How do calcium salts in hard water reduce soap’s cleaning ability?

Calcium salts in hard water reduce the cleaning ability of soap through a chemical reaction that renders the soap ineffective. When soap is introduced to hard water, the calcium and magnesium ions present react with the soap’s higher fatty acids.1

This reaction produces an insoluble, gelatinous curd—often observed as a scum or precipitate—rather than the desired cleaning lather. Because the soap is consumed to form this precipitate, it is no longer available to act as a surface-active agent to disperse soil or reduce surface tension, leading to a significant waste of the product. This process also results in undesirable side effects, such as a dull appearance on hair, yellow spots on laundry, and a sticky residue on surfaces like glassware and bathtubs.123