[mike]

I have been working on an experiment to investigate the ideal gas law using popcorn kernels.

Basically the experiment goes:

Weigh popcorn kernels, determine their volume, heat them until they pop, weigh them again.

Use PV=nRT to approximate the internal pressure of the kernel as it pops (knowing V, R, T, and n(from mass difference)). Unfortunately this approximates a pressure of about 200-300 atm which is too high.

The"literature" states that popcorn should pop at 175C and 9atm. I would realy like to know how they determine this (to me it looks like they have just taken the boiling point of water at 9atm, ie 175C).

I am aware that water is not an ideal gas and that there are assumptions being made regarding the volume and mass etc. Using van der Waals equation of state still only gets me to 40-90 atm.

Any ideas? I have access to some equipment if people can think of another way to test this (or a more interesting way to show ideal gas law in the vain of consumer chemistry).

Cheers,

Mike

[themark]

hello,

probably a very crude method to do this would be to put a kernel(probably repeat a few times to get an average) under a press with some kind of pressure read-out and thus get a poping pressure.  then, use a furnace fitted with a thermocouple and glass window to determine kernel poping temperature.

I would treat this problem as a constant volume and mass system before rupture; ideal gas equation reduces to

P2(poping pressure) = P1*T2/T1
assumptions:
P1= 1 atm; T1= 25 C and T2= 175 C (from your post)

P2 = 1.5 atm

I think the listed literature result was obtained using the steam tables and making assumptions about a closed system and constant volume.

hope this helps

[Dude]

I agree with your logic and assessment that the literature listed pressure is too low.  Do you have more than one literature source?

1.  Need to know the volume of the popcorn kernel and the mass percent water contained in the kernel (typically 12-14 % H2O) - might be obtainable by before and after popping weights if all of the gas coming out of the kernel is steam.

2.  As themark indicated you will need to determine the temperature just before popping from a thermocouple.

3.  The volume is constrained to the volume of the kernel and the temp is from 2.  Convert the mass of water into a number of moles and assume that it is all vapor before popping.  Solve for pressure.

[mike]

Hey thanks guys. This is a tricky one as you can see. I have actually done the experiment about 4 time with the same results each time. I weigh by difference for mass of water, measure displacement for volume and measure temperature at time of popping. I have also done gravimetric analysis of the kernels to get the water content.

I also though maybe put the kernel in a vacuum chamber and determine the pressure the kernel ruptures at room temp.

I think the trick is that only a very small amount of the water content is turned to vapour. ALso it is not an "ideal" gas as the equations require. ALso the volume is a tricky one because the kernel is full of starch etc.

Its an interesting one though, if you have anymore thoughts let me know.

Mike

[Dude]

Is the expansive (tensile) strength of a popcorn kernel the same as it's compressive strength?  Perhaps another approach would be to measure how much compressive force (using a Carver laboratory press for example) is required to rupture the popcorn kernel at some temperature.  I checked Google and it appears that the bite strength of the average human is in the 150-200 psi range.  Converting the 9 atm to psi yields about 130 psi, which is consistent with the observation that most people can rupture (with pain) popcorn kernels with their teeth.  The pressure calculated by the ideal gas law would make popcorn kernels impossible to chew (assuming a popcorn kernel is somewhat isotropic)  

Tires and insulating glass windows (if you live in an area with cold winters) are good for demonstrating the ideal gas law in consumer products.

[mike]

Hey great work, that is some interesting thoughts there Dude. Yes I agree that the pressure calculated by the ideal gas law is too high for a lot of reasons, that is clever finding the bite pressure, good one!

hmmm insulating windows, I will have to investigate that one (living in adelaide, basically a desert there is noth much need for these )

[mike]

Update: Well today is the first day that the "popcorn" experiment is running and already I have TA's coming in with problems, mainly due to poor choice of equipment for the experiment and my guess is they are using the old popcorn from when I first tested this experiment. Anyone ever done this experiment?

Second update: Well they did the experiment and it went really well, students and demonstrators enjoyed it.

[Dude]

Mike,

I was surveying some journals today and came across this.

March 2006 issue of the Journal of Chemical Educations.

http://jchemed.chem.wisc.edu/HS/Journal/Issues/2006/Mar/clicSubscriber/JCESupp/JCE2006p0414W.pdf

[mike]

That is wicked man, thanks a lot, you are a legend, I missed that article.

mike