How is water hardness defined?

Water hardness is defined specifically as the sum of calcium and magnesium ions present in the water. These two elements are the primary contributors to water hardness and are typically measured in milligrams per liter (mg/L) or parts per million (ppm). When water passes through deposits of limestone or chalk, it dissolves these minerals, resulting in increased hardness.

Hard water can pose challenges in various contexts, such as scaling in pipes and appliances and the reduced effectiveness of soap and detergents.

The other options do not accurately represent the concept of water hardness. For instance, the total number of dissolved solids encompasses all dissolved substances and not just calcium and magnesium. The concentration of dissolved oxygen is a critical parameter for aquatic life, particularly for assessing water quality, but it is unrelated to hardness. Lastly, the presence of heavy metals pertains to a different water quality issue, mainly concerning toxicity and pollution, rather than the hardness of water.

Thus, understanding water hardness as specifically related to calcium and magnesium provides clarity on how it impacts water use and treatment processes.