Some molecules just don’t play nicely with water. Because water is a polar molecule, it tends to stick to itself via hydrogen bonds. Other polar molecules also stick to water molecules and can mix right in, dissolv ing into the water. However, nonpolar molecules have evenly shared covalent bonds and lack the slight negative and positive charges of polar molecules. Because they’re uncharged, nonpolar molecules don’t mix well with water.

Nonpolar molecules are also called hydrophobic molecules because “hydro” means water and “phobic” means to fear.

When nonpolar molecules are placed in a watery environment, the polar mol ecules will all stick to each other and push the nonpolar molecules away. You can think of the scenario as if the polar molecules all belong to a clique that refuses to hang out with the nonpolar molecules. (The name of this clique, by the way, is the hydrophilic molecules.) Because the nonpolar molecules all get pushed together, they become associated with each other.

The interaction between nonpolar molecules is called a hydrophobic interaction.

You can easily demonstrate a hydrophobic interaction to yourself. Just go into your kitchen, put some water in a cup, and then add a little oil. Even if you stir the mixture vigorously to mix the oil into the water, as soon as you stop stirring, all the oil will gather together on top of the water.

The water molecules all stick to each other and push the oil molecules away. Hence the saying, “They get along like oil and water!”

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