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The Sound of Science - "Marshmallow Explosion"

J: Welcome to The Sound of Science on WNIJ. I’m Jeremy Benson from NIU STEM Outreach, and I’m in the studio with Kate Powers. Kate, today we have a question from Lydia that is making me laugh just thinking about it. Lydia wants to know why Peeps marshmallows expand so much when you microwave them.

K: I love this question Jeremy! Not only is it fun to blow up a marshmallow by nuking it, but it is a great example of one of the basic laws of chemistry.

J: Microwaving a marshmallow demonstrates a basic law of chemistry? That seems…strange.

K: Well, in chemistry there is a set of relationships called the ideal gas law. The ideal gas law describes how gases react to different types of changes.

J: But a marshmallow isn’t a gas Kate.

K: But it kind of is! A marshmallow is a foam. It is a sticky candy that is whipped up and filled with lots of tiny air pockets. Those air pockets expand in a microwave. As the heat expands the pockets, the air pushes out on the walls of sticky candy, expanding the candy until the marshmallow is big and bloated.

J: Oh, I suppose there is a lot of air in a marshmallow. But why does heat make the gas particles expand?

K: When I imagine gas particles reacting to temperature I like to imagine kids on a playground at recess. If their teacher makes them go outside for recess in really cold temperatures the kids are more like to huddle together and not run around much. They will group together to stay warm and take up less space. Now on a hot day those same number of kids on the same playground will run around and take up all the space on the playground. It’s the same with gas particles in a marshmallow.

J: Okay, so I understand that gas expands due to warmer temperatures. But when I roast a marshmallow on a camp fire it doesn’t blow up the same way as the marshmallow does in the microwave. Both marshmallows are heating up, but they don’t react the same way. Why?

K: Another great question! Microwaves are able to penetrate about 1 cm into the food, warming the food from the inside as well as the outside. It is this warming of the inside of the marshmallow that makes the all the air pockets expand, not just the ones near the surface.

J: That makes sense! Now if you excuse me, I need to find a marshmallow and a microwave.

K: Okay Jeremy, have fun. I’m Kate and Jeremy just left to go microwave a marshmallow. Keep your questions coming to us at stemoutreach@niu.edu. This has been the Sound of Science on WNIJ, where you learn something new every day.  

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