CHEMISTRY IN FOCUS
Snacks Need Chemistry, Too!
Have you ever wondered what makes popcorn pop? The
popping is linked with the properties of gases. What happens when a gas is heated? Charles’s law tells us that if
the pressure is held constant, the volume of the gas must
increase as the temperature is increased. But what happens if the gas being heated is trapped at a constant volume? We can see what happens by rearranging the ideal
gas law (PV nRT ) as follows:
Pa
nR
bT
V
When n, R, and V are held constant, the pressure of a gas
is directly proportional to the temperature. Thus, as the
temperature of the trapped gas increases, its pressure also
increases. This is exactly what happens inside a kernel of
popcorn as it is heated. The moisture inside the kernel vaporized by the heat produces increasing pressure. The
pressure finally becomes so great that the kernel breaks
open, allowing the starch inside to expand to about
40 times its original size.
What’s special about popcorn? Why does it pop while
“regular” corn doesn’t? William da Silva, a biologist at the
University of Campinas in Brazil, has traced the “popability” of popcorn to its outer casing, called the pericarp. The
Popcorn popping.
molecules in the pericarp of popcorn, which are packed in
a much more orderly way than in regular corn, transfer
heat unusually quickly, producing a very fast pressure jump
that pops the kernel. In addition, because the pericarp of
popcorn is much thicker and stronger than that of regular corn, it can withstand more pressure, leading to a more
explosive pop when the moment finally comes.