The chemical formula for magnesium sulfate heptahydrate is MgSO4x7H2O or MgSO4(H2O)7. This indicates that each magnesium sulfate atom is bound to seven water atoms. Heating the material releases the water as steam, resulting in anhydrous magnesium sulfate.
Consumers find magnesium sulfate heptahydrate in pharmacies and with cleaning products under its common name, Epsom salts. It is useful as a laxative and an additive for soaking baths to relieve sore muscles. In the garden, magnesium sulfate provides nutrients that encourage plant growth. Anhydrous magnesium sulfate is very hygroscopic, absorbing moisture from the air and making it difficult to weigh accurately, so the more stable magnesium sulfate heptahydrate is the preferred form for medical preparations.