A Styrofoam coffee cup, or a Styrofoam peanut is placed in a clear liquid. The coffee cup slowly melts, reminiscent of the wicked witch in The Wizard of Oz.
StyrofoamŪ is the trade name for polystyrene, a polymer made from styrene. Polymers are long chains that intertwine with each other, creating a plastic material. By itself, polystyrene is a hard plastic used often for packaging, like many of the plastic bottles you find in the grocery store. It's also used for model airplanes. But when a gas is bubbled through it as it is polymerizing, it produces a foamy, compressible plastic, the kind you find in things like coffee cups, coolers, and building insulation.
When a polystyrene cup is placed in acetone (found in nail polish remover and in lacquer thinner), the acetone sort of serves as a molecular "lubricant" between the polymer chains, allowing them to slide around each other. The Styrofoam becomes soft, releasing the air bubbles trapped in the foam, and the polystyrene ends up as a soft blob in the acetone. When the blob is removed and the acetone is allowed to evaporate, it solidifies into a piece of hard plastic.
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explanation furnished by UCCS's Dr. David Anderson