Theory

Expanded polystyrene articles are manufactured from polystyrene granules that incorporate a blowing agent - a substance which, when heated, gives off a gas.

This may be a volatile liquid (such as pentane) or a carbonate.

These granules are then steam-heated and the gas from the blowing agent produces a foam. This gas is eventually exchanged with air. Thus the gas in the solid foam is largely air.

The expanded polystyrene does not actually dissolve in propanone; it merely softens and, visibly, allows the air to escape thereby collapsing the foam.

Extensions

The resulting gel of polystyrene can be left to dry out altough this takes some time.


Safety

Wear eye protection.
Dispose of the resulting gel by decanting any remaining propanone down the sink and flushing it away with water. The gel can be poured into a throwaway container such as a screw top coffee jar and placed in the solid waste bin.