I. Water

To the left is a model of a water molecule. Identify the type of bonds present, types and numbers of atoms.

Now look at a cluster of water molecules. Rotate them and notice how they are oriented towards each other. Describe the pattern. What do you think causes this pattern?

These bonds orienting the water molecules towards each other are hydrogen bonds. They form between the oxygen which tends to hog electrons shared with hydrogen within its molecule, and a hydrogen from another molecule, which tends to lose its shared electron to the oxygen in its molecule. Water, with positive (hydrogen) and negative (oxygen) ends, is a typical polar molecule.

II. Polar and Nonpolar Molecules

Decane is a larger molecule. Again, identify the type of bonds present, types and numbers of atoms. Notice it lacks any oxygen that might hog electrons. This is a typical nonpolar molecule. How do think the attraction between decane molecules compares with the attraction between water molecules?

Notice below the water molecules cluster together underneath the decane molecules. The water is pulled to each other by hydrogen bonding while the decane, being less dense, floats to the surface. Decane is like oil, and this is why oil and water don't mix.

Now look at ethanol, another molecule. How do you think it will interact with water?

Notice below the ethanol mixes easily with water, which decane wouldn't. This behavior of polar and nonpolar molecules is summarized in the saying: "Like dissolves like."

III. Soap

This is a molecule of palmitic acid. Identify the type of bonds present, and the types and numbers of atoms. How is this molecule like decane, how is it like water? How would this molecule interact with water differently than decane or ethanol?

Notice below how soap molecules have one end that is polar and attracted to water, while the other end is like decane, and is nonpolar. Such molecules are amphipathic. It is not that the nonpolar end repels water, but water is much more attracted to other polar molecules than to nonpolar substances. Since dirt frequently contains grease or other nonpolar substances, how does soap "clean them"? Look carefully at the cartoon on the bottom. The squiggly lines that look like pollywogs are the soap molecules, the pink molecules are water, and the double line is some material with dirt on it.

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V. References

http://www.sdahq.org/cleaning/chemistry/

http://pathfinderscience.net/teachers/stain/background.cfm

http://www.iupac.org/didac/Didac%20Ned/Didac03/Content/L16.htm

http://www.cleaning101.com/cleaning/history/

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