Polystyrene foam revealed by a demonstration

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

There is an interesting article by Joe Schwarcz at the McGill Office for Science and Society on June 9, 2026 titled The Case of the Missing Coffee Cup. He describes a demonstration:

 

“It is a demonstration that never fails to draw oohs and ahhs from students. Pour a small amount of acetone into the bottom of a beaker and drop in a foamed plastic coffee cup. It instantly seems to melt into the liquid and within seconds just vanishes.

Of course, matter cannot just vanish, but it can change from one form into another. Like sugar dissolving in water, the plastic dissolves in the acetone. How can so much plastic dissolve so quickly in so little acetone? Because there is actually very little plastic in that coffee cup.

Yet that little plastic has been expanded by being filled with gas bubbles, much like blowing up a balloon. As the plastic dissolves, the air is released, and we have an apparent magical effect.”

I repeated it using a cubical sample of polystyrene foam insulation placed in a glass custard cup as shown above. When acetone was added, it became just a small puddle. That got even smaller after the acetone evaporated.

A Wikipedia page for Polystyrene has a section on Expanded Polystyrene (EPS) – commonly known as Styrofoam.