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

Sodium chloride does not contribute to water hardness because it does not contain calcium or magnesium ions, which are the primary contributors to hardness in water. Water hardness is typically defined by the presence of dissolved minerals, primarily calcium (Ca²⁺) and magnesium (Mg²⁺) ions.

Calcium sulfate, magnesium sulfate, and calcium chloride all release calcium and/or magnesium ions into the water when they dissolve, leading to an increase in hardness levels. Sodium chloride, commonly known as table salt, dissolves to produce sodium (Na⁺) and chloride (Cl⁻) ions, neither of which affect the hardness of the water. Thus, it is the absence of these hardness-causing ions that makes sodium chloride a compound that does not contribute to water hardness.