Heavy water is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium (2 H or D, also known as heavy hydrogen), rather than the common hydrogen-1 isotope (1 H or H, also called protium) that makes up most of the hydrogen in normal water.[3] The presence of deuterium gives the chemical different nuclear properties, and the increase of mass gives it different physical and chemical properties compared to normal "light water". Heavy Water Wikipedia