What primarily causes hardness in natural water?

Hardness in natural water is primarily attributed to the presence of polyvalent metallic cations, primarily calcium (Ca²⁺) and magnesium (Mg²⁺). These cations can dissolve in water from geological formations, leading to the characteristic hardness of the water.

When water passes through soil and rock formations, it has the potential to dissolve minerals that contain these cations, such as limestone (which contains calcium carbonate), dolomite, and gypsum. The higher the concentration of calcium and magnesium ions, the harder the water becomes. This hardness can affect various applications, such as soap and detergent efficiency, scaling in pipes, and impacts on aquatic life.

While carbonate compounds can contribute to total hardness in the form of carbonate hardness (temporary hardness), they are not the primary cause but rather a means through which the specific cations contribute to overall hardness. Acidic compounds and organic pollutants play different roles in water chemistry but do not directly cause hardness.