Thermodynamics

I asserted to my daughter last week that a paper cup with water in it will not catch fire if placed directly in a fire. So, of course, we had to try it.

I was a little nervous, but it turns out this is completely true. The cup burned down to the water line, and then didn’t burn until the water had completely boiled off. The *instant* the last of the water boiled off, the cup burst into flames and was gone almost immediately. (Animated version of image is here.)

Why? Well, it’s because water boils at 212°F (100°C) and paper combusts at 451°F (843.8°C) so as long as there is water in the cup, the heat of the cup is being convected away into the water to heat it towards boiling and the cup remains too cold to ignite. Once the water starts boiling, the cup is full of steam, which is quickly carrying away the heat. The moment the water has all evaporated, though, the cup is abruptly at combustion temperature and goes up in a flash.

You should try it. It’s a great way to impress your kids. Or win a bet.

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