科普习作：

Because of the segmental specific-heat disparity of the hydrogen bond (O:H-O) and the Coulomb repulsion between oxygen ions, cooling elongates the O:H-O bond at freezing by stretching its containing angle and shortening the H-O bond with an association of larger O:H elongation, which makes ice less dense than water, allowing it to float.

Hitorical background:

Galileo On Ice _ August 26, 2013 Issue.pdf

Ref:

1) Hidden force floating ice, http://arxiv.org/abs/1501.04171

2) Density and phonon-stiffness anomalies of water and ice in the full temperature range. J Phys Chem Lett, 2013. 4: 3238-3244.;

3) Hydrogen-bond relaxation dynamics: resolving mysteries of water ice. Coord. Chem. Rev., 2015. 285: 109-165