What characterizes "hard water"?

Hard water is characterized primarily by high concentrations of calcium and magnesium ions. These minerals are typically dissolved in the water as it moves through soil and rock, where they originate from the weathering of minerals containing these elements. The presence of these ions contributes to the hardness of water, which can lead to various impacts, such as scale buildup in pipes and appliances, and can affect the efficiency of soaps and detergents.

The other options do not define hard water. For instance, high concentrations of sulfate ions or chlorine do not relate to water hardness—sulfates and chlorine are different compounds with distinct effects on water quality. Similarly, low levels of dissolved solids pertain to soft water rather than hard water, indicating that hard water contains a greater concentration of dissolved minerals. Therefore, the defining factor of hard water is indeed the presence of elevated levels of calcium and magnesium ions.