4°C (39°F)
Usually when something freezes, it becomes more dense. For water, however, this is not true. Ice floats in water meaning that it is actually less dense than the water. Water actually reaches its densest (999.972kg/m^3) point at 4°C (39°F), just before freezing. The reason for this is the odd 104.45degree angle bend in the water molecule (picture?). This angle means that when the water molecules form the crystal structure of ice, there is a certain amount of space left in between them. It turns out that there is more space left over within the ice crystal than there was in the 4°C () water.
This phenomenon is very important to the survival of freshwater dwelling life forms in parts of the world where lakes and rivers freeze. Above 4°C (39°F), water becomes more dense as it cools. In a lake warm water comes to the surface, is cooled by the cold air above it, and since it is then more dense, sinks down into the depths of the lake in a convection current. If this just continued to happen, then lakes would quickly freeze out. The trick is that below 4°C (39°F), colder water is no longer more dense than warmer, but in fact slightly less dense. This means that a layer of water on the surface will finally freeze, leaving most of the rest of the lake at around 4°C (39°F), with an insulating layer of ice on top. Heat continues to leak out of the lake for the rest of the winter, but unless the lake is very shallow or the winter is exceptionally cold this usually just leads to a thicker layer of ice on top and not to the entire lake freezing.
In saltwater, however, this does not work in quite the same way. When water has salt added to it, its freezing point is actually lowered to about -1.9°C(28.6°F), and the density relationship changes as well. The densest temperature for salt water is actually right about at this freezing point. As ice forms in saltwater, however, the salt is forced out of it leaving frozen and essentially salt-free water. This means that while normal ice floats quite well in the more-dense saltwater, convection currents will continue to flow as long as the air above the water is still cooling it.
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