What Is a Hydrated Salt?

••• marekuliasz/iStock/GettyImages

A hydrated salt is a crystalline salt molecule that is loosely attached to a certain number of water molecules. Salt is created when an acid’s anion and a base’s cation are combined to produce an acid-base molecule. A salt molecule that is not bound to any water molecules is an anhydrate, and a salt molecule that is bound to water molecules is a hydrated salt. In a hydrated salt, the water molecules are incorporated into the crystalline structure of the salt.

Occurrence of Hydrated Salts

Hydrated salts naturally occur all over the world -- including in freshwater. For example, compounds in an area’s soil or rock may dissolve into the groundwater, where the free-floating chemicals bind to create salt molecules and hydrate with the groundwater’s molecules. One place where that occurred, to create naturally occurring Epsom salt, or magnesium sulfate heptahydrate, is Epsom, England. Since the human body needs many of the chemicals that make up different salts, but those chemicals may be difficult to obtain or absorb through diet alone, places where hydrated salts naturally occur often are traditionally viewed as places for people to heal and take curative baths. For example, such is the case with Epsom. Salts that have a loose enough crystalline structure to incorporate water molecules and become hydrated salts may absorb the water molecules from the water vapor in the air, or become hydrated when in contact with liquid water.

Naming Hydrated Salts

When hydrated, magnesium sulfate becomes magnesium sulfate heptahydarate. The chemical compound is represented as MgSO4(H2O)7. The magnesium sulfate molecule is the MgSO4 portion of the symbol of Epsom salts, and the (H20)7 is meant to show that the magnesium sulfate molecule is bound to seven water (H2O) molecules. The ratio of the salt molecules to water molecules may be more complex -- for example, the simplest ratio for hydrated cadmium sulfate is three cadmium sulfate molecules to eight water molecules, so the simplest chemical symbol for the hydrated salt is (CdSO4)3(H2O)8.

Methods of Dehydration of Hydrated Salts

The severing of the bonds between a salt molecule and the water molecules bound to it in a hydrated salt is called dehydration. The application of relatively gentle heat is usually enough to sever the bonds between the water molecules and the salt molecule of a hydrated salt, though how much heat is necessary is specific to the salt. When the hydrated salt is heated and the salt separates from the water molecules, the proportion of water molecules to salt molecules may be used to determine what the ratio of H2O to salt is in that particular hydrated salt compound.

Reasons for Dehydration of Hydrated Salts

Dehydrating a hydrated salt may allow the freed salt to be more easily ingested or absorbed. For example, people who wish to ingest magnesium and sulfate for the health benefits the molecules are meant to provide may dissolve the magnesium sulfate heptahydrate into a hot bath or combine it with hot water to make a poultice. When a person soaks in the bath with the MgSO4 that has been severed from its bonds with water through the heat of the bath, he is able to absorb the free-floating salt through his skin.


About the Author

Sasha Rousseau began writing in 2003. She won the best fiction award from "Thoroughfare Literary Magazine," placed in the Sir Martin Gilbert Churchill National Essay Competition and has been published in the "Washington Post." She graduated magna cum laude with a Bachelor of Arts in writing seminars and English from Johns Hopkins University.