Which of the following compounds contributes to water hardness?

Water hardness primarily results from the presence of dissolved minerals, particularly calcium and magnesium ions. Calcium sulfate is one such compound that contributes to water hardness because it dissociates in water to release calcium ions. These calcium ions are one of the key factors that define water hardness.

Hard water can lead to various issues, such as scale buildup in pipes and appliances, which can affect their efficiency and lifespan. It also interacts with soaps and detergents, making them less effective. By understanding the role of compounds like calcium sulfate in contributing to water hardness, one can better manage water quality and address issues associated with hard water.

While sodium chloride, potassium nitrate, and chlorine are important chemicals used in various contexts (such as water treatment or food preservation), they do not significantly contribute to the hardness of water as they do not release calcium or magnesium ions when dissolved.