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
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
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
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 firstname.lastname@example.org.