EXPERIMENT: DEMONSTRATING POLAR PROPERTIES

Some substances are polar and some are not. It is not so difficult to demonstrate a difference between these two classes of materials. This experiment is designed to help reveal the properties of polar and nonpolar substances.

Here are your goals for this lesson:

Describe experimental outcomes in terms of molecular shape and polarity

Apply ideas to the findings of other scientists

These supplies are needed:

acetate (overhead transparency material) strip and tissue paper vinyl strip and woolen cloth slow, steady stream of water from a faucet

Follow these directions and answer these questions.

1. Rub the acetate strip with the tissue paper. 2. Bring the strip near a slow stream of running water.

What happened when the strip was brought near the stream?

From your ideas about charges on acetate and vinyl strips, predict what will happen if a charged vinyl strip is brought near the slow stream of water. (Review Unit 4.)

3. Test your hypothesis. Rub a vinyl strip with a woolen cloth. 4. Bring the charged strip near a slow stream of water.

Now analyze your prediction.

a. Was your prediction correct?

b. If not, propose a reason for the difference.

Analyze the shape of H₂O. a. What is the shape of the H₂O molecule? b. Is it symmetrical? c. Does this mean that the H₂O molecule is polar or nonpolar?

Analyze the bonds of CCl₄. a. What is the shape of the CCl₄ molecule? b. Is it symmetrical? c. Does this mean that the CCl₄ molecule is polar or nonpolar?

Assume that the water stream is replaced by a stream of CCl₄. Predict what would happen in each case.

a. charged acetate strip:

b. charged vinyl strip:

c. Explain your predictions.

Chemists have found that charged acetate and vinyl strips have no effect on a stream of CCl₄. Does this fact match your prediction?

Develop a model or picture of water, H₂O, and carbon tetrachloride, CCl₄, that would account for observations with the charged strips for the two compounds.