What can act as either an acid or a base

Water (H ₂ O) is an interesting compound in many respects, including its ability to behave as an acid or a base.

In some circumstances, a water molecule will accept a proton and thus act as a Brønsted-Lowry base, which happens when HCl dissolves in H₂O:

In other circumstances, a water molecule can donate a proton and thus act as a Brønsted-Lowry acid. For example, in the presence of the amide ion (see Example 4 in Section 10.2 “Brønsted-Lowry Definition of Acids and Bases”), a water molecule donates a proton, making ammonia as a product:

So, depending on the circumstances, H₂O can act as either a Brønsted-Lowry acid or a Brønsted-Lowry base. Water is not the only substance that can react as an acid in some cases or a base in others, but it is certainly the most common example—and the most important one. A substance that can either donate or accept a proton, depending on the circumstances, is called an amphiprotic compound.

A water molecule can act as an acid or a base even in a sample of pure water. About 2 in every 555 million water molecules undergo the following reaction:

This process is called the autoionization of water (Figure 10.2 “Autoionization”) and occurs in every sample of water, whether it is pure or part of a solution.

Figure 10.2 Autoionization Note: Each H + ion in the diagram above could be viewed as attaching to an H₂O molecule, forming H₃O + . Also, it would require 555 million water molecules to have only one each of H₃O + and OH – , so the picture vastly over-represents the amount of autoionization of water.

Example 5

Identify water as either a Brønsted-Lowry acid or a Brønsted-Lowry base.

1. In this reaction, the water molecule donates a proton to the NO₂ − ion, making OH − (aq). As the proton donor, H₂O acts as a Brønsted-Lowry acid.
2. In this reaction, the water molecule accepts a proton from HC₂H₃O₂, becoming H₃O + (aq). As the proton acceptor, H₂O is a Brønsted-Lowry base.