Calcium sulfate is not readily soluble in water, although it does dissolve to a limited extent.
While not completely insoluble, calcium sulfate's limited solubility is a significant factor in water hardness and scale formation. This is because when dissolved calcium sulfate concentrations exceed its solubility limit, it precipitates out of solution as a solid, forming scale deposits on surfaces like pipes, boilers, and appliances. This scale buildup reduces efficiency, increases energy consumption, and can ultimately lead to equipment failure.
Here's a breakdown:
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Solubility: Calcium sulfate has a relatively low solubility in water compared to other salts. The exact solubility varies with temperature, but is typically around 2.0 g/L to 2.6 g/L at room temperature (20-25°C).
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Water Hardness: The presence of dissolved calcium (Ca2+) and sulfate (SO42-) ions contributes to the permanent hardness of water, as boiling does not remove them.
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Scale Formation: When water containing dissolved calcium sulfate is heated or evaporated, the concentration of calcium sulfate increases. If it exceeds the solubility limit, calcium sulfate precipitates out as a solid, forming a hard, crusty scale.
Therefore, while calcium sulfate does dissolve in water, its limited solubility makes it a concern in various applications, particularly in water treatment and industrial processes where scale formation can be problematic.
