Most substances expand when heated. But for the volume of water, when it is heated at
the volume decreases until .
And above it behaves normally, expanding when heated.
So, a given amount of water has least volume at meaning
that the volume of water is most dense at .
This fact has a great affect on the aquatic organisms of a lake.
During winter the surface of the lake cools towards .
This makes the surface water sink because it is denser than the water below it.
This process of cooling and sinking repeats until the entire lake turns into
When that happens the surface will cool down to and
form ice. Since ice is less dense than water, it will float on top insulating the lake from further cooling.
This makes aquatic life survive the winter.