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Is popcorn popping a chemical or physical change?

When popcorn “pops” is it a chemical or a physical change?  It seems a bit tricky, but it’s a physical change.  Let’s explore why.

Popcorn “pops” when a tiny amount of water contained in the kernel is superheated and turns to steam.  When enough pressure builds up inside the kernel, it ruptures, and some starch previously contained within the kernel fuses together as it cools to form a fluffy solid.

Because the driving force behind this change is water being converted to steam (a change of state), and nothing chemically new is made, this is a physical change.

iStock_000015614224Small

Misunderstandings can arise when we try to use the common “clues of chemical change” as the sole deciding factor in classifying something as a chemical or physical change.  What really distinguishes them is whether or not something chemically different is made.

If so, then it’s a chemical change.  If not, it’s a physical change.

While that can be a bit difficult to discern just from observation, it really helps us to avoid some of the common traps that the clues of chemical change sometimes set for us.

For example, it is commonly noted that popped popcorn seems to have changed color, possibly produces a “gas”, and can’t be changed back to its original form.  Really, the color doesn’t change, as the popcorn kernel is just inverted—what used to be on the inside is now on the outside, thereby exposing the previously hidden color.  Also, a gas isn’t produced, only steam is produced.

Even if those were true, however, it wouldn’t necessarily mean that a chemical change is taking place.  These common observations can only give us clues about what might be happening, but in order to really classify a change, we need to know whether something chemically distinct has been made.

Consider, for example, adding Kool-Aid mix to water.  The color of the water changes, but it’s a physical change.

Kool Aid

Kool Aid by Bobby Chromik, on Flickr.

Or think about a block of dry ice exposed to open air.  A gas is produced, but again, it’s a physical change.  The carbon dioxide just sublimes (changes to a gas from a solid without passing through the liquid phase) to form the mist or “smoke” that we observe.

dry ice is fun

by Herkie, on Flickr.

Sometimes, the clues of chemical change are really handy, and they can help us to venture good guesses about whether something is a chemical or physical change, but in the end we always have to ask ourselves the question: was something chemically new made?  If the answer is no, then it’s a physical change.

You can test your skill at identifying chemical and physical changes using the link below.

http://www.quia.com/quiz/303980.html

Adam M. Boyd is a Sr. Education Associate in the office of K–8 science at the American Chemical Society. In his work at ACS, Adam focuses on chemistry, writing, and the web.

5 Responses to “Is popcorn popping a chemical or physical change?”

  1. phillmon smart says:

    good example

  2. Steve P says:

    Hi,

    I’m a middle school teacher and our students do a lab where we mix a solid tablet of alka seltzer with water and trap the gas to demonstrate physical and chemical changes. Generation of carbon dioxide gas from this is an obvious chemical change; the two reactants undergo a chemical reaction and one of the products is CO2. Since we are generating a product that was not there in the beginning in another state, I do not consider it a physical change.

    I explain that the other product is a solution of the medicine. However, some of the alka seltzer comes in flavors and causes the solution to turn red or yellow. Should these color changes be taught as both chemical and physical changes, or purely a physical change?

    Likewise, placing a few drops of food coloring into water is simple dilution. Do you consider it a physical change since you could recover the food coloring and no rxn has occurred?

    • Adam says:

      Hi, Steve. You are correct that an Alka-Seltzer tablet reacts with water to form carbon dioxide, which is dissolved into water as carbonic acid. This is a chemical change because something *chemically distinct* was made. The product that is formed is unlike either of the reactants.

      For other types of colored Alka-Seltzer, the process remains a chemical reaction, but you get a corresponding color change in addition. Although a change in color is one of the classic clues of chemical change, in order to truly classify something as a chemical or physical change, we ultimately have to answer the question: was something new made? In the case of colored or uncolored Alka-Seltzer being dropped into water, the answer is yes (carbonic acid was formed), and that is why it is a chemical change. Sometimes, concurrent physical changes may signal that a chemical change has taken place. That is the case here.

      Adding food coloring to water is a physical change because nothing chemically distinct is made. As you point out, we once again have a color change, but the key point is that we always have to consider whether or not something *new* was made in order to definitely classify something a physical or chemical change. Because nothing new is made when food coloring is added to water, this is a physical change.

      Although the classic clues of chemical change (color change, formation of a precipitate, formation of a gas, temperature change) are trusty, we can’t rely on them (or any combination of them) to classify changes. The only way we can say for sure that something is a chemical change is if something new is made.

      I hope this helps!
      Adam

  3. John says:

    Hi, can something be both a chemical and a physical change? What would be an example?

    • Adam says:

      Hi John. According the definition we’ve outlined in this post, no, something can’t be both a chemical and a physical change. If anything chemically new is formed in the products, it’s a chemical change. If nothing chemically new is formed, it’s physical change. This definition is exclusive and binary, such that a change may not be simultaneously chemical and physical.

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