Popcorn Is A Gas

Katz, Linda                       Orr Academy                                   
                                  265-5000
                          

Objectives:

Students will understand why popcorn pops.
Students will be exposed to questioning how Chemistry/science is 
   integrated into everyday affairs.
Students will learn to take concept and apply to other phenomenon.
Students will understand how pressure, volume, and temperature are 
   related.

Apparatus needed: 

Popcorn popper (any type)
popcorn
sharp knife to cut popcorn
petri dishes or paper muffin cups 
balance
hot plate
250 ml Erlenmeyer flasks

Recommended Strategy:

Preplanning:
Cut kernels of corn - some longitudinally, some transversely (cutting
just the bottom tips off will be fine.  Make sure you save all the 
pieces.)  About ten (10) of each will do the trick.
Have popcorn popping/popped as students enter the room!

Intro:
If you allow students to eat in your lab class, pass out a handful to 
each student.  Elicit comments from students about popcorn.  It's tasty, 
fun to eat, reminds them of movies, etc.  Then ask: 
What causes popcorn to pop??
Pass out unpopped kernels for them to compare with popped kernels.  
Have them make observations comparing the two.  Remind them they are 
looking for clues as to how one gets from the unpopped to the popped 
stage. 
Have a student make a list on the board of observations/suggestions what 
causes the popcorn to pop.  Write down all of students ideas. Have 
students review list and decide which they think is a plausible cause 
for the pop.  If need be, coax them to think about what is inside the 
kernel. 
Once they have started to think about the internal kernel, ask what 
should be tested for and how.  One thing students will probably 
recommend doing is massing the kernels before and after popping the 
corn.  Have students predict which will mass more and why.  Then get a 
few students to volunteer to mass the kernels before and after.  (The 
popped corn will mass significantly less due to a loss of water.) 
Test the differently cut kernels to see which pop.  Do each 'type' of 
kernel in different flasks on a hot plate.  You will find the only ones 
that do not pop are the longitudinally cut kernels.  From this discussion 
you will be able to discuss:  There is water inside the kernel, which 
when heated increases the movement of the molecules.  As the molecules 
move, they increase the pressure on the sides of the kernels, so that 
the volume must increase - therefore the popping!! 

References: 
CHEM MATTERS,October, 1984.

Strongin, Herb.  Science on a Shoestring.  Addison Wesley: Menlo
Park, California,  1985.