What's happening here...

Factual Questions
1

So I want to steam some chinese buns, but I don’t have a steamer. So I improvise a deep pan, with 2+ inches of water in the bottom (bring to the boil), then invert a cereal bowl, place in bottom of pan, put a side plate atop that, the chinese buns atop that, and the pan lid atop the whole lot.

Okay, you got the picture. And it works wonderfully I have done this many times.

So when I go back to clean up after this little snack the boiling water has, some how, sucked itself all up and into the inverted bowl. Creating a kind of vaccuum, that lets go with a loud sucking noise as you pull the bowl away from the bottom of the pan.

Now the water doesn’t do this as the buns are boiling, and the bowl doesn’t sit all that tightly to the bottom of the pan. So…

What’s happening here?

I am wont to attribute this to magic but suspect there is some other explanation.

How about it, can you explain this to me in language I can understand please?

2

It’s fairly straightforward, and I hope I can explain.

As the water boils, the air inside the inverted bowl is replaced by steam.

When the system cools, the steam condenses back to liquid water.

When the steam condenses, the pressure inside the inverted bowl goes down.

Since the pressure on the water surface outside the inverted bowl is lower than the pressure on the water surface inside the inverted bowl, the water is pushed up into the inverted blow until the pressures are equal on both water surfaces or the inverted bowl is completeley full of water.

3

There is air space inside the bowl, that gets filled with steam. As the whole setup cools, the steam contracts, causing the space inside the bowl to have a low pressure. The higher pressure in the ambient air pushes down on the water, the water moves into the low pressure bowl. Voilá.

The hot steam is very low density, so once the water condenses out of it, the bowl has a near-vacuum in it.

4

Oh, and the noise …

When you lift the inverted bowl, the water filling the bowl goes with it because water doesn’t expand noticably under these conditions and there’s no source of any gas or anything else to fill any space in teh inverted bowl.

The instant one edge of teh inverted bowl clears the water’s surface, air can get into the inverted bowl, and the water inside the bowl falls out (and is replaced by air).

5

Well, sure, it all seems so easy now that you’ve explained it, and very nicely too.

Thanks

6

One cool experiment to try which demonstrates this phenom. is to do it with an aluminum can.

All you need is a pan of cool water, a pan, and an electric stove.

  1. Put a small amount of water in the can.

  2. Put it on a burner and turn it on.

  3. When steam starts to come out of the can, pick up the can (with the tongs), invert it, and place it (face down) into the cool water.

  4. Enjoy.

  5. Be careful! I disclaim all responsiblity for ruined stoves, burnt or crushed fingers, etc.

Rav