If heat makes things expand why does plastic shrivel up when exposed to it?
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Heat usually makes things expand because it’s making the molecules of the thing you’re heating more energetic so they spread out a tiny bit.
This isn’t always the case though, depending on how those molecules are arranged. Sometimes the molecules are already spread or stretched far apart, and heating them loosens them up enough to pull back together.
Plastic is made of really long molecules that look like chains (polymers). When we make stuff out of plastic we usually stretch and push those chains and mold them into the shape of whatever we’re trying to make because the chains are strong even when they’re spread over a wide thin area (like plastic bags or bottles).
When you heat up a plastic bag or bottle, those long chains loosen up and pull each other back together so the plastic seems to take up less space because it used to be spread out in the shape of a bag or bottle, but now it’s all blobbed together.
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