| Both of the above cans are being heated on the hot plate.
One can has a small amount of boiling water in it, the other is dry. Both cans must remain
on the hot plate for about 3-5 minutes after vigorous boiling starts in the
one can. Why?
In any case bring along
something to read or someone to talk to while you wait for the fun to begin.
Using crucible tongs, first the dry can is placed mouth down into the adjacent basin of water. Upon cooling to water
temperature and while still upside down in the water basin, its opening is
"tightly" covered by a large rubber stopper. The can and contained water is
lifted out and set down upright. Make sure no water sucked in during the cooling is lost
during this process. Also you will need the water temperature.
Then in the same way, the boiling water can is placed mouth down in the room temperature water basin. A second
can with boiling water can be placed mouth down in an ice bath. Its
reaction can be compared with that of the first.
| Using Charles' Law, the three weights,
the density of water and a little logic, the average temperature
while still on the hotplate of the hot gas in the dry
can can be calculated to a reasonable approximation. For
this calculation, assume the atmospheric pressure remains constant
and ignore the vapor pressure of water. Also the density of water
can be rounded to 1.00 g/cc.
Charles' Law is part of the Ideal
where pressure (P), the number of moles
of gas (n) as well as the gas constant (R) remain constant.
What is this "constant pressure"?
As for the can(s) with the boiling water in it.......observe and explain..
- What is the initial
temperature of the water vapor in the can with the boiling water?
- Why does the can crush rather
than fill with water?
- What does the size of the
drinking hole in the top have to do with what happens?
- What effect does the
temperature of the cooling water have on the result?
How Accurate is this
Don't throw away your thermometers yet!
The biggest error is probably caused by ignoring the partial pressure of water vapor.
Depending on the water bath temperature, an error in pressure of up to 3-5% can be
made by ignoring the partial pressure of water at room temperature.
approximately 760 mm Hg
Vapor Pressure Water
@ 25oC = 23.76 mm Hg
Vapor Pressure Water
@ 0oC = 4.46 mm Hg
A 5% error in pressure translates into a 5%
error in the temperature.
This is a 5% error in the
This means at 100oC or so, the error is not 5o but almost 20o!
You can figure out for yourself if neglecting the vapor pressure of
water makes the calculated temperature too high or too low.
Smaller errors usually occur due to
some cooling while the
can is being transferred from the hot plate to the water
bath. This error can be big if you hesitate too long.
A very small error is due to not
leveling the liquid levels in the can and bath.
water at 25oC is about 0.996 g/cc, rather than 1.00 g/cc
Measuring the temperature
of the air in cans heated in an oven set at 127oC gave
temperatures of 120-125oC when measured by this method using
P1V1/T1 = P2V2/T2.
Atmospheric pressure was corrected for the vapor pressure of water and
the correct density of water was used.
To view some crushed cans, please click
When you are
finished, remember to.....................RECYCLE the ALUMINUM
© R.W. Kluiber 1/13/2000;