You and Your Students!
Vicki Cobb, Education World Science Editor
Popcorn teaches history, physics, and the process of science.
This lesson requires a bag of unpopped popcorn kernels prepared as follows:
--- Spread on a cookie sheet 1/3 of the kernels. Heat in a warm oven (200 F) for 2-3 hours.
--- Two days before the lesson, place 1/3 of the popcorn in a plastic container with two tablespoons of water. Put the lid on and shake the kernels so that they are all coated with water. Over the next two days, give the kernels a shake from time to time. All the water will be absorbed by the corn, and the kernels will appear dry when it is time to do the lesson.
What makes popcorn pop? I recently presented a lesson in which we explored that question to several classes of second graders. It worked so well that I'm happy to share it with you.
Popcorn originated with the Indians of Mexico and the southwestern United States; it was the first way people ate corn. To start this lesson, I showed two ears of unpopped popcorn on the cob. (I bought it from http://www.bigredpopcorn.com/.) That helped students to see how popcorn grows. You might be surprised how many student do not make that connection!
We popped one ear of the popcorn in the microwave. I popped the corn as a demonstration. We discussed what makes popcorn pop -- and we were on our way to a lesson in science.
Put 1 tablespoon of oil in a corn popper with 1/3 cup of the unpopped popcorn (right out of the package). This is your control experiment. Have the kids listen and watch as the corn pops. Point out the condensation that forms on the inside of the popper. That condensation is proof that moisture in the seeds is responsible for the explosion. As that moisture changes into a gas, it makes the corn pop. Put the popped corn into a bowl labeled "control" or "regular popcorn."
Next pop 1/3 cup of the oven-dried popcorn with 1 tablespoon of oil. Have students guess what will happen. Those kernels pop a lot more quietly. Put this popcorn into another bowl you have labeled "dried popcorn."
Next pop 1/3 of the popcorn that has water added. You are in for a surprise! The popcorn is explosively loud, and the popped corn is fragmented and very small. Put that popcorn in a bowl labeled "water-added popcorn."
After all the corn is popped, have students measure which popcorn pops best by counting 20 popped kernels of each type of popcorn into a clear glass and measuring the height of the column of the popcorn.
I had the students work in groups of two or three to measure the popped kernels. I had other adults helping me; at the second grade level, it took a lot of "floaters" to help the kids get their results.
We measured the popcorn in centimeters.
When all the measurements were recorded, we tallied the every groups' data and figured the mean (average) for all the groups.
Popcorn is the result of an explosion. Water inside a popcorn kernel must be heated to about 450 F (232 C), at which point the pressure is about 135 pounds per square inch (or 9 times atmospheric pressure). The tough outside hull of the kernel acts like a watertight container, keeping the steam confined. Since the water is spread throughout the soft starch of the kernel, the expanding steam makes tiny bubbles in the hot starch. Pent-up steam builds up in pressure, putting more and more force on the hull until it can't take it anymore and it ruptures. This foam cools quickly to become the firm white mass that we like to eat.
Popcorn is not the only seed that pops. You can buy Amaranth, food of the Aztecs, at some health food stores. Kids will find the popping of these tiny seeds very interesting. And since they are seeds, you can also experiment with sprouting them. Add water and the seeds will grow tiny root hairs in about a week.
For more information about popcorn, see Vicki's book Junk Food, which is part of her "Where's the Science Here?" series published by Millbrook Press.
Article By Vicki Cobb
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