Why did the lid of a heated container cave inwards?

[Eve Litman]

I microwaved a container of soup with a lid sitting on top. When I tried to remove the lid, a suction had formed and the lid was very difficult to remove, as if the volume and pressure of the container had both decreased. However, heated gas (in this case, steam from the soup) expands. So why did the lid of the container cave inwards instead of bulge outwards?

 

[russ_watters]

Welcome to PF!

I microwaved a container of soup with a lid sitting on top. When I tried to remove the lid, a suction had formed and the lid was very difficult to remove, as if the volume and pressure of the container had both decreased. However, heated gas (in this case, steam from the soup) expands. So why did the lid of the container cave inwards instead of bulge outwards?

When you tried to remove the lid, the soup wasn't heating, it was cooling!

 

[Dale]

To add to what @russ_watters said, the lid was acting as a valve, letting steam out but not letting air back in.

 

[Eve Litman]

That makes sense. Thank you guys!