Hardness values in water are commonly expressed as equivalent to which compound?

Hardness values in water are primarily associated with the presence of calcium and magnesium ions. The correct answer, calcium carbonate, is often used as a standard reference when expressing water hardness because it represents the combined effects of both calcium and magnesium ions in the water.

When hardness is measured, particularly in terms of its concentration, it is typically reported in milligrams per liter (mg/L) as calcium carbonate. This is done because calcium carbonate is a well-known compound and gives a clear indication of the amount of hardness attributable to these hardness-causing ions. In practical applications, using calcium carbonate allows for easier communication of hardness levels and enables standardized comparisons across different water sources and studies.

Other compounds listed, such as calcium sulfate, magnesium chloride, and sodium bicarbonate, may contain the ions that contribute to hardness but do not provide the same universal basis for expressing hardness values as calcium carbonate does. Therefore, calcium carbonate is the conventionally accepted compound for expressing hardness in water analysis.