Which chemical is commonly used to soften water through precipitation?

Calcium hydroxide is frequently used in water treatment processes, particularly for water softening through precipitation. When calcium hydroxide is added to water containing hardness ions, such as calcium and magnesium, a reaction occurs. The calcium hydroxide reacts with these ions, causing them to precipitate out of solution in the form of insoluble compounds, which can then be removed from the water.

The precipitation process primarily involves the formation of calcium carbonate when bicarbonate ions are present. This not only reduces the hardness of the water but also helps in maintaining a balanced pH. The effectiveness of calcium hydroxide in reducing water hardness makes it a preferred choice in many water treatment facilities, particularly in areas facing issues with hard water.

Other choices, such as calcium carbonate, sodium bicarbonate, and magnesium sulfate, do not play a similar role in softening water through precipitation. Calcium carbonate is generally used to control pH and hardness but does not effectively precipitate hardness ions in the same manner. Sodium bicarbonate can raise the alkalinity and pH, while magnesium sulfate is more associated with adding hardness rather than removing it. Thus, calcium hydroxide stands out as the primary agent used in this specific water softening process.