Hard Water : It does not produce latter with soap, but forms a scum.
Hard water contains soluble salts of Calcium and Magnesium, Which forms an insoluble precipitable with soap.
Soft Water : It readily yields plenty of latter with soap and is suitale for washing.
Heavy Water : The deuterium isotope of hydrogen is heavier than protium and if protium is replaced by deuterium, the resulting water is called heavy water (D₂O).
It is useful in nuclear reactors. It is usually obtained from ordinary water by electrolysis.