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As anyone who's put marshmallows in the microwave knows, they expand a ton! Sometimes they puff up to literally more than twice their original size (YouTube video for those who haven't seen it).

So, why?
At first I assumed it was because they had a lot of air in them, but that doesn't make sense. There's no way that amount of air can puff up that much from the heat!

What makes marshmallows poof up so much when they are microwaved?

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Marshmallows expand so much because the water in them becomes steam, and gas takes up a LOT more volume than liquid. Specifically, 1 mL of water becomes ~1.36 LITERS of vapor, before it gets heated further. That's 1000-fold expansion, before you add additional expansion as the gas is heated.

Marshmallows don't have all that much water content, but when it's trapped in a stretchy gelatin matrix that holds gas readily, it only takes a bit to blow the whole thing up like a balloon.

You are correct that the gas expansion on its own is insufficient; unless I've badly muddled my calculations, gas expansion from 20C to ~150C (caramelization temperature) will increase the gas volume by under 50%.

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    Vacuum works as well as heat: youtube.com/watch?v=OHY9fFQhX68 Same principle, the water boils. You can tell the marshmallows leak vapor because when vacuum is released they return to less than original size. – Wayfaring Stranger Jan 23 '15 at 5:31
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Because the water becomes gas and takes up more space than a liquid, and then puffs up, and then when its not being heated anymore, it shrinks.

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    Try to explain your answer a little more – Sid Jan 31 '13 at 1:23
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Because the molecules inside the marshmallow moves faster and faster as the heat rises and the little bubbles of air inside the marshmallows grow bigger and bigger and then it expands. only 14 years of age

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    Welcome to Seasoned Advice! It turns out that this isn't the whole story - see BobMcGee's answer. It's not just air expanding, but also water expanding as it turns into steam. – Cascabel Jan 23 '15 at 0:48

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