||Introduction: Have you ever gotten clothes out of a
dryer and found that some are "stuck" together? Or, have you tried to "stick"
a balloon to the ceiling after rubbing it against your hair or a sweater?
(If not, try it!! The balloon trick is fun.) Both of these examples rely
on two simple facts of nature:
These ideas are important in hydrogen bonding and can be demonstrated by
simply sticking two pieces of tape together as described below.
- Electrons are relatively easy to remove from atoms.
- Some materials (or atoms) attract electrons better than others.
Materials: Before you begin, you will need to do the following:
Action: Follow these steps:
- Get some clear plastic tape (sometimes called Scotch Tape).
- Find a hard smooth surface nearby on which you can temporarily stick
some tape, maybe a desk top, a table (not wood), a window, the floor,
or a door.
- Tear off two pieces of tape. Make them about as long as your finger.
- Fold the top little bit of each tape down so that it sticks to itself.
- Press one piece of tape down on a hard smooth surface (such as a table
top.) Lay the second piece of tape directly on top of the first, with
the sticky side down and the little folded parts together. The picture
below shows how this looks.
- Rub the top piece of tape so that it is well stuck to the one on the
bottom. (While you are doing this, something has happened that you can't
see with your eyes. Try to imagine a whole bunch of electrons that are
part of the atoms that make up the clear plastic tape. These electrons
aren't stationary; they are migrating toward the material that attracts
them most. Can you see a picture of this in your mind?) Before doing
the demonstration, make a guess; will the glue side of the tape or the
smooth plastic side attract electrons more?
- Leaving the two pieces of tape stuck together, peel them both off
the smooth surface.
- Grab the folded parts of each piece of tape and very quickly
rip the two pieces apart.
- Now, slowly bring the two pieces of tape nearer and nearer to each
other without touching them together.
Explanation: If the demonstration worked and the two pieces of tape
became attracted to each other, then you saw the effects of electrostatic
attraction. In other words, because the electrons moved toward one side
of the tape, this side became negatively charged. The migration of electrons
left the other side of the tape with a deficiency of electrons, causing
it to become positively charged. Opposite charges attract, so the two oppositely
charged pieces of tape were attracted together.
- What happened?
- Did the two pieces of tape behave this way before you pressed them
Extension: From this demonstration alone, it is not possible
to know if the glue side or the plastic side of the tape is better at
attracting electrons (i.e., negatively charged). Can you think of a way
to determine which side of the tape was a better attractant for electrons?
To more demonstrations
Return to Kevlar--Clue 4