What two substances' concentrations are measured to determine water hardness?

The concentrations of calcium and magnesium are measured to determine water hardness because these two cations are the primary contributors to the hardness of water. Hardness is typically defined as the concentration of divalent metal ions in water, which are primarily calcium (Ca²⁺) and magnesium (Mg²⁺). When water comes into contact with certain rocks and minerals, it can dissolve these ions, thereby increasing its hardness.

Calcium is often abundant in limestone and other mineral deposits, while magnesium can be found in various geological formations as well. A higher concentration of these ions in water indicates greater hardness, which can affect water quality, scale formation in pipes, and the effectiveness of soaps and detergents.

Sodium and potassium do not contribute significantly to water hardness and are instead related to different aspects of water chemistry, such as salinity and alkalinity. Therefore, focusing on the levels of calcium and magnesium provides a clear and accurate measure of water hardness.