What are soaps? Why do they form scum with hard water?

(N/A) Soaps are defined as the sodium or potassium salts of long-chain carboxylic acids (higher fatty acids).
When soap is added to hard water,the calcium $(Ca^{2+})$ and magnesium $(Mg^{2+})$ ions present in the water react with the soap molecules.
This reaction results in the formation of insoluble precipitates known as scum (calcium or magnesium stearate).
Because these insoluble salts do not dissolve,they interfere with the cleaning action of soap and form a sticky residue.