How do household water softeners operate in treatment mode?

Household water softeners operate by removing hardness ions, such as calcium and magnesium, from the water and replacing them with sodium ions. This process is known as ion exchange. In treatment mode, the softener uses resin beads that are charged with sodium ions. As hard water passes through the resin, the hardness ions are attracted to the beads and are exchanged for sodium ions, thereby softening the water. This effectively reduces the hardness level, which can help prevent scale buildup on plumbing fixtures and improve the effectiveness of soaps and detergents.

The other options do not accurately describe the primary operation of a water softener during treatment mode. Filtering out hardness refers to a different method of water treatment that does not involve ion exchange. Exchanging hardness for salt is somewhat misleading; the process is not about replacing hardness with salt directly but rather about using sodium ions from salt to replace hardness ions. Therefore, the first choice correctly captures the fundamental principle of how water softeners function in treatment mode.