How is hardness expressed in water analysis?

Hardness in water analysis is commonly expressed as milligrams per liter (mg/L) of calcium carbonate (CaCO3). This is because calcium and magnesium are the two primary cations that contribute to water hardness. When evaluating the hardness of water, using CaCO3 as a standard simplifies comparisons and assessments across different water sources and treatment processes.

Calcium carbonate is a widely understood compound in both chemistry and environmental science, enabling analysts to convey the concentration of hardness-causing elements in a familiar and practical manner. Reporting hardness as mg/L CaCO3 provides clear information about the potential impact on scale formation, plumbing systems, and overall water quality.

The other options represent different substances that do not provide the same context for water hardness. For instance, sodium chloride (NaCl), magnesium oxide (MgO), and potassium chloride (KCl) do not relate to the specific ions that define water hardness in the way that calcium carbonate does. Therefore, the use of CaCO3 as the basis for expressing hardness conforms to industry standards and practices in water analysis.