Epsom salts are hydrate of magnesium sulfate. The formula is MgSO_{4}.7H_{2}O. It has 7 water molecules.
Here, hydrate means it is bind to water molecules. Dehydration means removal of water molecules. Thus, Epsom salt in its complete dehydration loss all its water molecules and the remaining formula of dehydrated Epsom salt will be MgSO_{4} also known as magnesium sulfate.
