Why do synthetic detergents work better than soap in hard water?

Synthetic detergents perform more effectively than soap in hard water because of how they interact with dissolved minerals.

Soap is composed of salts of fatty acids that react with calcium and magnesium ions—the primary components of water hardness—to form an insoluble, gelatinous curd or precipitate. This reaction not only wastes soap but also creates undesirable residues, such as bathtub rings, dullness on hair, and spots on laundry.123

In contrast, synthetic detergents are generally unaffected or only minimally affected by these metal salts. Even if a chemical reaction occurs between a detergent and the minerals in hard water, the resulting compounds remain soluble or stay dispersed in a colloidal form within the solution. Consequently, detergents do not form the problematic precipitates that interfere with cleaning performance in hard water environments.123