride is also a popular ice-melter for roads and driveways. Zeolites are aluminosilicate minerals with numerous microscopic pores that help them effectively absorb various liquids and gases; this property makes them useful both in filtration and as desiccants.
Some chemicals absorb moisture effectively but are seldom employed as desiccants, either because they react with water, are highly reactive in general or have other undesirable properties. Potassium and sodium hydroxide pellets, for example, readily absorb moisture from the atmosphere, but both are strong bases and once dissolved in the water they absorb become corrosive liquids. Lithium aluminum hydride absorbs water, but it is a potent base that reacts violently with water, so as a desiccant it would be unsuitable. Some salts like magnesium sulfate (Epsom salt) are typically available in the hydrated form, where the salt crystal already contains a specific ratio of water molecules for each formula unit of ionic compound, and these salts can be used as desiccants if dried to the anhydrous form as well.
Uses in the Lab
Water can interfere with many reactions in the lab. As already mentioned, lithium aluminum hydride and metals like sodium react violently with water. Desiccants can help to remove moisture from the air if water would be an undesirable ingredient in the reaction mixture. Water can also affect the weight of an object like a crucible containing a chemical that must be weighed; a desiccant can help dry an object out to ensure all the water is removed. Desiccants often contain indicator crystals, salts that change color as they absorb water, to show when the desiccant needs to be changed out.
Uses Outside the Lab
Outside the lab, desiccants are often used in packaging commercial products like vitamin tablets. Excess moisture inside the packaging could accelerate spoiling, while preserving a dry atmosphere will help retard microbial growth. Cases for certain musical instruments often include desiccants to help prevent moisture damage. In 2010, the National Renewable Energy Laboratory even proposed liquid desiccant air conditioning units as a way to achieve a more efficient cooling process for homes and businesses.