Cooling Curves

Imagine we boiled some water so that all of the water particles turned into water vapour (steam) and then we put that steam into a very, very cold room (let's say the temperature of the room was -20^oC - chilly!). Over time, we know that the temperature of the gas would start to cool down as heat energy was lost to the room.

Below, is a picture of how the temperature of the steam (to start) would change as the time goes on.

1. As the steam starts to cool down over time, it particles lose their kinetic energy to the surroundings, so its temperature drops. This is seen by the first fall on the curve.
2. When the temperature of the steam drops to 100^oC, this is the condensation point of water, so the steam starts changing state and condenses into a liquid. But notice something VERY strange. Whilst it is changing state over time from a gas (steam) to a liquid, its temperature stays the same! It only starts dropping again when ALL the steam has condensed into liquid water. It stays the same during the change of state because the kinetic energy of the particles stays the same (temperature is a measure of kinetic energy). During the change of state, chemical bonds change and their is a change in potential energy, but this does not affect the temperature.
3. Then you get another drop, as the liquid water starts cooling down over time. This happens because, like before, the kinetic energy of the liquid water is being released to the surroundings and so the liquid temperature drops.
4. Then it reaches 0^oC, which you know is the freezing point of water, and at this point the liquid water starts to freeze and turn into ice (solid water!). Once again, the temperature stays constant whilst it is doing the freezing. This is because, like before, the kinetic energy of the particles stays the same during the change of state and only rearrangement on bonds and a change in potential energy takes place.