(b) Vinegars contain the weak acid acetic acid. (a) Fruits such as oranges, lemons, and grapefruit contain the weak acid citric acid. (The use of a double-arrow in the equation above denotes the partial reaction aspect of this process, a concept addressed fully in the chapters on chemical equilibrium.)

When dissolved in water under typical conditions, only about 1% of acetic acid molecules are present in the ionized form, CH 3 CO 2 − CH 3 CO 2 − ( Figure 21.2). A familiar example of a weak acid is acetic acid, the main ingredient in food vinegars:ĬH 3 CO 2 H ( a q ) + H 2 O ( l ) ⇌ CH 3 CO 2 − ( a q ) + H 3 O + ( a q ) CH 3 CO 2 H ( a q ) + H 2 O ( l ) ⇌ CH 3 CO 2 − ( a q ) + H 3 O + ( a q ) Weak acids are commonly encountered in nature, being the substances partly responsible for the tangy taste of citrus fruits, the stinging sensation of insect bites, and the unpleasant smells associated with body odor. A far greater number of compounds behave as weak acids and only partially react with water, leaving a large majority of dissolved molecules in their original form and generating a relatively small amount of hydronium ions. Acids that completely react in this fashion are called strong acids, and HCl is one among just a handful of common acid compounds that are classified as strong ( Table 21.1). The nature of HCl is such that its reaction with water as just described is essentially 100% efficient: Virtually every HCl molecule that dissolves in water will undergo this reaction. When hydrogen chloride gas dissolves in water, (a) it reacts as an acid, transferring protons to water molecules to yield (b) hydronium ions (and solvated chloride ions). When dissolved in water, H 3O + ions are produced by a chemical reaction in which H + ions are transferred from HCl molecules to H 2O molecules ( Figure 22.1). The process represented by this equation confirms that hydrogen chloride is an acid. HCl ( a q ) + H 2 O ( a q ) ⟶ Cl − ( a q ) + H 3 O + ( a q ) HCl ( a q ) + H 2 O ( a q ) ⟶ Cl − ( a q ) + H 3 O + ( a q )

Hf acid or base