On the subject of cooking, someone once told me "If you have too much water in your pot after cooking a dish, just turn up the heat and let it boil without a lid for a few minutes".
Now I've thought about this, and as far as I can tell, it shouldn't matter if there is a lid or not. In both situations the pot will stay at the same temperature (around 100 degrees Celsius), and therefore the amount of heat energy entering the pot will be the same. The lid may cause some drops of water to condense and fall back in, but then it also traps heat and results in a higher rate of boiling inside the pot — but the amount of water exiting the pot as vapour shouldn't change.
So by that reasoning it seems that it doesn't matter. Or does it?
EDIT: Please note, I'm not talking about the time it takes for the water to reach its boiling point; I'm talking about the rate of water vapor leaving the pot when it is boiling.
Best Answer
When cooking, keeping the lid on a pot does a few things if you think about it:
I can't prove that with science, it's just my observation. I cook a lot. Often when I lift a lid off a stew or broth, the lid is wet and dripping. Curiously, what this means is that water will get to a boil faster with the lid on, but it will boil away faster with the lid off.
* On Air pressure: I have a hard time believing that, unless it's a pressure cooker, this would be a significant increase, as air pressure is 14.4 lbs per square inch and what does the average lid weigh - a pound? But there might also be a small decrease in pressure (er, I think), similar to Bernouli's paper lift.
The boiling of water creates a wind direction of sorts off what you're cooing up through the pot and that wind direction should decrease air pressure, but that effect would likely also be small. Perhaps, the boiling point - no lid might be 98 degrees C and with lid, 101 degrees C - that's a guesstimate, but it shouldn't be a huge difference I would think.