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Activity

Popcorn

Summary: 
Make popcorn, and learn about the science of popcorn and popcorn-popping: Find out what makes a popcorn pop/popcorn kernals as seeds on a corn cob.
Science content (2016 curriculum): 
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
  • optional: dried corn kernel to observe, if learning about seeds
  • dixie cups to distribute popcorn in
Procedure: 

Compare popcorn kernels with the kernels on a corn cob. (If learning about seeds.)

While the popcorn is popping, talk about what is happening:

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.

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. Then gather to view each others' skits.

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

Show students slow motion videos of popcorn kernel popping (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

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.

Grades taught: 
Gr K
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Teacher: 
Ingrid
Julie Johnson
Mona Francis
Ramona Smith
Sonia Ko
Sonja Watson
Teaching site: 
Bayview Elementary Science Club
Champlain Heights Annex
General Gordon Elementary
General Gordon Elementary Science Club
ingridscience afterschool
JEMZ+ After school science
Laurier Elementary
New York Botanical Garden
ProD for Elementary teachers
Activity originally developed and delivered: 

New York Botanical Garden