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Summer of STEM: Disappearing Cups

Posted 07/09/2018 by Erick Archer

How do you make a foam polystyrene cup disappear? Put it in an organic solvent such as acetone!

Important note: Doing the project in a well ventilated space and using gloves are smart ways to avoid unnecessary exposure to acetone. Only use enough acetone to cause the cup to dissolve. Follow your school’s chemical safety guidelines at all times when using this or any chemical available to the school’s science department.

What you’ll need:
  • Foam polystyrene cup(cups or packaging material)
  • Container (pie plate, large bowl or bucket).
  • Acetone
  • Gloves and goggles
What to do:
  • Pour the acetone into the container to about ½” depth.
  • With gloves on, add the cup to the liquid while applying a little downward pressure.
  • Notice the polystyrene is dissolving into the acetone.
How it works:
  • Polystyrene is made up of monomers of styrene, an organic compound (C6H5CH=CH2).
  • When polystyrene is being produced it exists in a liquid state (like a gooey, thick liquid).
  • As polystyrene is produced, air is injected into the mixture creating small balls that are pressed together to form shapes like cups and the packing peanuts you use to keep your items safe in shipment.
  • When polystyrene is added to acetone (an organic solvent), the chains between styrene molecules are broken – dissolving them and releasing the air that is trapped inside.
  • You may notice that the polystyrene actually forms a solid structure in the acetone since the air has been released; the styrene re-bonds to make a hard dense plastic. Polystyrene without air can be used to make plastic forks, spoons and knives!

Below, see a video of this experiment and others in action: 

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