Heavy water

Heavy water (deuterium oxide, 2
H
2O, D
2O) is a form of water that contains only 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.[4] The presence of the heavier hydrogen isotope gives the water different nuclear properties, and the increase of mass gives it slightly different physical and chemical properties when compared to normal water.