Which of the following is the principal scale-forming substance in water?

The principal scale-forming substance in water is calcium carbonate. This compound is a common constituent of hard water, where calcium ions combine with carbonate ions. The presence of calcium carbonate can lead to scaling in pipes and on heating elements, which is a significant concern in water treatment and distribution systems.

Calcium carbonate can precipitate out of solution under conditions such as increased temperature, concentration, or changes in pH, leading to the formation of scale. This phenomenon is particularly prevalent in areas with hard water, where the concentration of calcium and carbonate ions is high.

While calcium itself is a necessary element that contributes to hardness, and other compounds such as zinc orthophosphate and sodium carbonate have roles in water treatment, they do not primarily cause scaling. Instead, they are often used to prevent or control scale formation by sequestering calcium or altering water chemistry. Thus, calcium carbonate is specifically recognized as the main substance responsible for scaling in water systems.