The temperature at which water is most dense is 3.98 degrees Celsius (39.164 degrees Fahrenheit, 277.13 Kelvin). At this temperature water’s density is about 999.9720 kilograms per cubic meter. At room temperature (20 degrees Celsius) water has a density of 998.2071 kilograms per cubic meter. Near boiling, water reaches a density as low as 958.4 kilograms per cubic meter (and even lower into the gaseous phase).
For almost all compounds, the solid phase is denser than the liquid phase. In fact, the colder a substance becomes the higher its density will become. A block of a substance in the solid phase will always sink in container of that substance in liquid phase. Water, on the other hand, does not follow this simple property. Instead, the solid phase of water (ice) floats on the liquid phase. Water’s density increases as it become colder until 3.98 degrees Celsius, which is just short of freezing. At this point, decreasing the temperature further will begin to decrease water’s density as it transitions to ice and then further into the solid phase.
The molecular arrangement of water in the ice phase creates this expansion and lowering of density. In liquid water, the disordered molecules are arranged in a mesh-work of hydrogen bonds based on the electrostatic interactions between the oxygens and hydrogens. Upon freezing, water molecules form a lattice that is more organized and structured than in water. This structured arrangement actually takes more space and decreases the density.
This interesting property of water has huge implications for many of the Earth’s ecosystems. If ice sank in liquid water then rivers and lakes would freeze from the bottom up. This would cause many liquid environments to freeze solid during prolonged periods of cold. These systems could not thaw again in warmer periods, because it would have to melt from the top down, probably leaving large ice blocks at the bottom of many lakes. Many organisms, specifically fish, survive freezing and thawing every year because of the pocket of water under the ice on most ponds and lakes.
Introducing salt into the system has further dramatic changes. Salt lowers the freezing temperature of water, which means that the highest density also occurs at a lower temperature. In salt water bodies, like oceans, this further allows organisms to survive without freezing (though they are surviving in water that is nearly 4 degrees Celsius colder).