Magnesium sulfite

Magnesium sulfite is the magnesium salt of sulfurous acid with the formula MgSO
3. Its most common hydrated form has 6 water molecules making it a hexahydrate, MgSO
3·6H
2O. When heated above 40 °C (104 °F), it is dehydrated to magnesium sulfite trihydrate, or MgSO
3·3H
2O. The anhydrous form is hygroscopic, meaning that it readily absorbs water from the air.