What is anomalous expansion of water?

The anomalous expansion of water is an abnormal property of water whereby it expands instead of contracting when the temperature goes from 4^oC to 0^oC, and it becomes less dense. The density becomes less and less as it freezes because molecules of water normally form open crystal structures when in solid form.

Water is denser than ice while almost all the other substances are denser in solid form than in liquid form. The maximum density of water occurs at a temperature of 4^oC and this is caused by the hydrogen bonding between the oxygen atoms that are negatively charged and the hydrogen atoms that are positively charged, from two different molecules of water.