Why is heavy water denser than ordinary water?

Heavy water is denser than ordinary water because it is composed of deuterium, an isotope of hydrogen that has a mass double that of ordinary hydrogen.12

While ordinary water ($H_{2}O$) has a molecular weight of approximately 18, heavy water ($D_{2}O$) has a molecular weight of about 20. This difference arises because each deuterium atom contains one proton and one neutron in its nucleus, whereas the nucleus of an ordinary hydrogen atom consists of only a single proton. Consequently, the increased mass of the constituent atoms results in a higher density for the resulting water molecule.123