International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Why is HCl considered acidic when dissolved in water?

1. It forms H⁺ and Na⁺ ions

2. It forms H⁺ and Cl⁻ ions

3. It remains as HCl molecules

4. It forms water and hydrogen gas

The correct answer is: It forms H⁺ and Cl⁻ ions

Hydrochloric acid (HCl) is considered acidic when dissolved in water due to its dissociation into hydrogen ions (H⁺) and chloride ions (Cl⁻). In an aqueous solution, HCl molecules interact with water, leading to the release of H⁺ ions. These H⁺ ions are what make a solution acidic, as they increase the concentration of hydrogen ions in the solution. The presence of Cl⁻ ions does not affect the acidity; they are simply the conjugate base that stays in the solution after the dissociation. In this scenario, the formation of H⁺ ions is the crucial aspect because it is the concentration of these ions that determines the acidity of the solution. Thus, when HCl dissociates, the creation of H⁺ ions signifies the acidic nature of the solution, explaining why the correct answer involves the formation of H⁺ and Cl⁻ ions.