I put water in a thermos and shut it tight. 2 says later I open it and there’s a small release of gas. Why?

Question

I put water in a thermos and shut it tight. 2 says later I open it and there’s a small release of gas. Why?

In: 1

Answer 1

You put cold liquid in the thermos, it heats up. The little amount of air in the thermos also heats up, so it’s pressure rises. Now the pressure inside the thermos is higher than ambient pressure. As you open the lid, higher pressure air trapped inside rushes out.

You put hot liquid in the thermos, it cools down. The little amount of air in the thermos also cools down, so it’s pressure drops. Now the pressure inside the thermos is lower than ambient pressure. As you open the lid, higher pressure ambient air rushes in.