Heavy water is water that contains the heavy isotope of hydrogen called deuterium in place of ordinary hydrogen. The deuterium atom has a mass about twice as great as an ordinary hydrogen atom. Heavy water is also called deuterium oxide. Heavy water is useful in some kinds of nuclear reactors called heavy water reactors.
Because of the difference between the masses of the two kinds of hydrogen atoms, the physical properties of heavy water differ from those of ordinary water. Heavy water freezes at 3.82 °C (38.88 °F), rather than at 0 °C (32 °F). It boils at 101.42 °C (214.56 °F), rather than at 100 °C (212 °F).
Chemical reactions occur more slowly in heavy water than in ordinary water. As a result, seeds cannot germinate in heavy water, and animals cannot live with it in their body fluids.
Heavy water was first separated from ordinary water in 1932 by Gilbert N. Lewis, a chemist at the University of California.
