Water is a polar molecule.

As we saw earlier, water molecules have polar covalent bonds, characterized by an uneven distribution of electrons. A molecule like water that has regions of positive and negative charge is called a polar molecule. Molecules, or even different regions of the same molecule, fall into two general classes, depending on how they interact with water: hydrophilic (“water loving”) and hydrophobic (“water fearing”).

Hydrophilic compounds are polar; they dissolve readily in water. That is, water is a good solvent, capable of dissolving many substances. Think of what happens when you stir a teaspoon of sugar into water: The sugar seems to disappear as it dissolves. What is happening is that the sugar molecules are dispersing through the water and becoming separated from one another, forming a solution in the watery, or aqueous, environment.

By contrast, hydrophobic compounds are nonpolar. Nonpolar compounds do not have regions of positive and negative charge. As a result, they arrange themselves to minimize their contact with water. For example, oil molecules are hydrophobic, and when oil and water are mixed, the oil molecules organize themselves into droplets that limit the oil–water interface. This hydrophobic effect, in which polar molecules like water exclude nonpolar ones, drives such biological processes as the folding of proteins (Chapter 4) and the formation of cell membranes (Chapter 5).