Popcorn is one of the most well-loved and well-known movie snacks out there. It is enjoyed outside of the theater as well as inside and there are so many different versions of it. There is caramel popcorn, cheese-flavored popcorn, plain salted popcorn, and so many other creative versions of it. One interesting aspect of popcorn is the change that happens when it pops.

Popping popcorn is a physical change. A tiny bit of water in the kernel heats up and turns to steam. As it gets hotter, the molecules in the water speed up and expand until they burst the outer shell of the kernel and the starchy inside of the kernel explodes out.

The process of popcorn popping is really interesting and there are so many different parts of it. There is a lot that goes on beneath the shell of the kernel when it is heated up, so keep reading to learn more about it!

Chemical vs. Physical Changes

Before we talk about how the popcorn pops and what happens on a deeper level, we need to explain chemical and physical changes and how they are different. Chemical and physical changes are present in many different situations, and although popcorn popping is only a physical change, it is important to understand the difference between the two.

Chemical Change

A chemical change is a change that happens when a new substance is formed. This means that after a chemical change happens, whatever substance you started with will be something that has a different chemical compound than it had at the beginning of the process. Sometimes a change in appearance is mistaken for a chemical change while it is actually a physical change.

Some examples of chemical changes are burning wood, digesting food, baking a cake, cooking an egg, and iron rusting. All of these are chemical changes because they each have a chemical reaction that turns them into something with a different chemical composition than they started with.

When a cake is baked, all of the ingredients mix together and heat up to form new substances that have a different texture because of the chemical reactions. When iron rusts, the metal oxidizes and becomes rough and rusty. It changes its chemical composition and becomes something else with the introduction of oxygen and moisture to the iron molecules.

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Physical Change

A physical change is what happens when the substance has the same chemical compound after changing. No new chemicals are created and no chemical reaction happens. Changing the state of a chemical compound is a physical change even though it often gets mistaken for a physical one.

An example of this is ice melting or water turning into steam. While the state of the compound changes, it is still a physical change.

Some other examples of physical changes besides changing states are crumpling aluminum foil, casting metal in a mold, breaking glass, carbon changing from graphite into a diamond, and the evaporation of dry ice into carbon dioxide.

All of these changes do not alter the chemical composition of the original substance. While they might change in color, shape, or state of matter, they are still made of the same chemical compound.

When graphite changes into a diamond, it is still just made of carbon even though the appearance has changed. The carbon molecules have taken on a different shape on a chemical level, but their composition is the same.

When molten metal is poured into a mold and hardens, it is still the same chemical composition as it was before, it is just a different shape and state.

The other clear indicator of a physical change is that it can be reversed. Not all physical changes are easy to reverse, some are not possible with today’s science, but theoretically, they could be changed back into their original form. An easier example of this is when ice melts, you can always freeze it again.

Changes within the Kernel

Now that you understand the difference between a chemical and physical change, we can explain how this is applied to popcorn popping. When popcorn pops, there is no chemical change involved, everything that happens is only physical. Unless you understood that a change of state is a physical change, it would seem like there is a chemical change as well.

To reiterate the basics of popcorn popping, there is a tiny bit of water inside the kernel when it is put in the pan, popcorn popper, or microwave. This tiny bit of water turns into steam as it heats up.

The water molecules in the steam move faster and faster and begin to expand. Eventually, the outer shell of the kernel is burst open by the inside of the kernel and then the popcorn is done!

The Physical Changes of Popcorn

Let’s analyze the changes that happen when popcorn pops. The small amount of water inside the kernel heats up and turns into steam. A change of state, like water into steam, is a physical change even though the form is different from the original.

When the molecules of the steam heat up, they move faster and expand this is still a physical change because the molecules are still just water molecules. There has been no chemical reaction.

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Once the expansion of the steam is too much for the shell of the kernel to handle, it will shatter and the inside, which is made of starches and other chemicals, will burst outwards. This is the “pop” of popcorn!

The bursting and shattering is a physical change as well. While the shell of the kernel is shattered and the insides are a different shape, the chemical compounds are still the same.

All of the changes that happen when popping a kernel are physical and they can all be reversed. The steam can be turned back into water, the broken shell could be put back together, and the starches that burst outward can be condensed back down to their original size.

While it takes a lot of work to reverse popping a popcorn kernel, it can be done, and is, therefore, a bunch of physical changes.


Victoria State Government

Torrey Pines Science

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