|In Kevlar, the attraction that holds the polymer strands together
works on a principle similar to electrostatic attraction. The difference
is that polymer strands of Kevlar are held together by hydrogen bonding
(which you have probably guessed involves hydrogen). A hydrogen atom has
only one proton and is not good at attracting electrons when it is bonded
to most other atoms, such as oxygen.
When oxygen and hydrogen are bonded together in a molecule (for example, in water), the oxygen attracts the electrons more than the hydrogen and they become unevenly distributed. Consequently, the oxygen becomes a little bit negatively charged, while the hydrogen becomes somewhat positively charged. This uneven charge distribution allows the hydrogen from one molecule to attract the oxygen of another and form bonds called hydrogen bonds.
Hydrogen bonding in water.
The next three demonstrations allow you to see evidence that hydrogen bonding exists between water molecules. This is helpful, since you can't see the individual molecules.
Materials: For the next three demonstrations you will need to collect:
Demonstration IAction: Take a shallow cup or petri dish and partly fill it with water. Now, for the hard part. Try to float a paper clip on top of the water. It really can be done!
Since the metal paper clip is more dense than water, why does it float?
Try to draw a picture of water molecules that shows how the paper clip is being held up on the surface of the water.
Demonstration IIAction: Take the shallow cup or petri dish and fill it to the top with water. Now, take a second cup and very slowly add water until you think the next drop will make the water spill. Look at the over-filled cup from the side, keeping your eyes level with the top of the water.
Do you notice the water bulging above the lip of the cup, like a flat ice-cream cone? How is this possible? Why doesn't the water spill over once it is filled to the top of the dish?
With your fingers, gently tap the sides of the cup. See how much you can get the water to slosh around without spilling. Watch how the water moves.
Use the idea of hydrogen bonding to try to explain why you can over-fill a cup with water and why it moves as a unit--like a blob.
Demonstration IIIAction: Turn on the water at your sink so that it makes a very slow steady stream. It should flow only slightly faster than dripping. With the small comb, vigorously comb your hair about twenty times. Then, bring the comb near the stream of water without touching it. Watch what happens to the stream of water.
If you don't notice any change, try combing your hair again. This time, when you bring the comb near the water, watch from a different angle.