Demonstration of the physical properties of heavy water ice (orange) compared to normal water ice (green). The heavy water ice is added to a beaker of normal water, and sinks to the bottom. Normal water ice floats in normal water. Heavy water is water in which the hydrogen atoms are replaced by atoms of the isotope deuterium, which contains one neutron as well as a proton in its nucleus. This gives it twice the mass of a normal hydrogen atom, and means the molecular mass of heavy water is around 20, rather than 18, atomic mass units. The crystal structure of the ice is almost identical in both types, but the extra mass in the heavy water ensures that the ice is slightly denser than normal liquid water.