Water molecules can act as acids with other molecules, too. For example, they can transfer a hydrogen ion to a nonbonding electron pair on a solute molecule like ammonia, NH3(aq), to form an ammonium ion, NH4+(aq), and a hydroxide ion.
This reaction produces an excess of hydroxide ions over hydronium ions in the solution of ammonia in water. Therefore, this solution is basic. Ammonia is called a base, because it can accept a hydrogen ion from water to form a basic aqueous solution. Water molecules can also act as bases, accepting a hydrogen ion from a solute molecule like acetic acid, HOOCCH3(aq), to form a hydronium ion and an acetate ion, OOCCH3–(aq).
This solution has a higher concentration of hydronium ions than pure water; it is acidic. Acetic acid is called an acid, because it can transfer a hydrogen ion to water to form an acidic aqueous solution.
Carbon dioxide, O=C=O, is an acidic solute that plays a particularly important role in the Earth’s oceans. The electrons in the carbon-oxygen bonds are attracted more to the oxygen atom giving the oxygen atoms a partial negative charge and the carbon atom a partial positive charge. Because carbon dioxide is a linear molecule these bond dipoles cancel one another, so the molecule has no permanent dipole moment and is usually said to be nonpolar. However, when dissolved in water, the partial positive charge on the CO2 carbon and the partial negative charge on the water H2O oxygen attract one another. Similarly, of course, the partial negative charges on the CO2 oxygens and the partial positive charge on the water H2O hydrogens attract one another. One result is that carbon dioxide is about 40 times more soluble in water than the truly nonpolar atmospheric gases, nitrogen, N2, and oxygen, O2.