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Anomalous expansion of water
Intermediate
Water is densest at 4 °C.
From 0 to 4 °C water contracts on heating; density peaks at 4 °C. As a result ice floats and lakes freeze from the top, protecting aquatic life.
A pond at 4 °C at the bottom under ice.
Most liquids don't behave like this.
In more detail
Water contracts when cooled from room temperature to about 4 degrees Celsius, and then expands as it cools further towards freezing. This is why its density is greatest at 4 degrees.
Real-world uses
- Lakes freeze from the top, because ice floats on the denser water below.
- Pipes can burst in winter when water expands on freezing.
- Lakes do not freeze from the bottom up because water is densest at 4 °C, so the cold water sinks and ice forms at the top.
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Frequently asked questions
Why does ice float?
Because ice is less dense than liquid water, due to its open crystal structure.
Why does this matter for aquatic life?
It keeps deep water above 4 degrees, so fish survive under ice.
Why does water expand when it freezes?
Ice has an open hydrogen-bonded structure that takes up more space than liquid water, which is why pipes can burst.
Related topics in Heat & Thermodynamics
Zeroth law · Thermal expansion · Area and volume expansion · Heat · Specific heat capacity · Molar heat capacity