The terms hydrophobic and polar refer to the overall distribution of charge in a molecule. If there are no local regions of high or low electron density in the molecule, it is called hydrophobic (Greek for "water-fearing"). This term arises because hydrophobic molecules do not dissolve in water.

If a molecule has areas where there is a partial positive or negative charge, it is called polar, or hydrophilic (Greek for "water-loving"). Polar molecules dissolve easily in water.

How can you tell if a molecule is polar or hydrophobic? There are two things to consider when trying to decide.

1. If a molecule has polar covalent bonds, then it may be polar. A covalent bond is polar if one of the atoms in it has a significantly higher affinity for electrons (electronegativity) than the other. Some examples of polar covalent bonds often found in biological molecules include C-O, C-N and O-H bonds. If all the bonds in a molecule are nonpolar, then the molecule itself is nonpolar. Some examples of nonpolar covalent bonds are C-C and C-H bonds.

2. Even if a molecule has polar covalent bonds, if these bonds are arranged symmetrically, the molecule overall will be hydrophobic. The best example is carbon tetrachloride (CCl₄, see the structure). Even though each of the C-Cl bonds is quite polar, they are arranged symmetrically around the central carbon atom, so the molecule as a whole is nonpolar or hydrophobic.