Hard water contains soluble salts of calcium and magnesium.
It does not give lather with soap. This is because of the precipitation of the insoluble soap.
To check the poisoning of water in lead pipes, hard water is used. when hard water flows inside the lead pipe, some lead salts dissolved in the water to make it poisonous calcium sulphate, present in hard water forms insoluble lead sulphate in the form of a layer inside the lead pipe.
Hard water is used to check the Poisoning of water in lead pipes