‘Dissolving’ polystyrene in acetone

In this experiment, students observe as expanded polystyrene is placed in propanone (acetone). They then attempt to solve the mysterious disappearance of the polystyrene – which seems to dissolve – and investigate the clue of the ‘fizzing’ gas bubbles released.

A small volume of propanone can absorb an impressive volume of expanded polystyrene, making this an attention-grabbing demonstration.

The experiment takes only a few minutes, and is readily repeated, making it a suitable demonstration for school open days and like activities. It is not really suitable as a class experiment as the gel produced forms a good adhesive as the propanone evaporates. This can make clearing up (and cleaning up) a major problem! However the teacher may wish to consider whether suitable students could perform the demonstration at a school open day.

During the demonstration, other activities can be interspersed as each addition of expanded polystyrene is left to be absorbed.

Equipment

Apparatus

• Eye protection
• Supply of paper towels, newspaper or similar (see note 5 below)
• Beaker, 1 dm³
• Large transparent container (see note 6)

Chemicals

• Propanone (acetone), (HIGHLY FLAMMABLE, IRRITANT), 50 cm³
• Expanded polystyrene pieces, large quantity (see note 7)
• Expanded polystyrene drinking cups, a few

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout and ensure there are no naked flames nearby as propanone is HIGHLY FLAMMABLE.  
• Propanone (acetone), CH₃COCH₃(l), (HIGHLY FLAMMABLE, IRRITANT) – see CLEAPSS Hazcard HC085A.
• Polystyrene – see CLEAPSS Hazcard HC071. 
• The supply of paper towels, newspaper or similar is to protect the bench and mop up any spillages. Warning: the polystyrene gel formed is an excellent adhesive, especially where it is not wanted, and can be very difficult to remove from clothes. 
• The large transparent container is to allow the class to observe the volume of expanded polystyrene pieces being added. A 5 dm³ beaker or a plastic fish tank would be suitable, but note that the latter might be damaged by any propanone accidentally spilt on it. Mark the side of the fish tank at approximately 5 dm³ intervals.
• For the expanded polystyrene, the most suitable material is found in the packing beads used to protect glassware, etc, in transit. Up to 10 dm³ of packing beads will be needed for each demonstration. Other forms of the material will need to be broken into small enough pieces for convenient addition to the 1 dm³ beaker. Provide the supply of material in a large box, from which the demonstrator can easily fill the transparent container with a scoop. 
• Dispose of the resulting gel from this demonstration by decanting and diluting any remaining propanone and then flushing it away with water – see CLEAPSS Hazcard HC085A. The gel can be poured into a throwaway container, such as a screw top coffee jar, and placed in the solid waste bin.

Procedure

1. Hold an expanded polystyrene drinking cup over the 1 dm³ beaker and slowly pour about 50 cm³ of propanone into it. The bottom will fall out of the cup and the propanone will pour straight through into the beaker.
2. Alternatively, simply pour about 50 cm³ of propanone directly into the 1 dm³ beaker.
3. Add about 5 dm³ of expanded polystyrene pieces to the large transparent container.
4. Add the expanded polystyrene pieces a handful at a time to the propanone. A suitable scoop may be useful for this. The polystyrene pieces will shrink and fizz, eventually forming a layer of sticky gel below the propanone.
5. Fill the 1 dm³ beaker with pieces; the level will gradually drop as the bottom layer dissolves. Refill the beaker at intervals.
6. Over 5 dm³ of beads may be absorbed although this will depend on the type of material used. The empty 5 dm³ beaker gives a striking reminder of the volume of beads that has been absorbed.
7. Afterwards the beaker can be passed round the class; but take care as the contents could be difficult to clean off clothes if spilled.
8. The resulting gel of polystyrene can be left in a fume cupboard while the propanone evaporates, although this takes some time. The resulting small volume of solid material contrasts sharply with the volume of the original expanded polystyrene.

Teaching notes

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 expands to produce a foam plastic. 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 the propanone; it merely softens as it absorbs the propanone and allows the air to escape, thereby collapsing the foam. An interesting example of a gas formed not by a chemical process, but by a physical process. The resulting colloidal gel consists of propanone molecules dispersed in a network formed by a tangle of large polystyrene molecules – a similar structure to ordinary jelly in which water molecules are dispersed in a network of protein molecules.