Law of Conservation of Mass?? We did a lab in my science class about the law of conservation of mass. We popped uncooked popcorn with oil, measuring the mass before and after. For my lab write-up, I have to explain why it weighed less after it was popped, but I don't understand why. Help? Thanks!! :)

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To add to the above answer, in terms of your results showing conservation of mass:

The popcorn itself now weighs less because the water has turned to steam, and is no longer weighed along with the popcorn. The mass still exists, though; if you could weigh the escaped steam somehow,...

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To add to the above answer, in terms of your results showing conservation of mass:

The popcorn itself now weighs less because the water has turned to steam, and is no longer weighed along with the popcorn. The mass still exists, though; if you could weigh the escaped steam somehow, you would find where the rest of the mass is. Even though it is invisible to you, the mass is still around.

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The reason that popcorn pops is because there's water inside.  When the water heats up and becomes steam, that puts pressure on the inside of the kernel.  When the pressure is put on, the kernel pops.

So why the lower weight?  Well, water is pretty heavy.  And so when the water boils off (popping the corn), the kernel has less mass than it used to.

Water makes up something like 14 percent of a kernel of popcorn.  Given that, you can see where the kernel would weigh quite a bit less with the water gone.

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