Re: Why does the temperature inside the balloon decreases when popped?

We were doing this experiment, where we were supposed to measure the temperature inside the balloon when it is popped. We stuck this sensor inside the balloon which measures temperature. Our result is that the temperature inside the balloon decreases when popped. Why is this so? Thank you!

Hi Dawn,

Thanks for asking. I taught for 38 years, and I never thought to try this. Neat!

Let me state a couple things that you may already know; but I’ll need to refer to them in my explanation. First, the “temperature” you measured was an indication of the average kinetic energy of the gas molecules striking the sensor inside the balloon. This is essentially caused by how fast they are moving around (like a swarm of bees) inside the balloon and how often they hit the sensor. Second, your referring to a balloon makes me think of an elastic container that was stretched to receive the air and, while inflated, is squeezing the air to be slightly compressed. If you had just filled a plastic bag, I don’t think the same thing would have happened. Does this sound about right?

Now, when the balloon is “popped”, the squeeze is over. The air will expand and fewer molecules will hit the sensor less often. This is what we refer to as “cooler” air. You were lucky enough to be able to see the effect.

The opposite is also true. But if you recorded the temperature go up as you inflated the balloon, people could say that your breath was warm or that a compressor machine caused the heat. So your experiment was better.

On a larger scale, the atmosphere is compressed at the bottom by the weight of the air on top of it … lots of molecules in a liter. Not so at the top of the atmosphere so airliners must be pressurized to provide enough oxygen in a lung-full for us to survive at 45,000 feet. When I was a child, cross-country flights provided oxygen masks as they do today. However, everyone used them for most of the trip. While only the fastest molecules can climb to the top of the atmosphere, there are so few per liter that they appear (on average) to be cold. Notice that the tops of many mountains retain their snow packs in the summer.

Nature can often be explained by citing different physical principles. A second and equally valid way of looking at the cooling of expanding air is to consider the original container. Rubber balloon or steel tank, the walls are fairly fixed to the enclosed air and they cause the bouncing gas molecules to rebound with the same speed, kinetic energy and apparent temperature. This is like playing racquetball with a solid wall. If the container expands (or pops in your case) the interior gasses will rebound with less speed (kinetic energy), as a ball will not bounce well from a wall that gives when hit since the walls will now vibrate a little. Because the gas is essentially doing work against their new boundaries, they will now have less energy and will exhibit a lower temperature.

Thanks again for asking and keep up the good work,

G Ewald