I don’t know if this should be a General Question, since it has become nearly a Great Debate in my house.
I have always learned that despite what we are inclined to think, water boils faster in an uncovered pot. This is because an uncovered pot has lower air pressure above the boiling water, making it easier for the water vapor to escape. A covered pot does give the impression of boiling faster because it causes water to boil at a higher temperature, so when you finally put your noodles or potatoes in that boiling water they cook faster.
My wife, however, refuses to believe this. Can anyone give me an easier logical proof, or some location where I can find a demonstration (at home the conditions wouldn’t be standard enough to prove anything - different pot sizes, burner sizes, etc), or <shudder> actually show me that I am wrong?
Sorry, curwin, but I’m guessing that you are wrong…
Seems to me that a covered pot would tend to hold in the heat, and thus encourage boiling at a more rapid pace.
Yes, an uncovered pot might have a slightly lower air pressure immediately above the water, but I think that would be an insignificant difference.
I make a lot of soup, and do so on a small, portable gas burner… which is severely strained to boil a big pot of water (I have no stove in my dorm-room!)! If the lid is off, I get a very slow boil… but if the lid is on, I get a fast boil!
Try it! Bring a pot of water to a slow boil on your stove, and then put a lid on… you will see that the rate of boiling increases within a few seconds…
Sorry!
Um… Interesting idea, I hadn’t thought of it before, but I’m pretty sure a pot lid does not form such a tight seal that it would significantly affect the air pressure inside it… But since it traps the heat in better, instead of letting it radiate out into the rest of the room so efficiently, it would heat quicker… I’m pretty sure it would be enough to compensate for any difference in pressure.
the pressure might be slightly diffrent but that is to your advantage (with the lid). the water will heat to 212F faster under pressure because less boiling/evaporating will occure. Even if the boiling point was higher as soon as you opened the pot it would boil (even if it wasn’t before)
Additionally the boiling takes a great amount of heat away from the water. by putting a lid on it some of the energy is recaptured and the water recondenses on the lid (not the best place for it but better then the window for the open pot). The pot itself will act like a coolong fin when opened (almost like your cpu heat sink) delivering room temp into your water - with a lid you cause the metal to heat up too.
Also the air is hotter and more humid right over the surface of the water.
to yor advantage is you are removing a volumn of water through evaporation so you are actually heating up less - but that’s a false argument since you could start with less overall in the covered pot.
like I said before - I really don’t know but you might just be wrong.
What we want to do is to heat up a potful of water to boiling temperature (100°C, 373K, 212°F or whatever - assuming sea level). Unfortunately we have to administer this heat to the pot containing the water, but as most metal conduct heat very well, there is normally no problem to get the heat to the water at the bottom of the pot. Now the water at the bottom gets warm, and due to convection it starts moving around, and there will be a more or less uniform distribution of heat in the water. This means that after a while there will be warm water at the top.
Now let’s look a bit closer to the interface water-air: As in any gas-liquid interface there will be a few water molecules jumping out of the water and becoming vapour. In a steady-state interface this will be matched with just as many molecules of water vapour leaping back into the water, so there will be no net transfer of energy. If the water is heated, however, there will be a net flow of water molecules jumping out into the air, at a substantial cost of energy to the warmer water.
It is thus clear that in the case of a lid it will be easier to maintain a saturated vapour close to the surface. Note that this has nothing to do with the actual air pressure, but the partial pressure of water vapour.
(Of course the air mass will not be at rest either. Most likely the lid will be substantialy cooler, and thus water will condense on it, bringing the temperature up.)
What you want (presumably) is hot liquid something.
If we go with the notion that putting a lid on increases pressure inside the pot then you’d be right in saying that a pot with the lid off boils faster (ignore for the moment the fact that it will hold heat in better than a pot with no lid).
So, what you get is equally hot liquid in both pots but one is boiling and one isn’t. IIRC water will not rise above 212[sup]o[/sup] Farenheit at sea level. All extra energy that would raise the temperature is lost as the water boils away. More energy means the water just boils away faster…it doesn’t get hotter.
With the pressure raised the water can reach higher temperatures. This can be dangerous since the water could flash boil if you remove the lid (it would essentially explode in your face once the pressure was released). This generally doesn’t happen with normal pots on your stove since a loose lid doesn’t allow the pressure to increase very much (pressure cookers are another story).
The reverse of this is true when you lower pressure. Some food instructions will tell you to cook your dinner longer if you are above a certain altitude. The boiling water doesn’t reach such a high temperature so it takes longer to cook your food.
I’d say in the interests of keeping heat in a covered pot would boil faster since I believe (but have no evidence to back this up) that the increase in pressure in a covered pot is too small to seriously raise the boiling temperature of the water. The extra trapped heat would likely more than compensate for the difference.
Everyone who’s answered here has been right (for the most part), but I’d like to add my $0.02.
Like Whack-A-Mole said, you want to get the water to 100C as quickly as possible, pressure or no (and no, a covered pot does not have higher pressure anyway). Getting the water to that temperature from room temperature takes a certain amount of heat, which will be applied by the stove burner.
The heat applied by the burner is equal to the heat that goes into the water, plus the heat that escapes into your kitchen. If the heat applied by the burner is constant, then to get the most into the water, you have to minimize the heat lost to the kitchen, so cover that pot!
After the water reaches boiling, it will boil away at virtually the same rate whether it’s covered or not. This result is a little counter-intuitive (to me anyway), but makes sense if you look at it from the conservation of heat viewpoint.
Wow, you guys really didn’t like my arguement, and my wife feels quite vindicated.
For what it’s worth, here’s my “proof”:
From Rainbows, Curve Balls & Other Wonders of the Natural World Explained, by Ira Flatow:
So I think it is an issue of semantics (although my wife says it is physics). In other words, if my pot was closed tight enough, it could make a difference, but usually pots aren’t closed that tightly.
In a pressure cooker, the water may take longer to reach boiling point, but that’s because boiling point is higher and takes more energy input=longer time from the same heat source, however, it will reach 100[sup]o[/sup]C quicker than it would in an open pot (it just won’t be bubbling at 100[sup]o[/sup]C).
All I know is, when I put a lid on the pot, it takes less time. Also, if it’s not boiling as much as I want too, and I want my noodles to cook faster, I put a lid on for a few seconds, and it boils faster.
Except that space is extremely cold, so any water that escapes into space freezes pretty much instantly rather than boiling. (Cf. pictures of the ice crystals after the astronauts’ urine dumps.) So even hot water freezes. The ‘blood boiling’ myth is just that; expose an astronaut to space and you’d get freeze-dried astronaut (after a few minutes; death is not instantaneous).
It depends what you want to do - get the water to a max temperature asap, or watch little bubbles rolling asap. True, the lower the pressure above the water surface the lower the temperature at which the water will start boiling. However, your regular pot lid won’t fit tightly enough to make much of a difference in pressure, so I suggest climbing Everest with a potatoe in a little pot to watch the water physicaly boil at a lower temp. Since you’re talking about using boiling water for cooking, you want to get the temp up asap; meaning keep the lid on to keep in both the heat and the steam. Once the space above the water is 100% saturated with water vapor (steam), you’ll stop loosing heat to evaporating all your water away instead of making it hot. With the lid off, there will be a lot of water evaporating and filling up the local atmosphere with water vapor… and it takes about 5 times as much energy to turn water into steam as it does to heat it all the way up to boiling. End result: heat only what’s in the pot, not your local atmosphere.
All the PC I’ve used you can’t remove the lid - the pressure prevents you from turning it (and newer ones have a lockout device to prevent this too.)
But it some you can remove the weight off the top and quickly releive the pressure sometimes causing a flash boil so that water starts comming out with the steam.
It also freezes so fast because heat is lost through boiling - I think most heat is lost this way in space. After water would go from ice directly to gas but it takes longer (sublimation).
