Which compound would not contribute to water hardness?

Water hardness is primarily caused by the presence of divalent cations, specifically calcium (Ca²⁺) and magnesium (Mg²⁺) ions. Compounds that contain these ions, such as calcium sulfate, magnesium sulfate, and calcium chloride, contribute directly to water hardness because they dissolve in water, releasing calcium and magnesium ions.

In contrast, sodium chloride does not contribute to water hardness. Sodium chloride is composed of sodium (Na⁺) and chloride (Cl⁻) ions. Neither of these ions is involved in the hardness of water, which is defined by the concentration of calcium and magnesium ions. Therefore, sodium chloride is unrelated to water hardness and is often used in processes such as water softening, where it can help displace calcium and magnesium ions from their compounds.

Understanding the factors that contribute to water hardness can help in managing water quality in various applications, including drinking water, industrial processes, and agriculture.