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States of Matter
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Temperature Changes
Most substances expand as they heat and contract as they cool. Recall, all matter is made of atoms. In a solid, the atoms vibrate next to each other, atoms in a liquid are jostling around, and atoms in a gas are whizzing past each other. As heat is introduced, the atoms begin to move around more due to the increase of kinetic energy.
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature. As the temperature of a substance increases, the molecules move faster and the substance expands.
You may have seen expansion joints (look at the picture below) on the sections of bridges and wondered why they look like zippers. These allow for the expansion and contraction of the bridge when it changes temperature. Without them, the stress of getting bigger and smaller would crack and destroy the bridge!
To account for thermal expansion, expansion joints like the one above are included on bridges and highways.
Anomalous Expansion of Water
In general, objects will expand with increasing temperature. However, some materials will contract upon heating and expand upon freezing. Water has this special property and it's called the anomalous expansion of water.
What happens to the density of water as the temperature changes?
- At temperatures greater than 4°C (40°F) water expands with increasing temperature (so its density decreases).
- However, it expands with decreasing temperature when it is between +4°C and 0°C (40°F to 32°F). Water is most dense at +4°C.
