Where did the coffee cup go?

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.

That demo is a great way to get into the fascinating story of polystyrene, a plastic that when foamed is used to make coffee cups, packaging peanuts and home insulation materials. In its rigid version, it is found in clear plastic glasses, disposable cutlery, laboratory Petri dishes and toy model kits.

Our story begins in 1839, when Berlin apothecary Eduard Simon distilled an oily substance from the sap of the sweetgum tree and days later found that it had turned into a thick jelly-like material. He assumed that the oil had reacted with oxygen in the air, but by 1845, German chemist August Wilhelm von Hofmann had shown that the reaction would occur even in the absence of air.

After the concept of molecules had been introduced by Englishman John Dalton, and........

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