Polar and nonpolar liquids
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Materials needed:
Balloon
Sink faucet
Wool sweater
Pen
Paper cup
Cooking oil
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Have you ever wondered why water and oil do not mix, or how soap combats oily dirt? To learn more about the properties of water and oil, try this experiment. (Note: this experiment works best on a cool, dry day when the humidity is low.)

Begin by inflating a balloon and tying it closed. Turn on a sink faucet and adjust its flow to get a thin stream of water. Rub the balloon on a wool sweater or in your hair to charge the balloon. Move the charged balloon near the stream of water. What happens?

Next, use a pen to punch a small hole in the bottom of a paper cup. Hold the paper cup over the sink and add cooking oil to the cup until a thin stream of oil starts to flow from the hole in the bottom, as shown in figure A. What happens when you bring a charged balloon near the stream of oil?

Both water and oil are made up of molecules. Molecules are combinations of atoms held tightly together through chemical bonds. A good analogy for a molecule is a word, which is a combination of letters.

Within molecules are positive and negative charges. If the positive and negative charges in a molecule are not distributed evenly, one end of the molecule will be slightly negative and the other end slightly positive. Molecules with charged ends are called polar molecules. If the positive and negative charges in a molecule are distributed evenly, the molecule is nonpolar.

Rubbing the balloon on a sweater (or in your hair) causes electrons to move from the sweater and collect on the balloon, charging the balloon. Since electrons are negative charges, the balloon becomes negatively charged. You use a charged balloon in this experiment to learn whether water and oil are polar and nonpolar.

When the charged balloon is brought close to the stream of water, the stream bends toward the balloon, showing that water is polar-the positive ends of the water molecules are attracted to the negative charges on the balloon. What would happen if the balloon had a positive charge instead of a negative charge? Since the stream of oil is not affected by the charged balloon, the oil must be nonpolar.

Much of what we call dirt is made up of oil and grease. Water alone will not dissolve oil and grease because water is polar and oil and grease are nonpolar-"like dissolve like." Adding soap or detergent to the water, however, will cause the oil and grease to dissolve.

Soap and detergent molecules are unique in that they each contain a large, nonpolar tail and a smaller, polar head. The nonpolar tail combines with oil and grease while the polar head combines with water. Figure B shows how soap or detergent molecules cause oil to dissolve in water. As you can see in the figure, the nonpolar tails of many soap or detergent molecules stick into a tiny oil particle. The polar heads of the soap or detergent molecules stick out into the water, causing the oil particle to be suspended in water. The suspended oil particle can now be washed away with the water.