What is a common scale forming substance found in water?

Calcium carbonate is commonly found in water and is a significant scale-forming substance. When water that contains dissolved calcium ions and carbonate ions evaporates or is subjected to certain temperature and pressure conditions, calcium carbonate can precipitate out of solution, leading to the formation of scale. This scale can build up in pipes, boilers, and heat exchangers, potentially causing operational issues and reduced efficiency.

In contrast, magnesium sulfate, sodium chloride, and potassium nitrate do not form scale in the same manner as calcium carbonate. While magnesium sulfate and sodium chloride can contribute to the overall mineral content of water, they do not typically precipitate to form hard deposits like calcium carbonate does. Potassium nitrate is soluble and remains in solution; it also does not contribute to scale formation. Therefore, the prominence of calcium carbonate in contributing to scaling problems in water systems makes it the correct choice.