K-6, Middle school, High School
Polystyrene is a polymer that can be processed with air bubbles to produce Styrofoam, that amazingly light weight material that keeps the contents of our packages safe in the mail and our coffee warm in the winter.
A polymer is long, chain-like, molecule composed of the same unit, styrene in this case, repeated many times like beads on a string.
In this experiment, you will partially dissolve polystyrene using the organic solvent, acetone.
SUPPLIES, TOOLS, MATERIALS
- A wide mouth glass container or large beaker
- Styrofoam cups or packing peanuts
- Stir rod or disposable spoon
- Disposable Aluminum tray or pie pan
*available in the solvent section of most hardware stores
Acetone, being an organic solvent, might dissolve or mar just about anything made of plastic. Be careful when working on laminate flooring and be especially careful with cell phones or any electronic devices having plastic parts or covers!
Incidental contact of acetone with your skin is perfectly safe. In fact, you might notice that it feels very cold to touch (why?). You should avoid extended contact or submerging your hands in the solution.
As with any time you work with chemicals, be sure to wear eye protection and wash up before you eat.
- Fill your glass container with about 200 mL of acetone.
- Drop in your Styrofoam cup or packing peanuts.
- Record your observations.
- Once dissolved, you can transfer the semi-solid polystyrene to a disposable aluminum tray or pie pan, shape it as you wish, and allow it to dry overnight.
- In the morning, you should find a glassy, relatively dense (compared to Styrofoam), polystyrene sculpture. What a great way to re-purpose a used cup!
WHAT IS HAPPENING?
Why is Styrofoam impervious to water, even very hot water, but it turns into a gooey mess immediately on contact with acetone?
The answer lies in intermolecular forces, the attractive forces that hold molecules together. These are the forces that determine the temperature at which a substance melts or boils. These are also the forces that determine which solvents will dissolve which substances.
Whenever a solvent (like acetone) comes into contact with a solid solute (like Styrofoam), there are three types of intermolecular forces at work:
- How attracted is the solute to itself? These are solute-solute interactions.
- How attracted is the solvent to itself? These are solvent-solvent interactions.
- How attracted is the solute to the solvent? These are solute-solvent interactions.
In this case, the interactions between styrofoam and acetone are stronger than those between acetone and itself or Styrofoam and itself. Thus, it "dissolves"!
But, did it really dissolve? Did it dissolve in the same way a spoonful of sugar or table salt would dissolve in a glass of water? Not even close! This is because the Styrofoam molecules are a tangled spaghetti-like mess that makes them nearly impossible to separate completely one from another (which is required to become fully dissolved). Individual styrene molecules would certainly dissolve in acetone, but once they are polymerized into long chains, dissolving them completely becomes impossible.
QUESTIONS TO CONSIDER
- Once dry, would your new polystyrene sample have the same heat insulating properties as Styrofoam? If not, why not?
- What would be more dense, the Styrofoam you started with or your new sample of polystyrene?
- Did the polystyrene really fully dissolve in the acetone? What did it do? What would your experiment have looked like if it did fully dissolve?