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(Watch our video below to see this project in action.)
What to do:
When the water inside the test tube reached boiling point, it changed into water vapor or steam. The vapor expanded in the heat and tried to escape, exerting pressure on the balloon until it inflated. If you left the water boiling for a few minutes, the pressure got so great inside the balloon that it exploded.
This is what happens to popcorn. Inside each kernel is a little bit of water, making up about 13-15 percent of the whole kernel. As the popcorn heats up, the water inside the kernel changes to steam and exerts great pressure on the outer covering of the seed, called the hull. (Unlike the balloon in our demonstration, the hull doesn't expand with the air inside it.) The pressure builds until the kernel explodes, shattering the hull and allowing the steam to escape. As it does so, the inner starchy parts of the kernel puff out with air and expand into the fluffy popcorn we like to eat.
Check it out: put a single kernel of popcorn in a dry test tube and cover the mouth of the test tube with aluminum foil. Hold the test tube over a flame, shaking it a little bit periodically to keep the kernel from burning. (Keep the test tube pointed away from you!) After a few seconds the kernel will pop, releasing the steam from inside. You'll see the steam condense on the sides of the test tube. (If your kernel doesn't pop, it might be because there was not enough water inside. If it doesn't pop but you see steam condensing on the sides of the test tube, the kernel has a hole in the hull, so the pressure can't build enough to make it pop.)
See this project in action with our video:
Now that you know what makes popcorn pop, do some experiments to see if altering the kernels will cause them to pop faster or bigger. Since water inside causes them to pop, would they pop better if you soaked the kernels in water first? What if you pre-heated them before trying to pop them? (Don't let them get over the boiling point, 212 degrees Fahrenheit.) What if you used a needle to poke a hole in the kernels before trying to pop them? Does microwave popcorn pop a higher percentage of kernels than popcorn popped over the stove or in a popper? Predict what you think the result will be with each of these tests and then see if you're right or not. Can you explain what happens in each test?
Producing popcorn is a precise business. The kernels can't be cracked or broken and must be dried to contain a very specific amount of moisture. If they contain too little they won't pop, and if they contain too much the popcorn will be dense and chewy rather light and fluffy.