What is precipitated when magnesium hardness is treated with lime?

• A. Calcium hydroxide
• B. Magnesium hydroxide
• C. Calcium carbonate
• D. Copper sulfate

Answer: (B)

When magnesium hardness in water is treated with lime (calcium hydroxide), magnesium hydroxide is precipitated as the primary product. The chemical reaction involves the addition of lime, which raises the pH of the water and allows magnesium ions (Mg²⁺) to react with hydroxide ions (OH⁻) from the lime, resulting in the formation of magnesium hydroxide, an insoluble precipitate. This process is significant because it helps to soften water by removing the hardness caused by magnesium ions.

Calcium hydroxide, although it is the reagent used in the treatment process, does not form a precipitate itself as magnesium hydroxide does. Calcium carbonate can form in certain conditions when calcium ions react with carbonate, but that is not the primary reaction in the treatment of magnesium hardness. Copper sulfate is unrelated to this context as it does not play a role in the precipitation of magnesium hardness when lime is added.