Lime can only be used to soften water that contains what?

Lime is used in water treatment processes primarily to soften water by removing hardness caused by calcium and magnesium ions. The correct answer highlights that lime can effectively soften water that contains carbonate. When lime (calcium hydroxide) is added to water with bicarbonate and carbonate ions, it reacts to precipitate calcium carbonate, which helps in reducing water hardness. This process is part of the carbonate hardness removal, as carbonate ions can lead to scale formation and other issues in water systems.

Water that contains carbonate ions is specifically suited for treatment with lime because the lime will react with these carbonate ions to form insoluble precipitates, enhancing the softening effect. This is why the presence of carbonate is crucial for the effectiveness of lime softening processes. In contrast, water that contains heavy metals, organic matter, or simply calcium carbonate would not utilize lime in the same beneficial way for softening purposes. Heavy metals require different treatment methods for removal, organic matter can complicate the treatment process, and while calcium carbonate itself is what lime targets, simply stating "calcium carbonate" does not encompass the broader context of the softening action involving carbonate ions.