Identify an example of a weak base.

In the context of acids and bases, weak bases are substances that partially ionize in solution, leading to a lesser degree of hydroxide ions (OH⁻) compared to strong bases, which completely ionize. Ammonia is an example of a weak base because when dissolved in water, it does not fully dissociate; rather, it establishes an equilibrium between undissociated ammonia (NH₃) and ammonium ions (NH₄⁺) along with hydroxide ions (OH⁻). This results in a relatively modest increase in the pH of the solution compared to stronger bases.

In contrast, the other options listed—sodium hydroxide, calcium carbonate, and potassium hydroxide—are strong bases or behave as such in aqueous solutions. Sodium hydroxide and potassium hydroxide completely dissociate into their constituent ions when dissolved in water, contributing a significant concentration of hydroxide ions. Calcium carbonate, while it does act as a base in certain scenarios, is typically not classified as a strong base due to its limited solubility in water. Thus, ammonia stands out as the correct example of a weak base due to its incomplete ionization in solution.