Explain the formation of scum when hard water is treated with soap.

(N/A) Soap does not work properly in hard water. Soap is a sodium or potassium salt of long-chain fatty acids. Hard water contains dissolved salts of calcium $(Ca^{2+})$ and magnesium $(Mg^{2+})$. When soap is added to hard water,the calcium and magnesium ions present in the water displace the sodium or potassium ions from the soap molecules. This reaction results in the formation of an insoluble white precipitate known as scum. Because of this reaction,a significant amount of soap is wasted before any lather can be formed.