Ontario Operator-in-Training (OIT) Practice Exam

What is the effect of adding an acid like HCl to water?

1. It increases hydroxyl ions

2. It releases hydrogen ions

3. It reduces pH

4. It forms stable compounds

The correct answer is: It releases hydrogen ions

Adding an acid like hydrochloric acid (HCl) to water primarily results in the release of hydrogen ions (H+). When HCl dissociates in water, it splits into hydrogen ions and chloride ions (Cl-). This increase in hydrogen ions is significant because it's the defining characteristic of an acidic solution. The concentration of hydrogen ions directly affects the pH of the solution. A higher concentration of hydrogen ions corresponds to a lower pH value, indicating an increase in acidity. This is why the introduction of HCl leads to a reduction in pH, but the focus on the release of hydrogen ions highlights the primary chemical process taking place when an acid is dissolved in water. While it is true that adding an acid decreases the pH (which reflects increased acidity), and the possibility of forming stable compounds exists in various contexts, the most direct and evident effect of adding HCl to water is the release of hydrogen ions. This fundamental understanding of acid-base chemistry is crucial for managing water treatment processes and understanding aquatic chemistry in general.