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Boiling Water Experiment and Temperature Measurement
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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