What is the only physical difference between heavy water and pure water?

Heavy water contains deuterium instead of common hydrogen-1. In its pure form it has a density about 11% greater than water, but otherwise, it is physically and chemically similar. It is not radioactive. Because of greater density and stronger hydrogen-oxygen bonds, its behaviour is slightly different in some biochemical reactions. It may be toxic, but only when a large fraction of water (> 50%) in higher organisms is replaced by heavy water.