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Name: Hayley
Status: student
Grade: 6-8
Location: AK
Date: Winter 2009-2010

Question:
My friend and I have found that water in a pot with a lid begins to boil (starts a rolling boil) at a lower temperature than water in a pot without a lid. Could the lid somehow affect (lower) the air pressure within the space under the lid, at least initially? Or can the lid somehow affect the mixing of the water within the pot, which would in some way lower the temperature measured? We have been unable to find any references that address this question. We are using a LCD thermometer with a probe to take the temperature of the water. The probe is stationed in the middle of the water, not touching the bottom or the sides of the pot. When we take our measurements, we don't remove the lid or the probe on the thermometer - we watch through the lid for a rolling boil. We measure three cups of water into the pot for our experiment and we use the same pot to boil water in with and without the lid. We leave the pot outside to cool for 10 minutes the pot is at 20 C. We do the replicates in pairs - one trial with the lid, one without, and we have replicated the experiment 5 times.



Replies:
Hayley,

Interesting question. I see no way that the lid could lower the pressure surrounding the water, in fact the theory states that a tightly fitting lid could increase the external pressure on the water as the water warms up, INCREASING the boiling point, but there are some other things that could be going on.

The first thing that comes to my mind is that the covered the pot will boil more quickly than the uncovered one and this shorter period of time may mean there is a less consistent temperature throughout the pot so your thermometer may be recording a lower temperature not because the water is boiling at a lower temperature but because the water in the pot is not all at the boiling temperature. I am not sure how likely this is though, if you have a rolling boil.

I am stumped! But how did you have the thermometer held in the water with a lid on top? Could this be to do with your experimental design? It is often difficult to control all the variables whilst changing just one (the lid). Could I suggest you measure the boiling point with the lid off, then pop the lid on whilst it is still boiling and see if the boiling point goes down - this would be pretty convincing evidence!!

Best wishes,

Tom Collins


Haley,

First let me congratulate you on a well-designed experiment, it seems you had thought about everything and so this interesting result can be analyzed in more detail (since we can remove possible errors in experimental design).

The result is puzzling because - as I am sure you already know - high pressures should result in higher boiling points. The lid should have caused a higher than atmospheric pressure and a higher boiling point.

The big differences (that I can think of) between the two systems is that the open container: (1) allows the atmospheric pressure to control the boiling point, and (2) the evaporation from the open container allows some evaporative cooling so that the temperature rises slower.

So here is my hypothesis: "rolling boil" in the two systems is not indicative of the same state. In the open container, rolling boil might occur at the boiling point, whereas in the closed container, rolling boil is at a state that is lower than the boiling point.

I am inclined to think that the open container, because of evaporative cooling (as the water evaporates it cools a little bit), the rolling boil is reached later because there is a simultaneous heating and partial cooling. This longer time allows the heat to permeate the whole vessel and so when the "rolling boil" is reached, it is actually the boiling point of the open system. On the other hand, in the closed system, because very little vapor escapes, cooling does not happen as much, higher temperatures are reached sooner, and bubbles form *before* the boiling point is reached. These bubbles could be a combination of water turning into steam and dissolved gases coming out of the water. It just appears that you've reached the boiling point - because of the outgassing. As such, this temperature, which is reached sooner and is lower than the boiling point, is actually lower in the closed lid then in the open container.

To test my hypothesis, I suggest you do an additional experiment. When you have reached the "rolling boil" take a note of the temperature, and keep noting the temperature for about five - ten minutes more. Be VERY careful as there will be a lot of splattering of very hot water at this point - use a much larger container than the amount of water to help contain the splattering. Note if the temperature of the water continuous to rise. Since a boiling liquid cannot go much higher in temperature than its boiling point, if the temperature in the open container does not go much higher - it is already at its boiling point and the "rolling boil" is equivalent to the "boiling point". If my hypothesis is correct, then the closed lid container should still continue to rise in temperature since the "rolling boil" is lower than the boiling point and can still go up in temperature until it does reach its boiling point and remains steady.

I am curious to know what you find.

By the way - what were the temperatures of the "rolling boil" that you observed for the two systems?

Greg (Roberto Gregorius)
Canisius College


Hi Hayley,

You are witnessing the importance of "evaporative cooling". It takes a lot of energy to convert liquid water to water vapor. When you heat water in your pot, even before it starts a rolling boil, some of the water vaporizes at the water warms up. The water that leaves as vapor takes a lot of heat with it, which causes the liquid water remaining in the pot to heat more slowly. However, with a lid, that vapor is kept near the water, reducing the amount of water that vaporizes (and thereby keeping more heat in the liquid). More heat in the liquid means its temperature rises faster -- and that means a faster rolling boil. Some terms for you to read more about include "enthalpy of vaporization" (http://en.wikipedia.org/wiki/Enthalpy_of_vaporization) and "boiling" (http://en.wikipedia.org/wiki/Boiling). Enthalpy is a scientific term for heat.

You can "feel" the same effect when you get out of a swimming pool or shower - the reason you feel so cold is because as the water evaporates from your skin, it takes a lot of heat with it, making your skin feel cold.

I think that mixing is likely not strongly affected by the lid -- but I am impressed that you are aware of this. As you heat the water, it will "autoconvent" -- which means it will mix on its own. As you heat the water, and especially once it starts a rolling boil, the water will mix strongly. As vapor condenses on the lid and drops back into the heating water, it will slightly increase convection, but I think this effect is much, much smaller than the evaporative cooling.

Last, I want to compliment your excellent experimental description -- it is fantastic that you are running and documenting your experiment so carefully! Very nice work!

Hope this helps,

Burr Zimmerman



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