Activity

Popcorn

Summary
Make popcorn and learn about the chemistry of the popping.
Science content
Chemistry: States of Matter, Properties of Materials (K-7)
Chemistry: Physical Changes, Solutions, Mixtures and Separating (2, 4, 5, 6)
Materials
  • popcorn (1/2 cup in a medium pan, 1 cup in large casserole-sized pan)
  • vegetable oil, two tablespoons per half cup of popcorn
  • popcorn maker, microwave, or pan and stove top/hot plate
  • hot pad to put pan on
  • oven gloves
  • optional: larger bowl to dump popcorn in
  • sprinkle of salt, to flavour popcorn if necessary
  • optional: dried corn kernel to observe, if learning about seeds
  • dixie cups to distribute popcorn in
Procedure

While the popcorn is popping, talk about what is happening to make this food:

Each kernel has some water in it. (Corn that is used for popcorn has just the right amount of water: 13.5%.) As the hot oil heats up the kernal, the water inside it evaporates to form a gas (water vapour). The shell is strong and watertight so the gas cannot escape.
As the heat increases further, the water vapour molecules move around more and more vigorously, exerting more and more pressure on the inside of the shell. Eventually the pressure inside the kernel is great enough to burst the shell. (This happens at about 180 centigrade, when the pressure inside is 135psi.)
As the shell bursts the pressure suddenly drops again. This causes the water vapour to expand which makes the starch and proteins inside the kernel expand into an airy foam.

Students can inspect the kernels as they eat them, and see that the shell turns inside out from the force of the explosion.

If students have done some acting out the sates of matter already, they can do skits in small groups on what is happening inside the popcorn to make it pop - each student can be a water molecule, or the kernel shell, or a narrator. Gather to view each others' skits.
(Students can prepare their skits as the popcorn pops if it takes a while.)

Show students slow motion videos of popcorn kernel popping (maybe while they eat their popcorn):
http://www.youtube.com/watch?v=CXDstfD9eJ0
http://www.youtube.com/watch?v=NCSr18vtjeo
http://devour.com/video/popcorn-popping/
good video, but avoid the subtitled language near the beginning by starting at 40 seconds: https://www.youtube.com/watch?v=4rPMIkN5fR4

For a lesson on heat:
Popcorn can be made as part of a lesson on heat. Students brainstorm whether the popcorn is being popped as a result of conduction, convection, radiation or all of them.

For a lesson on seeds:
Compare popcorn kernels with the kernels on a corn cob.

Notes

Using my cast iron pot, preheated for 5 mins, it takes 35 to 40 mins for the popcorn to pop.
The lesson plan I have settled on: Act out states of matter. Start popcorn heating while watching a couple of minutes of videos of popcorn popping. Ask students to group up and prepare skits of popcorn popping. Watch the skits while popcorn finishes popping. Eat popcorn while watching videos again, and/or discussing other popcorn/states of matter topics.

Grades taught
Gr K
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Teaching site
Bayview Elementary Science Club
Brock Elementary
Champlain Heights Annex
Gordon Elementary
Gordon Elementary Science Club
ingridscience afterschool
JEMZ+ After school science
Laurier Elementary
Lord Roberts Elementary
New York Botanical Garden
Shaughnessy Elementary