I guess my point is that we put a pot of water on to boil, then do other things, measure other ingredients, fix a drink, then keep checking…oh, it’s boiling, time to put in the pasta. We rarely stand over it with a thermometer and stopwatch and turn off the gas at the boiling point. I just thought it was pedantic for him to stress that I’m wasting energy when there is no way we’re going to significantly measure how much gas we use, when making a batch of macaroni and cheese for instance, that would make much difference whether I put the lid on the pan or not.
However it does seem that it would boil faster with the lid on, based on the thoughts so far expressed.
I’ll try to perform, as best I could, and if I remember, a test tonight using three gallons of water or so. I’m guessing the amount of time saved is significant–like 5 minutes. All I know is that when I boil water for pasta, it takes forever if I leave the pot unlidded.
I brought 10 quarts of water to a boil in a 16-quart pot. I used the same pot and performed the experiment one after the other (after cooling off the pot, of course). I started with the lid off and water at 72 degrees F. It took the water 26 minutes to come to a boil. I then emptied the pot, rinsed it off (to cool it), and refilled it to the 10 quart fill line (which I had marked before). I put a lid on it. It took about 25 minutes to boil. Only one minute fast, although my temperature readings look a bit odd:
Time Lid On Lid Off
Start 72 F 72 F
2 min 82 F 86 F
4 min 95 F 99 F
6 min 109 F 111 F
8 min 124 F 124 F
10 min 136 F 135 F
12 min 151 F 147 F
14 min 160 F 160 F
16 min 171 F 172 F
18 min 181 F 180 F
20 min 192 F 190 F
22 min 201 F 203 F
24 min 208 F 210 F
25 min 210 F 212 F
26 min 212 F 212 F
Considering the amount of waste heat from the typical stove burner that never is applied to the water and that most sauce pans don’t hold 10 quarts of water, I doubt that putting the lid on makes much difference as to the amount of heat actually used or the time taken to boil water.
My experience is based on camping, when fuel (propane or Coleman fuel) is limited to what you have carried in with you. It’s absolutely the first rule to cook with a cover over the pot, in order to avoid wasting fuel. (You will also want to shield your stove from wind, which also carry away heat.) A covered pot will come to the boil substantially faster than an uncovered one, and you will save fuel, which can be critical in some circumstances.
This may not be of such concern in a house where you essentially have unlimited fuel and unlimited time. However, it is also my experience that when cooking on a kitchen gas stove a covered pot will come to the boil substantially faster than an uncovered one. I have to say I am very surprised by pulykamell’s results, and may try to do some experiments myself tomorrow.
The real issue though, especially when camping, is usually not boiling per se but cooking time. If the food boils at a higher temperature it will also cook faster. The boiling point for a particular pressure just represents the maximum temperture you can achieve for those conditions. If something boils at a lower temperature it will take longer to cook, which is why you need to boil rice longer at higher elevations.
I had the same thought, which made me wonder how the temperature readings were taken. Was the lid lifted every two minutes thereby negating most of it’s effect?
But to address the OP and how to tell a pot is boiling without looking, you can do it by ear. I’ve noticed that as a pot of water heats up, it will make a kind of speudo-boiling hissing/rumbling sound which will steadily increase in volume and intensity as it goes. At some point the noise will reach a peak, and quickly taper off. When it get’s quiet, your water is just starting to boil. Then you’ll hear actual boiling sounds which are similar to the previous sounds, but…wetter. This is very noticable on a whistle equipped kettle because just as the preboil fades out, the whistle begins it’s crescendo. There’s an explanation for this phenomenon.
When I thought about this, I wondered if the fact that leaving the lid off means that you ultimately don’t need to bring as much water to the boil makes a difference. Part of the water will evaporate before it’s boiling, which means that although you started out with 2 litres of water, you’ve now only got 1.9 (say).
However, the specific heat capacity of water is very small in comparison to its specific latent heat of vaporisation, and therefore I think the heat lost with the evaporating water far outweighs this benefit. It might explain the relatively small difference in boiling time found by pulykamell.
My readings were taken with one of them electronic probe thermometers, you know, the kind that Alton Brown uses on his Good Eats show.
Pot was not stirred. The probe was left hanging in such a way to minimize any contact with the metal of the pot.
Lid was not taken off to make any readings; the wire from the probe just ran out of the pot between the lip and the lid.
Fire was kept at highest setting. Pot was placed as much as possible in the same location both times. Temperature readings were made in two minute intervals.
The pot used was a cheap aluminum pot with a thin bottom, not the best for even heat distribution, but I don’t think that should affect the results much as the same pot was used for both trials. It was filled to 10 quarts, and has a total capacity of 16 quarts, meaning that there was about 5 inches or so of space between the top of the water level and the lid. Once again, I don’t think that should make too much difference. Ambient room temperature was approximately 72 degrees.
I tried to keep conditions as identical as posisble between the two trials. The graph you see is really not what I expected. Also, I was struck by its linearity. For some reason, I thought it was going to be more of a logarithmic curve.
In a situation where you’re boiling 10 quarts of water, the ratio of the surface area of the water (where the heat is lost) to the total volume of the water is probably too low for the lid to make a significant difference. Someone may want to repeat the experiment with a more customary household cooking quantity of water, like say 2 quarts.
I would suggest a couple of possible ways to “clean up” your experiment.
[ol]
[li]When you are looking for a difference that may be as small as this, you would need to repeat the experiment and find the statistical difference over several iterations. (The more the better, but realistically, two or three more identical tests would give you really solid data.)[/li][li]Were all of the conditions in your kitchen identical durring all of both trials? (e.g. air temperature, air movement, humidity, etc.) Anything that affects the rate of evaporatio nwill alter your results.[/li][li]The wire coming out of the pot will make a small gap for gasses to be lost. This may or may not be enough to affect your results. Try wrapping the edge in tin foil.[/li][li]Just how sure are you that you had identical volumes of water (to start with, anyway)? I.e. How accurate is your measuring device? How many times did you have to refill your measuring device?[/li][li]How did you ensure that the probe was not in contact with the pot? Can you wire it in place, so you know it is in the center of the liquid? (I would try to run wire from the two handles, assuming that they exist on your pot.)[/li][li]Lastly, I would also suggest that the larger pot will show a smaller difference between lid on and off. Any evaporation will take place at the surface; if you really want to see a difference, try a short, wide pot. For the camping folks, their conditions are drastically different than what you have been reproducing - much colder, possibly drier, lower air pressure, windier, etc. These effects will add up to increase the heat lost through evaporation and make difference much more noticable.[/li][/ol]
Sorry, I probably shouldn’t have gone on as long as I did. Despite what the above critique may look like, I really enjoyed your test, am glad that you reported it here, and don’t think that your results are wrong. (Your test could be more rigorous, but I don’t think - even if you took all of my suggestions - that your results would be much different.)
If you choose to repeat (with or without modifications) please let us know your new results.
-Geek
(Sometime) Professional Scientist*
Professional Scientists,
taking the fun out of experiments since 1214.
I suspect the curvie is exponential. After all, the fire temperature, or the electric heating element, is much, much hotter than the water and that’s the temperature for which the water is heading. That makes it an expeonential curve which looks linear over the first part. Of course, the curve stops at 212 F and stays there until all the water boils away.
I was expecting more than a minute difference, so if the difference really is this small, then it’s pretty much insignificant for my purposes.
The kitchen was probably a few degrees warmer for the lid-on experiment due to the burner being on for the last twenty minutes with the first trial. I would have expected the higher temperature to skew my results more towards the way I was expecting the experiment to turn out (lid on boiling much faster).
Oops. Forgot to mention that I tried to minimize this by putting a heavy cast-iron pan on the lid.
I marked the volume with a pencil on the pot to ensure as much accuracy as I could.
That’s basically what I did. Also, if it were touching the metal, it would be evident from a temperature reading of >212. At any rate, thermometer or not, I could see when the boil occurred.
This is what I suspect may be the case. I’ll try again with a smaller pot.
Maybe I’m confusing my terms. I was expecting a curve with a steep initial rise, and then a tapering as it gets closer to the boiling point–basically the curve mapped by y=1/x^2
What David is saying is that your experiment only views the left side of the curve, the straightest portion. The curve tapers, not in relation to the boiling point, but in relation to the relative temperatures of water vs. heating element. The water, even at boiling, does not get close enough to the temperature of the element to show the curvature. You’d have a better chance of seeing the curve if you did the experiment with oil, going up to 400+ degrees.
