Calcium sulfate (\(text{CaSO}_4\)) is an inorganic salt composed of calcium cations (\(text{Ca}^{2+}\)) and sulfate anions (\(text{SO}_4^{2-}\)). It is most commonly encountered as the mineral gypsum (\(text{CaSO}_4 cdot 2text{H}_2text{O}\)). Calcium sulfate is classified as a sparingly soluble compound, meaning it dissolves in water only to a very limited extent.
Defining “Sparing Solubility”
A sparingly soluble compound dissolves to such a small degree that it forms a solution with a low concentration of solute particles. Chemically, this classification applies to salts with a molar solubility of less than \(0.1\) moles per liter (\(text{M}\)) in water.
Gypsum dissolves at a rate of approximately \(2.0\) to \(2.5\) grams per liter of water at room temperature. This low solubility means the solution quickly reaches its saturation point, which is the maximum concentration of dissolved substance the water can hold. At saturation, a dynamic equilibrium is established where the rate of dissolution equals the rate at which dissolved ions precipitate back into solid form.
The anhydrous form of calcium sulfate, known as anhydrite (\(text{CaSO}_4\)), exhibits a slightly higher solubility than gypsum, dissolving at around \(2.4\) grams per liter. When the salt dissolves, it dissociates into calcium and sulfate ions. The low value of its solubility product constant (\(text{K}_{sp}\)) confirms that only a tiny fraction of the solid exists in the dissolved ionic form at any time.
Factors that Influence Dissolving
The solubility of calcium sulfate is affected by external variables, particularly temperature. Unlike most solids, whose solubility increases with rising water temperature, calcium sulfate exhibits inverse or retrograde solubility. This means that above a certain point, the salt becomes less able to dissolve as the water temperature increases.
Maximum solubility occurs at a relatively low temperature, typically between \(30^{circ}text{C}\) and \(40^{circ}text{C}\). This inverse relationship is attributed to the exothermic nature of the dissolution process, which releases heat into the solution. According to Le Chatelier’s principle, increasing the temperature shifts the equilibrium toward the reverse reaction, causing less solid to dissolve.
The Common Ion Effect further suppresses calcium sulfate’s limited dissolving ability. This effect occurs when a compound containing one of the salt’s ions, such as sodium sulfate (\(text{Na}_2text{SO}_4\)) or calcium chloride (\(text{CaCl}_2\)), is already present. The introduction of a common ion causes the dissolution equilibrium to shift back toward the solid form. This results in a decrease in solubility, causing more calcium sulfate to precipitate.
Where Calcium Sulfate Solubility Matters
The limited solubility of calcium sulfate affects both industrial and natural settings. In water systems, dissolved \(text{CaSO}_4\) contributes to permanent water hardness that cannot be removed by boiling. When this hard water is heated, the retrograde solubility causes calcium sulfate to precipitate, forming a hard, crystalline scale on surfaces that leads to clogging and reduced efficiency.
Low solubility is utilized in the construction industry, particularly with gypsum board and Plaster of Paris. Plaster of Paris (\(text{CaSO}_4 cdot 1/2text{H}_2text{O}\)) is created by heating gypsum, and when water is added back, it quickly rehydrates to form the dihydrate, gypsum, which sets into a solid mass. The resulting product’s low solubility ensures that finished walls and casts do not readily dissolve when exposed to moisture.
In agriculture, the sparingly soluble nature of gypsum makes it a useful soil amendment. It improves the physical structure of dense clay soils by promoting the aggregation of clay particles, enhancing water infiltration and drainage. The low solubility ensures that calcium and sulfur nutrients are released slowly over time, relying on rainfall or irrigation to gradually dissolve the gypsum.