Which of the following is a common method for softening hard water?

Distillation stands out as a common method for softening hard water because it effectively removes dissolved minerals, including calcium and magnesium ions, which contribute to water hardness. The process involves heating the water until it vaporizes, then cooling the vapor to collect it as liquid. During this transition, the unwanted minerals are left behind, resulting in softened water.

While other methods can also modify water quality, they function differently. Reverse osmosis is another commonly used technique to reduce hardness but involves a semi-permeable membrane rather than a phase change like distillation. Additives can include chemicals that bind to hardness-causing minerals, but this is more of a temporary solution rather than a purification method. Filtration may help remove some particulates and larger impurities, but it typically does not effectively address dissolved minerals that cause hardness. Thus, distillation is a straightforward and effective approach for achieving soft water through the removal of those dissolved solids.