Which of the following is NOT a cause of water hardness?

The correct choice identifies phosphate salts as not being a cause of water hardness. Water hardness primarily arises from the presence of specific dissolved minerals, mainly calcium and magnesium salts. These minerals contribute to water hardness because, when they dissolve in water, they form calcium and magnesium ions which directly relate to the hardness of the water.

Bicarbonate salts also contribute to water hardness as they can dissociate into calcium and magnesium ions when they react with other substances. However, phosphate salts do not contribute to water hardness in the same manner. Instead, phosphates are more commonly associated with issues such as nutrient pollution and can lead to algal blooms in water bodies, but they do not add to the hardness level of water.

Understanding water hardness is important for various applications, including maintaining the efficiency of plumbing systems and water treatment processes, which is why distinguishing the contributing factors is essential for effective water management.