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Name: Marlene K.
Status: student
Age: 50s
Location: N/A
Country: N/A
Date: 9/22/2004


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?


Replies:
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)


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


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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