10.4: Water - Both an Acid and a Base

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Learning Objectives

To write chemical equations for water acting as an acid and as a base.

Water (H₂O) is an interesting compound in many respects. Here, we will consider 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. We saw an example in the dissolving of HCl in H₂O:

\[\rm{HCl + H_2O_{(ℓ)} \rightarrow H_3O^+_{(aq)} + Cl^−_{(aq)}} \label{Eq1}\]

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), a water molecule donates a proton, making ammonia as a product:

\[H_2O_{(ℓ)} + NH^−_{2(aq)} \rightarrow OH^−_{(aq)} + NH_{3(aq)} \label{Eq2}\]

In this case, NH₂⁻ is a Brønsted-Lowry base (the proton acceptor).

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 6 in every 100 million (6 in 10⁸) water molecules undergo the following reaction:

\[H_2O_{(ℓ)} + H_2O_{(ℓ)} \rightarrow H_3O^+_{(aq)} + OH^−_{(aq)} \label{Eq3}\]

This process is called the autoionization of water (Figure \(\PageIndex{1}\)) and occurs in every sample of water, whether it is pure or part of a solution. Autoionization occurs to some extent in any amphiprotic liquid. (For comparison, liquid ammonia undergoes autoionization as well, but only about 1 molecule in a million billion (1 in 10¹⁵) reacts with another ammonia molecule.)

10.21.jpg — Figure \(\PageIndex{1}\) Autoionization. A small fraction of water molecules—approximately 6 in 100 million—ionize spontaneously into hydronium ions and hydroxide ions. This picture necessarily overrepresents the amount of autoionization that really occurs in pure water.

Example \(\PageIndex{1}\)

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

H₂O(ℓ) + NO₂⁻(aq) → HNO₂(aq) + OH⁻(aq)
HC₂H₃O₂(aq) + H₂O(ℓ) → H₃O⁺(aq) + C₂H₃O₂⁻(aq)

Solution

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.
In this reaction, the water molecule accepts a proton from HC₂H₃O₂, becoming H₃O⁺(aq). As the proton acceptor, H₂O is a Brønsted-Lowry base.

Exercise \(\PageIndex{2}\)

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

HCOOH(aq) + H₂O(ℓ) → H₃O⁺(aq) + HCOO⁻(aq)
H₂O(ℓ) + PO₄³⁻(aq) → OH⁻(aq) + HPO₄²⁻(aq)

Concept Review Exercises

Explain how water can act as an acid.
Explain how water can act as a base.

Answers

Under the right conditions, H₂O can donate a proton, making it a Brønsted-Lowry acid.
Under the right conditions, H₂O can accept a proton, making it a Brønsted-Lowry base.

Key Takeaway

Water molecules can act as both an acid and a base, depending on the conditions.