The excess lime-softening process is used so that which compound will precipitate out of the water?

The excess lime-softening process is used so that which compound will precipitate out of the water?

• A. calcium nitrate
• B. calcium chloride
• C. magnesium hydroxide
• D. magnesium sulfate

Answer: (C)

Raising the pH with lime causes certain ions to form insoluble compounds that can be removed as a solid. In excess lime-softening, the pH becomes high enough for magnesium to react with hydroxide ions and precipitate as magnesium hydroxide: Mg2+ + 2 OH− → Mg(OH)2(s). This solid can settle out of the water, taking magnesium hardness with it. The other options stay dissolved under these conditions: calcium nitrate and calcium chloride are highly soluble salts, and magnesium sulfate remains soluble at the pH range used. So the precipitating solid formed specifically due to the high pH from excess lime is magnesium hydroxide.

Raising the pH with lime causes certain ions to form insoluble compounds that can be removed as a solid. In excess lime-softening, the pH becomes high enough for magnesium to react with hydroxide ions and precipitate as magnesium hydroxide: Mg2+ + 2 OH− → Mg(OH)2(s). This solid can settle out of the water, taking magnesium hardness with it.

The other options stay dissolved under these conditions: calcium nitrate and calcium chloride are highly soluble salts, and magnesium sulfate remains soluble at the pH range used. So the precipitating solid formed specifically due to the high pH from excess lime is magnesium hydroxide.