Experimenting with gases and liquid nitrogen

This week Derek is with Dr Chris Muirhead from Birmingham University and student volunteer Mary from Hills Road Sixth Form College in Cambridge. They'll be using liquid nitrogen, which is very cold and can be very dangerous. Chris has special liquid nitrogen safety training, and you should NOT do this experiment at home.

To do this experiment, we used:

Balloon filled with helium
Balloon filled with air
Vat of liquid nitrogen

How we did the experiment:

1 - Chris took the balloon filled with helium and put it into the liquid nitrogen.

2 - We observed what happened and took the balloon out again, watching how the balloon changed.

3 - Chris the repeated the experiment with an ordinary air-filled balloon.

What's going on?

Firstly, we need to understand what liquid nitrogen is. 80% of the atmosphere is composed of nitrogen gas, and liquid nitrogen is this same gas cooled down to a temperature of -196 degrees Celsius. It then turns from a gas into a liquid.

When a balloon full of helium is lowered into the liquid nitrogen, it gets smaller. In fact, it shrinks to about a quarter of its original volume. Why is this?

Balloons are made of rubber, and rubber likes to contract. You can easily see this by stretching an elastic band - if you stretch it out and let go, it will contract back to its original size. The reason a balloon full of air doesn't contract is because it's being maintained at the larger volume by the pressure of the air inside. When you cool the gas down by plunging the balloon into liquid nitrogen, the atoms of gas inside move around less rapidly. This means that they bash against the walls of the balloon less hard and less frequently, and that produces less force on the walls of the rubber. When the force acting on the inside of the balloon decreases, the rubber is able to contract.

When the balloon is removed from the liquid nitrogen, the balloon begins to increase in size and start floating back up towards the ceiling. This is because as the gas warms up again, the atoms start to move around a lot more rapidly, the pressure on the inside of the balloon increases and the rubber is forced to expand.

Repeating the experiment with a balloon filled with air gives a slightly different result. Instead of contracting down to a quarter of the orignal volume, the air-filled balloon becomes much smaller and is pretty much flat. Inside the balloon you will see a small amount of liquid floating around in the bottom. What is it? It turns out that the liquid is liquid air, and herein lies the reason why this balloon became a lot smaller than the helium balloon.

When a gas turns into a liquid, its volume decreases dramatically. In fact, the volume of the liquid air is about a seven hundredth of the volume of the gaseous air. The reason why the same result wasn't seen with the helium balloon is that helium doesn't liquefy until much lower temperatures. In contrast, normal air will turn into a liquid at about the temperature of liquid nitrogen.

It's possible to demonstrate how much more space is needed when molecules exist as a gas by putting liquid nitrogen into an empty Pringles container - it's not long before the lid pops off. This is exactly the same process of gas contraction and expansion seen in the balloons - when the liquid nitrogen evaporates it takes up 700 times its original volume. The reason we use a Pringles tube is that the lid is made of a light plastic and can blow off safely. You must never put liquid nitrogen into a sealed can, as it will explode. And it goes without saying - don't try this at home!

Want to find out more?

The science of the very cold and superconductivity are some of Chris Muirhead's specialities! If you'd like to find out more about this kitchen science experiment or any of his research, then you can contact him through the Birmingham Portfolio Partnership at superconductivity@bham.ac.uk.