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Unit 5: The Structure of Molecules—Lewis Structures and Molecular Geometries

Section 10: Physical Properties of Molecules

Molecules have properties that can be directly measured in the laboratory, which are the results of their molecular structure. This is because molecular structure determines the types of intermolecular forces that hold molecules together. The stronger these forces, the harder it becomes—and the more energy it takes—to move the molecules apart from each other. Some of the properties of molecules that are dependent on these forces are the temperatures at which phase transitions take place, such as the boiling point and the melting point. It also affects how dense compounds will be or how viscous they will be. Viscosity is a resistance to flow, and molecules with strong intermolecular forces will be more viscous, that is, pour like maple syrup. In addition, solubility, the ability for molecules to form solutions with other molecules, is one of these types of properties. We will focus on two of them here, specifically, boiling point and solubility.

Boiling Point

When we bring enough molecules of a substance together in the form of a liquid, we can heat it up and measure the temperature at which it turns into a gas, that is, its boiling point. The strength of intermolecular forces and the amount of them in the collection of molecules determine boiling point. More or stronger intermolecular forces result in a higher boiling point because the molecules want to stay closer together, in the liquid phase, rather than spread out into the gas phase (see Table 5-2). Thus, compounds that form hydrogen bonds boil at higher temperatures than compounds that have only the weaker London forces or dipole-dipole attractions.

Table 5-2: Boiling points of Common Substances
SubstanceLewis StructureBoiling Point
°C°F
Oxygen (O2)Lewis structure for oxygen-183-297
Ammonia (NH3)Lewis structure for ammonia-35.5-28.1
Ethanol(C2H5OH)Lewis structure for ethanol79172.4
Water (H2O)Lewis structure for water100212
Glycerin (C3H8O3)Lewis structure for glycerin290554

The oxygen molecule, the one molecule that cannot make hydrogen bonds, has the lowest boiling point in the table. While both the ammonia and ethanol can make a hydrogen bond, the ethanol is a much heavier molecule and thus has additional, stronger van der Waals forces. Water, which has the ability to make two hydrogen bonds, boils at an even higher temperature. And lastly, glycerin, which has the ability to form up to three hydrogen bonds and has a much higher molecular weight, boils at the highest temperature of them all.

Solubility

Molecules are attracted to other molecules when there are strong intermolecular forces between them. Therefore, a polar molecule would like to find another polar molecule with which to make dipole forces. Water, which is one of the most polar molecules, therefore, is very good at dissolving other polar molecules; a solution with water as the solvent is called an "aqueous solution." For example, acetic acid (C2H4O2) dissolved in water makes an aqueous solution known as vinegar.

Polar and Non-polar Liquids

Figure 5-20. Polar and Non-polar Liquids

© Left: Wikimedia Commons, CC License 2.5. Author: John, 16 March 2007. Right: Wikimedia Commons, CC License 3.o. Author: PRHaney, 19 January 2009.

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Polar and Non-polar Liquids

Figure 5-20. Polar and Non-polar Liquids

Left: Diesel fuel, which is a nonpolar hydrocarbon floats on a puddle of water without mixing. Right: A separatory funnel is a common piece of glassware used in a chemistry lab. It allows for liquids that will not mix together, to separate into layers so that the more dense one can be drained out from the bottom of the funnel. In this funnel, oil floats on top of colored water.

Nonpolar molecules, like those found in vegetable oil, lack any intermolecular forces except London forces. As such, they interact much less strongly with water. In fact, the water molecules attract each other more than they do the nonpolar compounds; this means the nonpolar molecules are pushed to the side and excluded from the water, and therefore they are not soluble in aqueous solvent. Water's attraction for itself pushes nonpolar compounds off to the side. This is why the oil and vinegar in salad dressing separate, similar to the examples in shown in Figure 5-20.

Nonpolar molecules will dissolve only in solvents composed of other nonpolar molecules, that is, in nonpolar solvents such as turpentine (mostly C10H16), hexane (C6H14) or benzene (C6H6). Nonpolar solvents are also often called "organic solvents," because the most common ones are made of carbon, just like the common molecules that make up living things. Saying that a molecule is hydrophobic is another way of saying it doesn't dissolve in water (from the Greek hydros meaning "water," and phobos meaning "a fear of"); hydrophilic is another way of saying it will dissolve in aqueous solution (philia meaning "love of").

Glossary

Aqueous solution

A solution where the solvent (primary component) is water.

Hydrophilic

Description of a molecule that dissolves easily in an aqueous or other polar solvent.

Hydrophobic

Description of a molecule that dissolves readily in a nonpolar solvent.

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