214

THINK ABOUT IT

Not everything has a smell. In fact, there are more substances on the planet that we don’t smell than substances that we do smell. If everything around us had a smell, our noses would probably be overwhelmed most of the time.

Why do some molecules smell while others do not?

To answer this question, you will explore

EXPLORING THE TOPIC

Compounds That Do Not Have a Smell

Take a big whiff of air. Perhaps you are able to detect some odors—a freshly mowed lawn, somebody’s food cooking. But the air itself does not smell. Small molecules, such as nitrogen, oxygen, carbon dioxide, and water vapor, obviously fit inside the receptor sites. So, why can’t we smell them?

Perhaps these molecules are too small to stay docked in a receptor site. But some small molecules do have a smell. Hydrogen sulfide, H₂S, smells like rotting eggs and ammonia, NH₃, smells pungent.

So the molecular size of a compound does not explain whether we can smell it. It turns out that a molecular property called polarity is involved.

Polar Molecules

215

When two different materials are rubbed together, some electrons can transfer from one material to the other. The result is an imbalance of positive and negative charges. One material has an excess of positive charge and the other an excess of negative charge. This is known as static electricity.

Suppose that you rub a plastic wand on a piece of cloth. The plastic wand will end up with a charge. In class you tested a number of liquids by holding a charged wand near them and observed that some of the streams bend toward the wand.

In this table, all of the liquids in the first column are attracted to a charged wand. They are made up of polar molecules. The liquids in the second column are not attracted to a charged wand. They consist of nonpolar molecules.

Water is a polar molecule. As this illustration shows, there are partial charges at different locations on the water molecule. These partial charges are much smaller than the charge on an individual electron or proton. However, the charges are large enough to cause a stream of water to be attracted to a charged wand.

The oxygen atom has a partial negative charge, and the hydrogen atoms each have a partial positive charge.

Intermolecular Force

EXPLAINING THE CHARGED WAND

When a wand with a negative charge is placed next to a stream of water, the water molecules orient themselves in space so that their positive ends are lined up in the direction of the wand. As a result, the whole stream of water is attracted to, or pulled toward, the charged wand. The illustration on the next page shows a possible way to depict what happens to the molecules.

216

INTERMOLECULAR BEHAVIOR

The partial charges on polar molecules cause more than an attraction to a charged wand. They also cause attractions between molecules. As the molecules in a polar liquid tumble around, they tend to align with each other because partial negative charges are attracted to partial positive charges. This attraction between individual molecules is an intermolecular force. The prefix inter- means “between” and the force is a force of attraction.

Intermolecular attractions can be used to explain many observable properties.

Observation: Water beads up on waxed paper. Oil spreads out.

Explanation: The water molecules are polar, while the oil molecules are nonpolar. The individual water molecules are attracted to each other and “cling together.” Individual oil molecules are not attracted to one another to the same extent, and they spread out.

Observation: Water is a liquid at room temperature, but methane is a gas at room temperature. They have roughly the same molecular mass.

Explanation: Water is polar and methane is nonpolar. The individual water molecules are attracted to each other and stay together as a liquid. The attractions between methane molecules are much weaker. The methane molecules spread throughout the room as a gas.

217

Observation: Methanol dissolves easily in water. Oil floats on top of water but does not dissolve.

Explanation: The polar methanol molecules are attracted to the polar water molecules and go into solution. The nonpolar oil molecules are not attracted to the polar water molecules.

LESSON SUMMARY

Why do some molecules smell while others do not?

Molecules can be divided into two classes: polar and nonpolar. Polar molecules are attracted to a charged wand because they have partial charges distributed within the molecule. Nonpolar molecules are not attracted to a charged wand. Polarity is responsible for intermolecular attractions that affect many properties of molecules, possibly including smell properties.

219

220

221

222