8.1 Introduction to Acids and Bases

Defining Acids and Bases

The Arrhenius definition says acids produce H⁺ (H₃O⁺) in water and bases produce OH⁻. The broader Brønsted-Lowry definition defines acids as proton (H⁺) donors and bases as proton acceptors. This works beyond aqueous solutions and explains reactions where no OH⁻ is involved.

Conjugate Acid-Base Pairs

In a Brønsted-Lowry reaction, an acid donates H⁺ to become its conjugate base, while a base accepts H⁺ to become its conjugate acid. These pairs differ by exactly one H⁺. For example, HCl donates H⁺ to become Cl⁻ (conjugate base), while H₂O accepts H⁺ to become H₃O⁺ (conjugate acid).

Amphoteric Species

Some substances, like water, can act as either an acid or a base depending on the reaction partner — this is called being amphoteric (or amphiprotic). Water donates H⁺ to strong bases but accepts H⁺ from strong acids, making it central to most acid-base chemistry in solution.

Key terms

Arrhenius acid

A substance that produces H⁺ (H₃O⁺) ions in aqueous solution.

Brønsted-Lowry base

A substance that accepts a proton (H⁺) in a reaction.

Conjugate acid

The species formed when a base accepts a proton (H⁺).

Conjugate base

The species formed when an acid donates a proton (H⁺).

Amphoteric (amphiprotic)

A substance that can act as either an acid or a base, like water.

Try an AP-style question

[H₃O⁺] = 1 × 10⁻³ M. What is the pH?

1. 3
2. 11
3. −3
4. 0.001