Why balloons shrink when they get cold

The reason than hot air balloons float and chimneys suck the smoke out of a room, is that when you heat up a gas it gets bigger. So why is that?

Here you can see the air inside a balloon warming up after being put in liquid nitrogen:

The gas in the bottom
of the balloon is at
about -200°C
A bit warmer At about room temp

Now everything is made up of really little things called atoms, and these often go around in groups called molecules. Now in gases these molecules are normally quite small and roughly blob shaped, and these molecules don't stick together.

This means that they are wandering around the place bumping into each other the walls and whatever else gets in their way. If you run around and bounce off someone else you will push them back. This is why a balloon stays big and streched and doesn't collapse - there are loads of gas molecules bouncing into it pushing it out.

Have a play with this model of some molecules in a very small balloon. You can change the temperature by dragging the arrow at the right up and down

On a small scale temperature is just how much energy each molecule has - so it it is hot the molecules have loads of energy and bounce off things really hard pushing them back. If it is cold they have less energy so don't push as hard.

So when a gas is heated it expands and when it is cooled it shrinks.

David Ansell