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THINK ABOUT IT

You can smell chicken frying or someone’s perfume from across a room, but you can’t smell a piece of paper or a binder on your desk. And cookies right out of the oven are much easier to smell than cold ones. How can you use what you have learned about molecules to understand these observations?

What generalizations can you make about smell and molecules?

To answer this question, you will explore

EXPLORING THE TOPIC

Bonding and Smell

In Unit 1: Alchemy, you sorted matter into four classes based on whether the substance dissolved in water and whether the substance conducted electricity. This sorting led to four models of bonding: ionic, molecular covalent, metallic, and network covalent. How does the property of smell relate to bonding?

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Recall that all the substances you smelled in this unit were molecules with nonmetal atoms. This evidence suggests that substances that have a smell fit into the molecular covalent category.

Molecular covalent substances exist as gases, liquids, or solids at room temperature. Substances in the other three bonding categories are strictly solids (with a few exceptions, such as liquid mercury). It is reasonable to propose that substances that are ionic, metallic, or network covalent do not have a smell because they do not form gases easily at room temperature. This means that they cannot get into the receptor sites in our noses.

Molecular Size and Smell

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In the previous lesson, you learned that small molecules have a smell only if they are polar. What about medium-sized and large molecules? Take a moment to examine the data in this table.

Many pieces of information can be gathered from the table. First, small molecules are gases at room temperature, medium-sized molecules are liquids, and large molecules are solids. One reason for this sorting by size is that large molecules tend to have more intermolecular attractions. This means that larger molecules are less likely to be gaseous and are less likely to get to your nose.

Note that a molecule is considered medium-sized if it has about 5 to 19 carbon atoms and large if it has 20 or more carbon atoms. Carbon is a unique element because it bonds easily to other carbon atoms and forms four bonds. This makes it ideal for forming long and complex molecules.

In general, all medium-sized molecules have a smell, whether they are polar or nonpolar. The medium-sized molecules form a liquid because they are attracted to one another, but they are volatile, meaning they can go easily into the gas phase. Once these molecules get inside the nose, the intermolecular attractions between the molecules and the receptor sites allow you to detect a smell.

HEATING AND COOLING

What happens when you warm or cool molecular substances? Small molecules are already gases at room temperature. But larger molecules are liquids or solids. These molecules must undergo a phase change in order to be smelled. Warming a molecular liquid or solid will generally give it a more intense aroma. This is because heating a substance increases the rate at which a substance becomes a gas. Cooling a substance has the opposite effect.

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When there are more molecules in the air, you are more likely to detect them. So the temperature of a substance will usually affect the strength of its smell. For example, you are more likely to smell warm food than cold leftovers from the fridge.

LESSON SUMMARY

What generalizations can you make about smell and molecules?

Humans can smell some molecular substances. We cannot smell ionic, metallic, or network covalent substances. This is because the first requirement for a substance to be smelled is that it be in a gaseous form. The size of the molecule seems to be another important factor in predicting smell. Most medium-sized molecules will smell. Most large-sized molecules will not smell. Much of this is due to the effect of size on phase change. For small molecules, polarity also plays a role. Small nonpolar molecules do not have a smell. Small polar molecules do have a smell.

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