2 Ice floats

When water freezes, the molecules lock into a regular hexagonal lattice in which each molecule makes four hydrogen bonds at fixed angles. This arrangement leaves large empty spaces, so ice molecules are spaced farther apart than in liquid water: density of ice ≈ 917 kg/m³ (about 9 % less than water), volume increases by about 9 % on freezing. Therefore ice floats, with about 90 % of its volume below the surface (an iceberg). (Frozen water pipes burst and rocks crack for the same reason.)