$(a)$ Soaps cannot be used with hard water. Why?
$(b)$ Give the chemical composition of detergents.
$(c)$ Give one advantage of detergents over soaps.

(N/A) Hard water contains calcium $(Ca^{2+})$ and magnesium $(Mg^{2+})$ salts. These ions react with soap molecules to form an insoluble substance known as scum,which prevents the soap from forming a lather.
$(b)$ Detergents are generally the ammonium or sulphonate salts of long-chain carboxylic acids.
$(c)$ The main advantage is that detergents do not form insoluble precipitates (scum) with the calcium and magnesium ions present in hard water,allowing them to remain effective for cleaning even in hard water.