Ontario Operator-in-Training (OIT) Practice Exam

Which statement best describes an acid?

1. It has a pH of more than 7

2. It has fewer free hydrogen ions than hydroxyl ions

3. It has a pH less than 7

4. It increases hydroxyl concentration in water

The correct answer is: It has a pH less than 7

An acid is best described by a pH of less than 7. This characteristic indicates that the solution has a higher concentration of hydrogen ions (H⁺) than hydroxyl ions (OH⁻). In general, when substances are dissolved in water, acids will donate hydrogen ions to the solution, thus increasing the acidity of the solution and lowering its pH value. In addition, the other options provide definitions or characteristics of substances that are contrary to the nature of acids. For instance, a pH greater than 7 signifies a basic or alkaline solution, while stating that there are fewer free hydrogen ions than hydroxyl ions describes a basic solution as well. Similarly, saying that an acid increases hydroxyl concentration goes against the definition of an acid since they tend to decrease the relative amount of hydroxyl ions in solution. This distinction is critical for understanding the foundational concepts of acid-base chemistry.