Disappearing Styrofoam
Introduction
Styrofoam will rapidly dissolve, disappearing into a soupy mush when placed in
acetone, but not in water. Starch packing peanuts, however, break down in water but
not acetone.
Materials
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Styrofoam plates, cups, packing peanuts, packing chunks.
Starch packing peanuts. These will dissolve in water.
Acetone. Pure acetone works better than nail polish remover.
Opaque container (mug)
Glass cups, and something to poke the Styrofoam with (chopsticks, straws, etc.)
Safety
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Acetone is flammable and poisonous.
A few students may find the smell offensive – if so, keep them away from the
acetone.
It is not dangerous if it gets on the skin. It will sting quite a bit if it gets in the
eyes. This is not normally dangerous, but it can cause problems if wearing
contacts. Therefore, wear safety glasses when working near the acetone
containers.
Procedure
1) Pour 50 mL of acetone (volume is not too important) into a small beaker. Ask the
students what it is. Don't let them smell it! Get as many responses as you can.
2) Ask the students what would happen if you poured the liquid into a Styrofoam cup.
Holding a Styrofoam cup over a large beaker, pour the acetone into the cup. It should
rapidly dissolve a large hole in the bottom.
3) Act surprised! Ask the students what happened! Where did the Styrofoam go? You
can recycle the same acetone, and try using 2 or 3 cups.
4) Do the same demonstration, substituting water for the acetone. Act like you think the
Styrofoam cups won't hold the water. Ask them why you sometimes got a hole in the
bottom and sometimes didn't. Elicit as many responses as you can.
5) You can ask for a volunteer to smell the two liquids. But don't let your volunteer
inhale too deeply! Use a small amount of acetone in a beaker and a small amount of
water. This should confirm that the two liquids are quite different.
6) Show them that you can dissolve several Styrofoam packing peanuts in a fresh 50
mL acetone sample. You can use a glass stirring rod to stir the mixture. Also show them
that Styrofoam peanuts do not dissolve in water.
7) Pour a small amount of the acetone-Styrofoam mixture onto a watch glass and let it
evaporate. Ask them what remains on the watch glass.
8) Pass out a Styrofoam cup about 1/3 filled with water to each student. Ask them why
the cup didn't dissolve.
9) Give each student two starch packing peanuts and a soda straw. Tell them that these
packing peanuts are made of something different. Show them how to use the soda
straw as a stirrer to dissolve the starch peanuts. Ask them how they would get the
starch back.
10) Add a couple of starch packing peanuts to a small amount of acetone and have the
students record their observations.
Explanation
Styrofoam is made of polystyrene, which is a polymer. A polymer is a long string of
organic molecules linked together in a process called polymerization. Polystyrene is
used in plastic cutlery, CD jewel cases, and other clear plastics. In Styrofoam, the
polystyrene molecules are cross-linked, that is, the neighbouring strands of polymers
are joined to one another. This makes it stronger and more versatile, much like weaving
threads together makes cloth. As the polymer is made, gases are blown into it, making it
foam up into the familiar Styrofoam. Styrofoam is a non-polar molecule – the electrons
are pretty equally shared among the bonds in the molecule. Acetone is also a non-polar
molecule, with structure shown on right. “Like dissolves like,” and the cross-links
between the Styrofoam molecules are broken by the acetone. This releases the trapped
gases, reducing the volume of the Styrofoam. The polystyrene molecules themselves
are intact, and stay in a mushy puddle in the bottom of the glass. Styrofoam is not
dissolved by water because water is a polar solvent. Is it melting? No! It’s tempting to
say the Styrofoam “melted”, but this reaction does not involve a state change from solid
to liquid. Starch, on the other hand, is a polymer made of linking glucose molecules.
Starch packing peanuts are crosslinked starch molecules. It is an environmentally
friendly alternative to Styrofoam, because it can be broken down by water and bacteria.
It is polar due to the –OH groups (see structure below). Water breaks the crosslinks, but
acetone does not.
http://sciencestage.com/v/16592/eco-friendly-packing-peanuts-demonstration.html
http://www.sciencegeekgirl.com/activities/Wicked%20styrofoam%20activity%202.pdf
http://chemistry.lsu.edu/webpub/demo-3-styrofoam.pdf
Make your predictions:
Will there be a difference between Styrofoam and cornstarch peanuts?
Predict how many Styrofoam peanuts the teacher can put into this mug?
Observations:
Record your observation:
Styrofoam peanuts
Observations
In water
In acetone
Cornstarch peanuts
In water
In acetone
Observations
Summary
Summarize your findings and comment on your prediction and the outcome of this
experiment.
Question:
Explain which packing peanut is the more environmentally friendly choice.