Which intermolecular forces in h2o make ice less dense than liquid water: hydrogen bonding or dipole-dipole?
Hydrogen bonding make ice less dense than liquid water.
The solid form of most substances is denser than the liquid phase, thus, a block of most solids will sink in the liquid. But, when we are talking about water something else happens. That is anomaly of water.
The anomalous properties of water are those where the behavior of liquid water is quite different from what is found with other liquids. Frozen water or ice shows anomalies when compared with other solids.
A block of ice floats in liquid water because ice is less dense. Upon freezing, the density of water decreases by about 9%.