What does hard water contain that contributes to the creation of soap scum?

Hard water contains a high proportion of minerals and salts, primarily calcium and magnesium ions, which contribute to the formation of soap scum. When soap is used in hard water, the calcium and magnesium react with the fatty acids in the soap to form insoluble precipitates, commonly known as soap scum. This scum is often visible as a filmy residue that can build up in sinks, tubs, and on other surfaces.

In contrast, soft water, which has a low mineral content, allows soap to lather and clean effectively without forming these insoluble compounds. High pH levels may impact soap performance, but the primary concern with hard water is its mineral content. Chlorine is used for disinfection and does not contribute to soap scum formation. Therefore, the presence of a high proportion of minerals and salts in hard water is the correct reason for soap scum development.