Thawing a frozen balloon

The pressure, volume and temperature of a gas are all linked so it makes sense to keep one of them constant when investigating the interdependence of the other two. But we didn’t do that when we watched the effect of liquid nitrogen on a partially-inflated balloon – as shown in the animation below.

Before viewing the animation, we need to be clear about what is happening – and to do that we use the kinetic theory of gases. This states that the particles in a gas move faster at higher temperatures. (The full theory recognises that the particles have a range of speeds but we can assume, for simplicity, that the particles all have the same speed.)

When the conical flask was floated on liquid nitrogen inside the Dewar, the air particles lost energy and therefore moved more slowly. This in turn reduced the rate of collisions between the air particles and the inside surfaces of the flask and balloon. The drop in speed also meant that the air particles had less momentum, so not only did collisions happen less frequently but also each collision exerted a smaller force. The overall result was a drop in pressure that caused the balloon to deflate. The air pressure outside the balloon remained constant and forced the balloon to invert then “re-inflate” inside the conical flask.

That state provided the starting point for the animation shown below, which comprises 35 photographs that were taken at approximately 15 s intervals over a 10 minute period when the conical flask was removed from the Dewar.

It is clear that there is a major change in the first few frames of the animation as the temperature climbs very quickly from -196 degrees Celsius (the boiling point of nitrogen) towards room temperature.

When the air particles gain energy, they push the balloon out of the conical flask. At first, there is no change in pressure because the flexible balloon surface is able to move and is resisted only by atmospheric pressure, which has a constant value. The initial effect of increasing the temperature is therefore only to increase the volume of the gas.

Further warming causes the volume to increase to such an extent that the balloon loses its flexibility and the pressure starts to rise, causing the balloon to re-inflate. The transition from constant pressure to increasing pressure is marked by the balloon “falling over” before inflation begins.

Throughout this process, everything can be explained by using the kinetic theory of gases and remembering that the temperature of a gas is directly linked to the speed(s) at which the gas particles are moving. Hotter gases have faster moving particles that collide with interior surfaces more often, and with greater force.

You may wish to revisit the online simulation that we used to explore the behaviour of gases: you can access the animation at this link http://falstad.com/gas/.