SAFETY: This is a really cool experiment, but it requires a few safety precautions. Acetone should only be handled by an adult and due to the fumes of acetone, it should be done outside.
This solvent is very flammable. Keep away from all flames.
Have your kids predict what will happen to the Styrofoam cup when you put it into Acetone? Acetone is (Nail Polish Remover). Sounds so much more cooler saying Acetone.
This is a tricky one. It looks like a chemical reaction because it is so dramatic, but actually it is a physical reaction. Acetone is the main ingredient in nail polish remover. The acetone, which is a solvent, causes the styrene in the Styrofoam to dissolve. This releases the air pockets that have been trapped. The substance left is still the same substance; it has just lost all of the air pockets.
If you put it in some kind of machine and whipped it around, blowing air into the substance it would puff up again. This is why we say this is a physical change. The substance did not change on a molecular level and in theory, it could be reversed.
Since Styrofoam is mostly air, you may be surprised and how much Styrofoam you can dissolve in one cup of acetone. A cup of acetone is enough to dissolve an entire beanbag worth of Styrofoam!