Why does soap form scum in hard water?

Soap forms scum in hard water because of a chemical reaction between the soap and the minerals dissolved in the water. Hard water contains salts of calcium and magnesium.12

When soap is added to this water, the calcium and magnesium ions react with the higher fatty acids found in the soap. This reaction produces an insoluble, gelatinous curd—commonly known as scum—which manifests as a ring around bathtubs, a whitish residue on glassware, or a sticky substance in laundry rinse water.132

This process not only creates an unsightly residue but also results in a waste of soap, as the soap is consumed by the reaction with these metal salts rather than being used for cleaning. In contrast, modern detergents are generally unaffected by these minerals, as the compounds they form remain soluble or dispersed in the solution.123