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.^[1] The anhydrous form is hygroscopic, meaning that it readily absorbs water from the air.

Magnesium sulfite

Names
IUPAC name Magnesium sulfite
Other names Magnesium sulphite
Identifiers
CAS Number	7757-88-2 Y
ChemSpider	2282946
ECHA InfoCard	100.028.932
EC Number	231-825-6
PubChem CID	3014583
UNII	792J6K230W Y
CompTox Dashboard (EPA)	DTXSID50884418
Properties
Chemical formula	MgSO 3 (anhydrous); MgSO 3·6H 2O
Molar mass	104.368200 g/mol (anhydrous) 212.4599 g/mol (hexahydrate)
Solubility in water	5.2g/L at 298.2K (hexahydrate)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references