Tetrahedral Geometry

When water freezes at 0°C, its volume increases by about 9%. The packing density of ice at 0°C is 0.34. This means that 66% of the volume is unoccupied because the packing is restricted by the hydrogen-bonding geometrical constraints. Water has the highest density at 4°C, where the packing density is 0.37. As the temperature is lowered, hydrogen bonds break less frequently and become stronger. This causes a decrease in the packing density from 0.37 at 4° to 0.36 in liquid water at 0°C.