Like Dissolves Like
Ever have a nasty stain on your shirt that won’t come out in the wash, no matter how many
times you try, and yet that same stained shirt comes back from the dry cleaner looking like
new? If you have, you’ve experienced the chemical phenomenon of “like dissolves like”.
That is, substances tend to dissolve in things that are similar to them. By ‘similar’ in this case
we mean in terms of their polarity. Some stains dissolve better in a polar substance like
water and some stains require a more non-polar substance to dissolve them away.
Let’s consider two solvents that are pretty different in their polarities in order to explore
this topic. Water, which we said is a polar solvent, dissolves almost anything that is polar,
such as salt and many other ionic compounds. Water can’t dissolve everything, though. Try
removing fingernail polish with water and you’ll see what I mean. Acetone, a solvent with
some non-polar properties, is commonly used to do that job. Acetone is an effective solvent
for all sorts of non-polar substances.
Demo:
Place two large glass beakers side-by-side.
Pour water into the first beaker until it’s
about half full. Place a Styrofoam cup in the
water beaker. Nothing will happen.
Styrofoam is non-polar, water is polar and,
since “like dissolves like”, they will not
dissolve in each other.
Now pour some acetone into the other
beaker and place another Styrofoam cup into
that beaker. You’ll see the cup slowly break
down until it is just a glob of goo. Acetone can
get in between the components of the
polymer of plastic and allow the air in the cup
to escape (don’t worry, they don’t use CFC’s
in Styrofoam anymore so there is no harm to
the environment when doing this demo).
The goo you retrieve from the beaker is
actually polystyrene plastic (#6 in recycling
code) and is the same plastic used to make
plastic table ware, etc. You can shape it any
way you wish while it is wet and it will harden
over time as all the acetone completely
evaporates away. In order
to completelydissolve the plastic, you’d need
a stronger and more non-polar solvent.
Place starch packing peanuts (the
environmentally friendly packing option
commonly used today) in a beaker of
acetone. Since the starch packing peanuts are
polar, they will not dissolve in acetone. Put
the starch packing peanuts in a beaker of
water, mix around a bit and you’ll see they
dissolve readily
Starch peanuts in acetone
Old-fashioned Styrofoam packing peanuts are
fun to play with too. You’ll need a large
beaker filled about half full with acetone.
Have someone ready with a large wooden
spoon and start loading the Styrofoam
packing peanuts into the beaker as your
helper stirs like crazy. You’ll be amazed at
how many peanuts will fit into the beaker.