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