Why does a balloon deflate when brought to room temp after staying inflated in a cold car for weeks?


I was cleaning out my car yesterday and found [this balloon](https://imgur.com/a/xK98QRw) from a month ago that was somehow still mostly inflated. Then I brought it inside for my kids to play with and by this morning it had deflated to about ¼ of it’s previous size. Shouldn’t it have gotten bigger as heat makes things expand?

In: 1

Did you wait for the air in the ballon to warm up?

This may have something to do with the qualities of rubber itself. Rubber is weird because it heats when stretched and cools down when allowed to contract. As a result rubber behaves a bit unexpectedly when it is heated, actually *contracting* when heated.

So while the air in the balloon would have expanded when heated, the rubber of the balloon would be trying to contract more in the warm interior than it did when outside.

Now the above is likely the most interesting possible explanation of what you observed, but it is also just possible that the balloon developed a hole as the kids played with it and leaked some air overnight.

Air diffuses through a balloon. Left long enough in the car, this will tend toward the temperature, pressure, etc. of the car (which will be a net loss of molecules). This process is slower than just the temperature expansion. Brought back to room temperature, where the pressure is higher, the pressure outside the balloon will be higher on the inside of the balloon (even once the molecules inside heat back up) and shrink it.