Below 4°C (from 4°C to 0°C) water will undergo expansion rather than compression. Hence the volume of water gets increased and density is decreased. Above 4°C water molecules absorb energy from external source get expansion with increased volume. Therefore, density of water decreases. But at exactly 4°C, intermolecular hydrogen bonds in water get break down and thereby volume decreased. Hence, the density of water is maximum at 4°C.