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Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO 4 and related hydrates. In the form of γ- anhydrite (the anhydrous form), it is used as a desiccant . One particular hydrate is better known as plaster of Paris , and another occurs naturally as the mineral gypsum .
The following chart shows the solubility of various ionic compounds in water at 1 atm pressure and room temperature (approx. 25 °C, 298.15 K). "Soluble" means the ionic compound doesn't precipitate, while "slightly soluble" and "insoluble" mean that a solid will precipitate; "slightly soluble" compounds like calcium sulfate may require heat to precipitate.
The tables below provides information on the variation of solubility of different substances (mostly inorganic compounds) in water with temperature, at one atmosphere pressure. Units of solubility are given in grams of substance per 100 millilitres of water (g/(100 mL)), unless shown otherwise.
A few, such as calcium sulfate and cerium(III) sulfate, become less soluble in water as temperature increases (ΔH < 0). [15] This is also the case for calcium hydroxide (portlandite), whose solubility at 70 °C is about half of its value at 25 °C. The dissolution of calcium hydroxide in water is also an exothermic process (ΔH < 0).
Solubility products are often expressed in logarithmic form. Thus, for calcium sulfate, with K sp = 4.93 × 10 −5 mol 2 dm −6, log K sp = −4.32. The smaller the value of K sp, or the more negative the log value, the lower the solubility. Some salts are not fully dissociated in solution.
This transformation is reversible, with gypsum or calcium sulfate hemihydrate forming anhydrite by heating to around 200 °C (400 °F) under normal atmospheric conditions. [6] Anhydrite is commonly associated with calcite, halite, and sulfides such as galena, chalcopyrite, molybdenite, and pyrite in vein deposits.
Due to its relatively high solubility in water, Calcium sulfate is the easiest sulfate scale to remove chemically as compared to strontium and barium sulfate. [2] Scale crystals are initially dispersed in production systems until accumulation of stable crystals of insoluble sulfates and scale growth occur at nucleation centers. [ 8 ]
Calcium bisulfite (calcium bisulphite or calcium hydrogen sulfite) is an inorganic compound which is the salt of a calcium cation and a bisulfite anion. It may be prepared by treating lime with an excess of sulfur dioxide and water. As a food additive it is used as a preservative under the E number E227.