Chemistry

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Video

Materials

• Polystyrene (also called "styrofoam")
• Acetone
• 1 small container (such as a film canister)
• 1 knife
• 1 stick (or something similar)

Short explanation

Acetone dissolves polystyrene because both of these substances are nonpolar. "Like dissolves like" is a rule of thumb in chemistry. When polystyrene dissolves, the many gas bubbles contained in it burst, and it therefore shrinks a lot.

Long explanation

Styrofoam is an old brand name for polystyrene that has been filled with gas bubbles. A better words for the material is foamed polystyrene, if you want to avoid brand names.

Polystyrene is a very common material in our everyday lives - CD cases, disposable razors, plastic cups and disposable cutlery consist of it. It can be transparent, white, hard and soft, all depending on how it's treated in the manufacturing process. And it can also be filled with gas bubbles - as in foamed polystyrene. Exactly which gas the bubbles consist of varies depending on the manufacturer, but often it's carbon dioxide or pentane.

Polystyrene is a polymer, which means that it consists of long molecules composed of smaller molecules (like pearl necklaces composed of pearls). The small molecules are styrene (C₈H₈). The styrene molecules are joined together by strong covalent bonds. The separate polystyrene molecules also attach to one another, like sticky spaghettis that attach to one another here and there, and that with relatively weak van der Waals bonds.

Polystyrene is a plastic. When polystyrene burns, carbon dioxide and water are formed, but unfortunately the combustion is often incomplete, which means that some by-products are formed that are dangerous to health and the environment.

Acetone is a colorless, volatile and flammable liquid consisting of rather small molecules (C₃H₆O). Acetone is made from propylene, which in turn comes from fossil fuels. Acetone is also a common substance in living organisms and in soil and water, and is broken down relatively quickly in nature, but larger amounts still cause great damage. The fact that acetone is volatile means that it evaporates quickly, and in the atmosphere it's broken down by sunlight to mainly ethane and methane.

Something that plays a key role in the demonstration is chemical polarity. Chemical polarity is how electrons are spread, across a molecule or something else in chemistry. Molecules with an even distribution of electrons are called nonpolar. Molecules with an uneven distribution of electrons are called polar.

When polystyrene comes in contact with acetone, bonds begin to break between the polystyrene molecules. This is because both acetone and polystyrene are nonpolar substances, which is the same as the electrons being fairly evenly distributed over their molecules - which in turn means that there are no distinct electrically charged places (poles) on the molecules. This means that the polystyrene molecules are almost as strongly (or rather weakly) attracted to the acetone molecules as to each other. So the polystyrene molecules begin to slide along each other, while van der Waals bonds are constantly broken and formed, and this makes the gas bubbles in the polystyrene break. What remains is a lump of compact polystyrene.

It's not true that polystyrene melts in this demonstration, as it remains in solid form. Closer to the truth is that it dissolves in acetone. Acetone acts as a solvent - a nonpolar one - which dissolves the nonpolar substance polystyrene. You may have heard that "like dissolves like", which means that nonpolar substances dissolve in nonpolar solvents, while polar substances (for example sugar) dissolve in polar solvents (for example water). The easiest way to understand this is to think of the water molecules, which have an uneven distribution of electrons and thus a negatively charged and a positively charged end (they have poles - are polar). This makes water molecules hold together tightly, like a bowl of magnets would. Only other polar molecules can attract water molecules, take over their bonds and thus mix with them. Dissolving a nonpolar substance such as polystyrene in water doesn't work well. Instead a nonpolar solvent is needed, which has weak bonds that the polystyrene can "take over".

However, no solution (homogeneous mixture) of polystyrene and acetone is formed. Acetone acts more as a lubricant to make the polystyrene molecules slide along each other.

Acetone also dissolves other things made of polystyrene, but the effect is greatest with foamed polystyrene because it's so porous and has a large contact surface.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What happens if you replace acetone with another nonpolar solvent, such as cooking oil?
• What happens if you replace acetone with a polar solvent, such as water?
• What happens if you try to dissolve a polar substance (such as corn starch or sugar) in acetone?
• What happens if you pour some acetone on something else made of polystyrene, such as a plastic cup?
• What happens if you try to dissolve another type of plastic, such as PET, in acetone?
• What is the mass of the polystyrene before, and after, it's been dissolved by acetone?

Variation

The "peanuts" used as packing material are available in two variants; those made of polystyrene (polar) and those made of starch (nonpolar). A variation of this demonstration is to compare how these two types dissolve in water (polar) and acetone (nonpolar).

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