Magnesium Sulfate Heptahydrate
Properties
| State | Solid at room temperature |
| Color | Colorless to white crystalline solid |
| Solubility | Very soluble in water (71 g/100 mL at 20 °C) |
| Melting Point | Loses water starting at 48 °C; decomposes at 200 °C |
About Magnesium Sulfate Heptahydrate
Magnesium sulfate heptahydrate is the laboratory and bathtub face of MgSO4 — the hydrate that crystallizes spontaneously from any aqueous solution at room temperature. The Mg2+ ion sits at the center of an octahedral [Mg(H2O)6]2+ cluster, with the seventh water molecule hydrogen-bonded to the sulfate framework rather than coordinated directly to the metal. That arrangement is the textbook reason for two of its better-known behaviors: dissolving 50 g of Epsom salt in a glass of warm water leaves the glass distinctly cold (the lattice energy released by breaking up the hydrate exceeds the hydration enthalpy released by re-solvating the ions, dH solution about +13 kJ/mol), and gentle heating drives off the water in stepwise plateaus visible by TGA at roughly 70, 150, and 200 °C. The compound was first concentrated by Nehemiah Grew from the bitter-tasting spring at Epsom, Surrey in 1695, and the historical name has stuck through three centuries of use as a bath additive and laxative. The mineral form, epsomite, is a common late-stage evaporite in saline lake deposits and on Mars, where the Curiosity rover has identified it in sedimentary mudstones. In horticulture, dissolving one tablespoon (about 15 g) per gallon of water gives roughly 0.4 g/L Mg2+, useful as a foliar spray on tomato and pepper plants showing interveinal chlorosis from magnesium-deficient soil.
Where you'll encounter it
If you have ever soaked sore quads in an Epsom salt bath after a long run, watered tomatoes with a tablespoon-per-gallon solution to fix yellowing leaves, or run a TGA on a hydrated salt to confirm stoichiometry by mass loss, you have been working with MgSO4.7H2O. A long-distance runner dumping 2 cups of Epsom salt into a warm tub gets the dilation-of-skin-vessels effect that reliably eases DOMS, even if transdermal Mg2+ uptake is a smaller piece of the picture than the marketing claims. A tomato grower spraying 1 tbsp per gallon onto interveinal-chlorotic leaves green-up the foliage within a week if Mg was actually the limiting nutrient — and learns it wasn't if nothing changes. A teaching-lab TGA run on the heptahydrate produces three textbook mass-loss steps near 70, 150, and 200 °C that exactly track the 7-water stoichiometry on the molar mass calculator.
Common Uses
- Epsom salt bath additive at roughly 1 to 2 cups per tub (about 250-500 g)
- Foliar magnesium spray for tomatoes, peppers, and roses at 1 tbsp per gallon
- Saline laxative dosed at 10-30 g in water for adults
- Float-tank density agent at roughly 250-300 kg per ~1500 L tank to give SG 1.25
- Demonstration substrate for endothermic dissolution and stepwise hydrate decomposition (TGA)
- Bath-bomb formulations combined with NaHCO3 and citric acid
- Source crystal for growing classroom single crystals from saturated solution
Safety Information
GRAS for oral use. External use is essentially without risk. Oral doses above about 30 g produce strong osmotic laxation; renal-impaired patients can accumulate Mg2+ and develop hypermagnesemia (sedation, hyporeflexia, cardiac depression), so chronic supplementation should be cleared with a clinician. Keep packaging away from young children — the salt looks like coarse table salt and the laxative dose for a toddler is small. Concentrated solutions can mildly irritate the eye conjunctiva. Not classified hazardous under GHS.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.