Ask A Scientist Chemistry Archive

 
name         Marlene K.
status       student
age          50s

Question -   When acetone is applied to Polystyrene foam, it "melts" or
changes form and is not reversible.  Is this a physical change or a
chemical change? Please explain.  Are any bonds being broken and any new
products being formed?
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Marlene,

Polystyrene foam is really just polystyrene (the same material you find in the
windows of some mailing envelopes) that have been processed to contain 
pockets
of gas (foam). Applying acetone to Polystyrene foam does not melt it, but rather
dissolves the material. You could just as easily apply acetone to the plastic
windows of mailers and see that the plastic dissolves there too. It is simply
more spectacular when applying acetone to Polystyrene foam because the gas pockets
collapse, the solvation is quicker (because of the larger area that comes 
into
contact with the solvent), and a large volume appears to dissolve into
nothing. However, if you were to weigh a piece of Polystyrene foam (and get a 
similar
mass of mailing envelope window), you will quickly realize just how much
material we are talking about here. I am sure you know whether a solvation
process is a chemical or physical change.

Greg (Roberto Gregorius)
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Polystyrene foam as the suffix "-foam" implies is a foam of gas in a matrix of
polystyrene, which is soluble in many organic solvents -- acetone in
particular. When the matrix softens and dissolves it cannot be re-formed.
It is similar to what happens to the foam on a soda or a beer. Once the
matrix that is holding the space together disappears, it's gone for good.
If you need to classify this, I suppose it is closer to a physical change,
but there is a spectrum of changes and some might call it a "chemical"
change since the styrene dissolves.
Vince Calder
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Marlene,

It is a physical change. The dissolving process simply causes the foam 
matrix to collapse, the blowing agent escapes -- some dissolves in the 
acetone, and a solution of polystyrene results.

The "irreversibility" observation is grounded in the apparent difference 
between the "before" and after" condition of the Polystyrene foam. Though it 
wouldn't be easy to accomplish in a home lab, the acetone/Polystyrene foam 
solution could be processed and re-blown into a foam.

Regards,
ProfHoff 915
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