Troy Fine, age 9, of Colville, Wash., for his question:

WHY IS SALT USED IN A WATER SOFTENER?

    A water softener is a unit of equipment used in removing from water the minerals that make it hard. The principal methods are the lime soda process and the ion exchange process.

    In the ion exchange process, the water filters through materials called zeolites. As the water passes through the filter, the sodium ions in the zeolites are exchanged for the calcium and magnesium ions in the water, and the water is softened. After household softeners become exhausted, a strong solution of salt, or sodium chloride, is passed through the filter to replace the sodium that has been lost.

    In the lime soda process, soda ash and lime are added to the water in amounts determined by chemical tests.