Which substance, when present in water as a salt, contributes to "hard water"?

The presence of calcium as a salt in water is a significant contributor to what is classified as "hard water." When calcium ions dissolve in water, they form compounds that interfere with the ability of soaps to lather effectively, resulting in the formation of soap scum. Hard water can have various implications for household activities, including increased soap consumption, scaling in pipes and appliances, and challenges in achieving effective cleaning.

Calcium, along with magnesium, typically comes from geological formations and can leach into groundwater supplies. This is why it's common to find hard water in areas with limestone or chalk bedrock. While magnesium and other elements like iron can also contribute to water hardness, calcium is one of the primary culprits specifically associated with the hard water phenomenon. In summary, the presence of calcium ions in water establishes its classification as hard water, making this the correct answer.