Pressure in microwave ovens

[Fezzik]

My fiancee recently reported this strange phenomenon at work.

She placed a tupperware container (with food) in a microwave, with the lid mostly sealed except for a small pulled-up gap. After two minutes of cooking, the lid had not only sealed itself entirely, but was inwardly dented as if the container had tried to suck down the lid.

Her co-workers said that that can happen even if the lid starts out firmly sealed.

I'm no physicist, but I seem to recall that the equation PV=nrT means that as temperature increases and volume stays constant, pressure has to increase. And inside the plastic container, isn't that what's happening? But if the pressure inside the container increased, wouldn't the lid want to pop off, instead of get sucked down into the container?

What's going on here? We'd appreciate any answers.

Thanks!

-Fezzik

[scotth]

I am sure it did not seal while it was actually cooking.

I am sure it happened after the microwave shut off and the contents started to cool a bit.

[Kamandi]

I've seen that myself. My guess is that as the contents are heated, they heat the air inside the container. The heated air expands and escapes through the tiny opening in your lid. I'm thinking the loose lid acts as a one-way valve, allowing air to escape but sealing up afterward. Therefore, when heating stops the air inside the container cools, lowering pressure. Atmospheric pressure pushes the lid in.

[Fezzik]

Originally posted by Kamandi
I've seen that myself. My guess is that as the contents are heated, they heat the air inside the container. The heated air expands and escapes through the tiny opening in your lid. I'm thinking the loose lid acts as a one-way valve, allowing air to escape but sealing up afterward. Therefore, when heating stops the air inside the container cools, lowering pressure. Atmospheric pressure pushes the lid in.

That was my first thought. But then there's the case (claimed by co-workers) that the same thing can occur even if the container starts out sealed, and there is no tiny opening.

I'll have to do some experimenting at home with my own microwave, to narrow down this phenomenon.

-Fezzik.

[Mr.Zambezi]

I have seen this many times. If it is sealed it will either pop open, or ir will just bulge a bit. While heating, teh lid usually softens a bit.

After teh microwave shuts off, the inside of the container begins to cool, the pressure decreases and it constrics. The result is a concave lid.

No matter what the employees say, heat expands the air, cooling contracts it. I defy them to heat a substance that causes the fluid and air inside of a sealed container to contract. Can't happen.

or are you talking about the Freezing Microwave being discussed on another thread

[astro]

Think about the parlor trick where you suck an egg into a soda bottle by heating it up and letting it cool. Some of the same principles apply.

[Fezzik]

Originally posted by Mr.Zambezi
After the microwave shuts off, the inside of the container begins to cool, the pressure decreases and it constrics. The result is a concave lid.

Thanks! This makes sense to me. The only thing about it that still puzzles me is why the heating and cooling isn't a zero-sum game as far as the lid is concerned. (That is, if the lid bulges when heated and contracts when cooled, shouldn't it end up in its starting state, and not more constricted than when it started?)

-Fezzik

[Padeye]

[quote]Originally posted by Fezzik
[b]

Originally posted by Kamandi
That was my first thought. But then there's the case (claimed by co-workers) that the same thing can occur even if the container starts out sealed, and there is no tiny opening.

Sealed containers aren't nearly as sealed as you might think, certainly not tupperware bowls and such. The lid acts as a one way valve even when sealed. Positive pressure on the inside tends to open the seal, positive pressure on the outside makes the seal better.

[Mr.Zambezi]

why the heating and cooling isn't a zero-sum game

it is. but what happens is much of the gas (air mainly) escapes as it is heated. But when it cools, air can't rush back in because the lid acts as a valve. Put the lid on sideways and it won't get sucked down.