Hi!
I recently heard the claim that if you are boling water in a pot, it is actually better to leave the lid off, since then the water will boil faster than if the lid was on.
Or, at least, that it would make no difference one way or the other.
Any truth to this? It seems counter intuitive, and I am doubtful.
Surely SDMB has the answer to this one…?
Water boils faster with the lid on.
Technically the lid raises the pressure in the pot some which increase the temperature at which water boils. But for a loose fitting lid this will not be very dramatic. The effect of the lid trapping heat inside that would otherwise escape has a much larger effect.
Also note that while a higher pressure raises the temperature at which water boils the water is still XXX degrees hot. So if the boiling point becomes (say) 102[sup]o[/sup] C stuff will still be cooking at 100 the same as if it boiled at 100 even though there is no boiling. This is what pressure cookers do. They allow the temperature to rise well beyond the boiling point so cooking ta higher temps can occur. Once boiling water does not get hotter…it just boils away faster if you add yet more heat.
So what evidence was presented for this claim?
Since it violates the known laws of physics (boiling occurs faster under higher pressure), they ought to have some pretty extraordinary evidence to explain how this would happen.
I think that if the rates that heat is travelling in or out don’t change, then boiling occurs slower under higher pressure, since it happens at a higher temperature. Whack-a-Mole pointed this out.
???
Am I misunderstanding you?
The higher the pressure the higher the temperature something boils at and vice versa. Lower the pressure enough and you can boil water at room temperature. This is why people who live at high altitudes need to modify how they cook because their water boils at lower temps (and as I said once boiling it will not increase in temp no matter how much heat you put on…it just boils away faster the more heat you add).
By “higher pressure”, do you actually mean: “lower pressure”?
Well it was actually my parents who had heard this from someone, so I can’t really say, unfortunatley. Apparently it was somehow important that he was a mathematician, but as for evidence, I can’t say.
Thanks for all the answers folks, it seems you agree with the few results google gave me.
Obvious: If you are boiling water with a lid on, how do you know when it is boiling?
Ignoring pressure effects, a lid also serves to trap hot moist air above the surface of the liquid; a layer of insulation, if you will. That reduces the rate of heat transfer to the environment, so the liquid heats faster.
What if you have “magic grits”? 
Real easy to test at home.
Glass cookware.
^:smack::smack::smack::smack:
By the sound, by the steam escaping around the edges of the lid, or (if it’s a glass lid) just by looking at it, same as without the lid.
Unless your lid is especially tight-fitting, you’d do well to completely ignore pressure effects on the boiling point. This site gives the data on the relationship between boiling point and pressure for water.
To raise the boiling point of water one single degree Fahrenheit, to 213 °F at sea level, you need to raise the pressure in your vessel by 0.3 psi. This may not sound like much, but on a 12" diameter lid, that means a force of 34 lbs pushing it up. And that’s for just one degree! If you wanted to raise the boiling point to something you might consider noticeably different, say 230 °F, you’d need a lid that could hold the inside more than 6 psi higher than the outside. That’s over 700 lbs of force on the 12" lid.
Suffice it to say that a normal lid can’t seal well enough to make any noticeable difference in boiling point by raising the pressure.
What the lid does well, however, is contain the hot gas above the liquid, reducing heat loss to convection. A shiny lid may also help by reflecting infrared radiation back into the container, instead of letting it radiate away into the kitchen.
So a lid helps reduce heat loss and lets the water boil faster. But it doesn’t delay boiling by increasing the pressure, because pot lids are horrible pressure containment devices.
(bolding mine)
It is not about radiation or convection, it is about steam and condensation.
It takes a relative big amount of energy to vaporize water, winning some of that lost energy back by letting it condense on the lid makes some of the difference, and the fact that water will vaporise less readily when the relative humidity is at 100% will also contribute a lot in making a pot with a lid boil faster (or with less energy added).
Some numbers:
It takes 418 kJ to raise the temperature of 1 kg of water by 100 degree K.
It takes 2260 kJ to vaporise 1 kg of (boiling) water.
How much pressure does an average lid add? I have a pressure cooker, so am very familiar with it’s pressury-goodness. But your typical lid – anything short of a heavy cast iron lid – will either have steam vents of a sort or will happily clatter away while things are boiling underneath. Is the “putting a lid on a pot raises the pressure” conventional wisdom akin to adding some salt to the pasta water? In other words, does it add such a marginal amount of pressure that it’s technically correct but observationally insignificant?
ETA: Oops… didn’t see aerodave’s post as I was writing. Thanks!
I think people tend to confuse “boiling” with “getting hot enough/finished” for whatever purpose the water was being heated for. Being practical, I don’t care whether the temperature of my tea water is 100.02ºC in an uncoverd pot or 100.04ºC in a covered pot. It doesn’t even have to be truely boiling, I’m not trying to sterilize surgical instruments or anything, just make a warm drink.
I’d prefer the water to get hot as fast as possible, and that obviously happens in a covered pot. But again speaking practically, it’s only going to be seconds difference anyways (considering hot much energy is put off by electic or gas burners), and I’ll either let it rattle away for 20 seconds while I finish buttering the toast or shut it of and let it sit there for another minute while I’m mixing the pancake batter; the small amount of saved or lost time is eaten up by other routine activities anyways. And by the way, don’t most people use some kind of mostly sealed kettle anyways?
I add salt to pasta water to salt the pasta, not to raise the boiling point (even though it does that, too).
I dunno the physics, but putting a lid on when I cook does seem to make it heat up more/more quickly. Things that would not burn without a lid, did burn (just over the weekend.)