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

Water hardness is primarily caused by the presence of certain metal ions in water, specifically calcium and magnesium ions. Compounds that dissociate in water to release these ions will contribute to the overall hardness of the water.

Calcium sulfate, magnesium sulfate, and calcium chloride all contain either calcium or magnesium, which when dissolved in water release ions that contribute to hardness. Calcium sulfate releases calcium ions, magnesium sulfate releases magnesium ions, and calcium chloride releases calcium ions; all of these ions increase water hardness.

On the other hand, sodium chloride, which is common table salt, does not contribute to water hardness as it dissociates into sodium and chloride ions. Neither of these ions is responsible for increasing hardness in water. Therefore, sodium chloride is the compound that would not contribute to water hardness. This makes it the correct choice in this scenario.