Which method is not suitable for softening water?

Adding lime is not considered a suitable method for softening water in the context of water treatment. While the addition of lime (calcium hydroxide) is effective for water treatment processes like removing hardness, it mainly precipitates calcium carbonate from hard water rather than effectively softening it. This method is more suited for reducing the calcium and magnesium levels in the water rather than providing a true softening effect.

In contrast, the other methods provided in the choices can actively soften water. For example, using sodium bicarbonate can precipitate some hardness but is primarily used to manage alkalinity. A water softener typically employs ion exchange to remove calcium and magnesium ions from the water, which is a recognized and effective method of softening water. Boiling can help in the temporary softening of water by precipitating calcium carbonate when the water cools after boiling.

In summary, while lime is beneficial in treating water quality issues, it does not serve as a practical method for achieving the softening of water in the way the other options do.