Question

Why soap does not work in hard water?

Answer 1

Hard water contains $C{a}^{2+}$ and $M{g}^{2+}$ ions,so when soap is dissolved inn hardwater these ions displace sodium or potasium from their salts and form insoluble $C{a}^{2+};M{g}^{2+}$ salts and they seperate as scum thats why soap does not work in hard water.