Deuterium can have kinetic isotopic effects different than hydrogen has, and the physical and chemical properties of the deuterium compounds differ from those of hydrogen compounds. For example, D2O (heavy water) is more viscous and heavier than H2O (normal water).
Heavy water is approximately 10% more dense than normal water, enough for the ice formed of heavy water to sink in normal water.
The differences in connection energies and lengths of hydrogen isotope compounds are greater than the isotopic differences for any other element. The bonds between deuterium and tritium are stronger than those with hydrogen, these differences being enough to produce important changes in biological reactions.
Heavy water is toxic to eukaryotic organisms. If 25% of the water in the organisms is replaced by heavy water, problems in cell division and reproduction appear. If 50% of the water is replaced, the eukaryotic organisms die.