An anhydrous salt is a salt without water.
To many of us, the word "salt" calls to mind a white crystal we add to potato chips and icy roads. For a chemist, however, salt is a broader term that describes all ionic compounds formed as the result of a neutralization reaction between an acid and a base.
Many salts are hydrated, meaning that a crystal of the salt contains water molecules in a definite ratio with the salt ions. The term "anhydrous" means the salt contains no water.
Cobalt(II) Chloride
Cobalt(II) chloride, or CoCl2, is an interesting example of an anhydrous salt. Cobalt(II) chloride is often found in the form of cobalt(II) chloride hexahydrate, which contains six molecules of water for each formula unit of CoCl2. The hydrated form has a different color than the anhydrous form; cobalt(II) chloride is light blue, while cobalt(II) chloride hexahydrate is bright pink. This makes anhydrous cobalt(II) chloride a good indicator for the presence of water.
Copper(II) Sulfate
In nature, copper(II) sulfate occurs in several forms, namely anhydrous copper(II) sulfate; copper(II) sulfate pentahydrate; and copper(II) sulfate trihydrate. The anhydrous form is pale green and powdery, whereas the pentahydrate is a beautiful blue color.
Calcium Chloride
Calcium chloride, or CaCl2, is so good at absorbing water that it's seldom (if ever) found in its anhydrous form in nature. However, chemists prepare anhydrous calcium chloride for use as a desiccant, which is a type of compound that dries out its immediate surroundings by absorbing water. It's also used on dirt roads, because it attracts enough moisture to the roadbed to prevent dust-related problems.
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