# Why don't air cool over time?

Say you have a balloon full of a type of gas, it is completely isolate from everything else. Let the temperature of the gas be 20 degrees (or any temperature, it doesn't matter)

Now, the ballon is blown up because the molecule of the gas is constantly colliding with the "skin" of the balloon, stretching it into a big, blown up balloon.

But as each time the molecules of the gas collide with the "skin" of the balloon and other gas molecules, it loses some kinetic energy, and will then begin to slow down.

Since temperature of a gas is dependent on the kinetic energy, it will cool down.

But from school, I remember learning that temperature of an isolated system will remain constant forever if heat is not transferred.

So why would the gas cool down? and why or why not?

• I'm not sure about the scenario, but I'd hardly consider a balloon someone is blowing into to be an isolate system. Also, isolate systems can be defined in ways were heat can be transferred: if you have a cold block that touches a hot block inside some very adiabatic big block, than both blocks are an isolated system... Even though they'll exchange heat and stabilize at a common, "warm" temperature. Jun 23, 2016 at 19:27
• But lets say we put the balloon in a vacuum, and make sure the gas pressure is not so high that the balloon will pop, what happens to the temperature of the gas in the balloon over time? Jun 23, 2016 at 19:45
• First off, a balloon in a vacuum would explode, since the pressure inside would be infinitely larger than on the outside. Secondly, as all answers above claim, the temperature of the air you blow into the balloon is warm because your lung is warmer than the air, and by exchanging heat with the environment it would cool down (or warm up, if it's in the middle of the Saara at noon). Jun 23, 2016 at 20:07
• @QuantumBrick, Re, "A balloon in a vacuum would explode, since the pressure inside would be infinitely larger than on the outside." The pressure inside the balloon would be whatever pressure you inflated it to. Jun 23, 2016 at 20:13
• @jameslarge You're quite right. I was sloppy. The pressure on the outside that would be infinitely small. Thanks! Jun 23, 2016 at 20:16

If you isolate the balloon from everything else then after filling it with gas the balloon will soon attain the temperature of the gas. Then the balloon and gas will be in thermal equilibrium and no more heating/cooling will take place.

Upon colliding with balloon skin the atom is not always lose energy but half the time it will gain energy (if the two systems have same temperature) such that net energy flow is zero over a period of time.

Your balloon is not an isolated system, it can exchange heat with the environment as the "skin" material is not a perfect insulator. The expanding gas inside a balloon will cool as it expands, but the outside air will warm it up again after some time, the speed will depend on the balloon material and volume, but eventually the heat exchange between atmosphere and balloon will make the gas inside it reach the same temperature than the outside.

• But lets say we put the balloon in a vacuum, and make sure the gas pressure is not so high that the balloon will pop, what happens to the temperature of the gas in the balloon over time? Jun 23, 2016 at 19:46
• if the vacuum chamber is located at earth, in the atmosphere, the effect will be to retard the warming up, because there is still heat exchange but slower, only the electromagnetic radiation that pass through the vacuum will warm up the gas. If it is in the moon, then the gas will freeze to the temperature of outer space.
– user65081
Jun 23, 2016 at 19:51
• but the freezing in the example of outer space is due to the temperature outside, the reduction of temperature as you inflate the balloon is limited, and only happens as the balloon expands.farside.ph.utexas.edu/teaching/sm1/lectures/node53.html
– user65081
Jun 23, 2016 at 19:53