WTF?!? I boil some water for mac and cheese, put the mac an cheese in (sorry, sorry- it’s Kraft “Cheese and Macaroni”) and cover with a lid.
Suds and bubbles fill the pot and boild over. As soon as I remove the lid, the foam subsides.
WTF?!?
jb
When the lid is off it boils less so there are less suds. . . or the steam has somewhere better to go. Some one will pop in and talk about heat, pressure and Boyle’s Law soon, I expect (having the lid on increases the pressure so temp is higher, I believe).
see, I thought about that though, and I don’t buy it. If there was that great a pressure build-up, the lid woulda popped up crazy like and I’d be covered in cheap macaroni.
As for the “more steam= more bubbles”, this is a huge bubble difference I’m talking about. If more steam were realy causing that much sudsing, wouldn’t a full (uncovered) boil cause my pot to overfloweth?
jb
Unless you make your Cheesy Macs in a pressure cooker, you won’t have that problem. The lid on a normal pot retains enough steam and heat to cause everything to boil over, but the lid has a loose enough fit to allow excess pressure to be relieved.
Okay, but what “cause[s] everything to boil over”?
still waiting,
jb
I think it’s higher temperature mostly, with maybe a little bit of pressure influence. Once you remove the lid the steam can evaporate, taking all the hottests water molecules away from the mix and so cooling it. With the lid on the heated water molecules are simply evaporating and bouncing straight back into the water, carrying their heat back in with them. the pot can’t cool.
Bubbles last longer in a more humid environment.
With the lid on the atmosphere over the pot is at least saturated and probably supersaturated with water vapor. Rather than losing water to the atmosphere, or by it moving to the bottom of the bubble and down to the next bubble in the heap,the bubbles are probably gaining water from the atmosphere.
When you take the lid off the excess water vapor wafts away into the room and you start to have liquid evaporate from the surface of the bubbles themselves. This causes the bubbles to pop.
ahh, that makes sense Squink. Any corroboration?
tanka,
jb
Will Julia Child do? On one of her cooking shows, years ago, she demonstrated how to get your water JUST at the boiling point for poaching quenelles Lyonnais. She put a lid on the pot, and it came up to boiling. She took the lid off, and it stopped boiling.
Thank you, Gaspode, for scientifically confirming what Aunt Julia done tole me. 
Add a drop or 2 of vegetable oil (canola or olive).
It’ll take care of the bubbles.