Synthetic detergents are preferred over soap for laundry in hard water because they don't form insoluble precipitates (scum) like soap does, remaining effective in the presence of calcium and magnesium ions. Hard water contains high levels of dissolved minerals, primarily calcium and magnesium ions. When soap (which is a sodium or potassium salt of a fatty acid) reacts with these ions in hard water, it forms insoluble precipitates, which appear as a scum or curd on clothes and in the washbasin. This scum reduces the cleaning power of the soap and can leave a residue on clothes.