Normally, calcium and magnesium ions are expressed as what when dealing with softening?

When discussing water softening, calcium and magnesium ions are typically expressed in terms of their contribution to water hardness, which is often represented as calcium carbonate (CaCO3). This is because calcium and magnesium, when present in water, combine with bicarbonate, sulfate, and chloride ions, ultimately influencing the total hardness of the water.

In softening processes, the goal is to remove or reduce these hardness ions, allowing the water to become "soft." Calcium carbonate is a common standard used to quantify hardness because it reflects the precipitating chemicals formed when calcium and magnesium react with carbonate ions in the water. It serves as a useful way to convert concentrations of calcium and magnesium into a single measure that is meaningful in terms of the potential impact on scaling and other issues associated with hard water.

There are other compounds listed that include calcium or magnesium, but they do not serve the same purpose in this context of expressing hardness or reactiveness in water treatment processes. CaCO3 is the most relevant compound for representing the total hardness that is targeted during softening processes.