Posted by: tpsciencefun | February 2, 2011

Is Popping Corn a Chemical or Physical Change!?

Is popping popcorn a physical or a chemical change?


Heat and pressure cause a physical change that results in popcorn.

The physical changes that occur when making popcorn is the same whether you put the kernels into the microwave, an air-popper or a pan of hot oil. The white, fluffy popcorn that we enjoy is created during heating when the inside of the kernel, known as the endosperm, blasts through the outer shell, called the pericarp.

How does heat cause this explosion? When the kernel is heated, some of the water (H2O) trapped inside the endosperm turns into steam. Molecules in the gas phase (steam) have more energy and collide more frequently with the outer shell creating pressure. The endosperm also contains starch. When the kernel is heated, the starch becomes more fluid like a jelly. At a certain pressure and temperature, the pericarp will suddenly break open and releases the pressure, steam, and starch. Rapid air cooling and drying of the starch creates the fluffy shape of popcorn. To summarize, making popcorn results in major changes to the physical shape, and the release of water, but there is no major change in chemical composition.

It is no secret how to get the most popped corn from your kernels; you need just the right amount of water in the endosperm and a robust pericarp. You can try some simple experiments at home to test these principles. In the first experiment, use a small sharp object (like a sewing needle) to puncture the outer shell of kernels. Can you still make popcorn? Since the holes allow the steam to escape you should not observe popcorn. In the second experiment, warm the kernels in an oven at 200°C for 90 minutes before trying to make popcorn. What did you observe? Very few of these kernels should pop because drying in the oven removed most of the water.





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