Which compound would NOT contribute to water hardness?

Water hardness is primarily caused by the presence of dissolved minerals, particularly divalent metal ions such as calcium (Ca²⁺) and magnesium (Mg²⁺). When these ions are present in significant concentrations, they lead to water hardness, which affects the quality of water for domestic and industrial use.

Sodium chloride, commonly known as table salt, does not contribute to water hardness. Sodium ions (Na⁺) are monovalent and do not have the same effect on water hardness as divalent ions like calcium and magnesium. Thus, sodium chloride, when dissolved in water, does not introduce hardness because it does not increase the concentration of calcium or magnesium ions.

In contrast, compounds like calcium sulfate, magnesium sulfate, and calcium chloride contain either calcium or magnesium and therefore contribute to overall water hardness. They release these ions into the water upon dissolution, which increases the hardness level. Understanding the role of different ions in water quality is crucial for effective water management, and recognizing which substances do not contribute to hardness is important for maintaining desirable water characteristics.