Magnesium sulfate

Magnesium sulfate or magnesium sulphate is a chemical compound, a salt with the formula MgSO₄, consisting of magnesium cations Mg²⁺ (20.19% by mass) and sulfate anions SO2−4. It is a white crystalline solid, soluble in water.

Magnesium sulfate is usually encountered in the form of a hydrate MgSO₄·nH₂O, for various values of n between 1 and 11. The most common is the heptahydrate MgSO₄·7H₂O, known as Epsom salt, which is a household chemical with many traditional uses, including bath salts.

The main use of magnesium sulfate is in agriculture, to correct soils deficient in magnesium (an essential plant nutrient because of the role of magnesium in chlorophyll and photosynthesis). The monohydrate is favored for this use; by the mid 1970s, its production was 2.3 million tons^{[clarification needed]} per year.^{[page needed]} The anhydrous form and several hydrates occur in nature as minerals, and the salt is a significant component of the water from some springs.

Magnesium sulfate can crystallize as several hydrates, including:

As of 2017, the existence of the decahydrate apparently has not been confirmed.

All the hydrates lose water upon heating. Above 320 °C (608 °F), only the anhydrous form is stable. It decomposes without melting at 1,124 °C (2,055 °F) into magnesium oxide (MgO) and sulfur trioxide (SO₃).

The heptahydrate takes its common name "Epsom salt" from a bitter saline spring in Epsom in Surrey, England, where the salt was produced from the springs that arise where the porous chalk of the North Downs meets the impervious London clay.

The heptahydrate readily loses one equivalent of water to form the hexahydrate.