Calcium sulfate

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Calcium sulfate
IUPAC name
Calcium sulfate
Other names
Plaster of Paris
Molar mass 136.14 g/mol (anhydrous)
145.15 g/mol (hemihydrate)
172.172 g/mol (dihydrate)
Appearance White solid
Odor Odorless
Density 2.96 g/cm3 (anhydrous)
2.32 g/cm3 (dihydrate)
Melting point 1,460 °C (2,660 °F; 1,730 K)
0.21 g/100 ml at 20 °C (anhydrous)
0.24 g/100 ml at 20 °C (dihydrate)
Solubility dihydrate
Slightly soluble in glycerol
Acidity (pKa) 10.4 (anhydrous)
7.3 (dihydrate)
107 J·mol−1·K−1
-1,433 kJ/mol
Safety data sheet Sigma-Aldrich (drierite)
Sigma-Aldrich (hemihydrate)
Sigma-Aldrich (dihydrate)
Flash point Non-flammable
Related compounds
Related compounds
Magnesium sulfate
Strontium sulfate
Barium sulfate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Calcium sulfate is a virtually insoluble inorganic compound with the formula CaSO4. It is commonly formed as a by-product of reactions. Since it's hygroscopic, it's commonly encountered as hydrate.



A very exothermic thermite reaction occurs with aluminium powder, which is used to 'heat boost' reactions such as a titanium dioxide thermite, where the extra heat is needed to reduce the oxide all the way to titanium metal

Double displacement reactions often are chosen to form calcium sulfate as the precipitation of the solid not only drives the reaction but makes the two formed chemicals easy to separate by simply filtering off the sulfate:

Ca(OCl)2 + K2SO4 → CaSO4 + 2 KOCl


It is generally considered to be insoluble, but does have a very low solubility of 0.21g/100ml @20 degrees (Wikipedia)

Calcium sulfate exists in three hydrates, a dihydrate, a hemihydrate and a anhydrous form. Over 100 degrees the hemihydrate is formed and it looses water to form the anhydrous over 180 degrees Celsius. These two forms will slowly absorb water at room temperature to revert back to the dihydrate.


The hemihydrate is easily available as 'plaster of Paris' in hardware stores. The dihydrate is heavily used as 'drywall', a calcium sulfate construction material. Chalk sticks tend to contain calcium sulfate rather than calcium carbonate (a chalk is not made from chalk, who knew), though some formulas may contain traces of calcium carbonate and in many locations it can be found in the ground as the mineral gypsum.


A solutions of a soluble sulphate salt such as ammonium sulfate and calcium chloride will precipitate calcium sulfate, which can be filtered and washed with cold water, then dried.




Anhydrous calcium sulfate will release large amounts of heat if mixed with water.


Should be stored in closed containers, away from moisture.


Unless it's contaminated with heavy metals or sodium ions, calcium sulfate can be safely dumped in the ground or trash. Do not pour it down the drain as it may clog the pipes.


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