Question

Why does hard water not produce foam with soap easily?

Answer 1

Soaps contain Na & K salt of long chain fatty acids . When they are dissolved in hard water (which contains Ca & Mg ions) ,these ions replace the Na & K from their salt to form insoluble Ca & Mg salts of fatty acids these insoluble salts separate as scum.

$2C_{17}{H}_{35}COONa$ + $CaC{l}_2$ $\to$ 2NaCl + $(C_{17}{H}_{35}COO{)}_{2}Ca$