D2O has higher m.p. (3.8 degrees C), b.p. (101.4 degrees C), and density (1.107 g/$cm^3$) than H2O. Prepared by repeated electrolysis of water (H2O is electrolysed preferentially, D2O concentrates). Primary use: moderator in nuclear reactors (slows neutrons without absorbing them). Also used as tracer in reaction mechanism studies. D2O reacts like H2O but more slowly (kinetic isotope effect — D-O bond is stronger than H-O bond). CaC2 + 2D2O → C2D2 + Ca(OD)2 (acetylene-d2). Living organisms cannot survive in pure D2O — biochemical reactions are significantly slower.