What primarily causes water hardness?

Water hardness is primarily caused by the presence of polyvalent metallic cations, predominantly calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These cations contribute to the formation of insoluble precipitates and scale when heated or combined with soap, which is the basis for describing water as "hard."

When water moves through soil and rock, it can dissolve minerals containing these metallic cations, thereby increasing the hardness of the water. Unlike organic compounds, which can affect water quality in different ways, and hydroxide ions or carbonic acid levels that relate more to pH and alkalinity, the direct presence of these polyvalent cations is the key factor defining water hardness. Thus, the focus on metal ions provides a clear understanding of what drives this characteristic of water.