CBSE Class 9 Answered

Water, when in solid state, has dominant hydrogen bonds existing between its molecules which are very strong. In solid state, when the water molecules come close to each other, the molecules are forced to align themselves into a very particular structure - a hexagonal lattice that is the basis for the six-sided snowflakes. Now, since Density = Mass/ Volume so due to the increased bond angles and expanded molecular structure and, this implies greater volume, and hence,  lower density.