Which of the following compounds would not contribute to water hardness?

Water hardness primarily refers to the concentration of divalent metal ions in water, particularly calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These ions are typically derived from compounds such as calcium sulfate, magnesium sulfate, and calcium chloride, all of which directly contribute to water hardness.

Sodium chloride, on the other hand, consists of sodium (Na⁺) and chloride (Cl⁻) ions. Sodium is a monovalent cation and does not contribute to the hardness of water because it does not precipitate as insoluble salts that are characteristic of hardness-causing ions. Therefore, the presence of sodium chloride would not increase the hardness of the water.

In summary, while calcium sulfate, magnesium sulfate, and calcium chloride introduce hardness-causing ions, sodium chloride does not influence water hardness, making it the correct choice in this context.