Popcorn Experiments

    Kids, what makes popcorn pop? This activity requires a bag of unpopped popcorn kernels divided in thirds. Two days prior, place 1/3 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. Shake from time to time. After 2 days the corn will absorb all of the water and the kernels will appear dry. Spread 1/3 of the original kernels on a cookie sheet and have an adult partner warm them in an oven at 200° F for 2-3 hours. Keep the last 1/3 aside as your control.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    Put 1 tablespoon of oil in a corn popper with 1/3 cup of the control unpopped popcorn. Listen and watch as the corn pops. Notice 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 one of 3 equal-size bowls labeled "control" or "regular popcorn."

    Next pop the 1/3 roasted popcorn with 1 tablespoon of oil. First guess what will happen. Do they pop a lot more quietly? Put this popcorn into a bowl labeled "dried popcorn." Now 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 it in a bowl labeled "water-added popcorn."

    Next measure which popcorn pops best. Do this by counting 20 popped kernels of each type into a clear glass and measuring the height of the column of the popcorn with a ruler. The results should be:

    • The regular popcorn is the largest.
    • The dried popcorn is slightly smaller.
    • The wet popcorn is very much smaller.

    What’s going on here? 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 is 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 ruptures. This foam cools quickly to become the firm white mass that we like to eat. Eating your results is perfectly fine – bon appétit!


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    January 2010



    Vicki Cobb’s book Junk Food, which is part of her "Where's the Science Here?" series published by Millbrook Press.