Advanced Placement (AP) Biology Practice Exam

Which condition indicates that a solution is acidic?

1. If [H+] is greater than [-OH]

2. If [-OH] is greater than [H+]

3. If [H+] equals [-OH]

4. If [H+] is equal to 0

The correct answer is: If [H+] is greater than [-OH]

A solution is considered acidic when the concentration of hydrogen ions \([H^+]\) is greater than the concentration of hydroxide ions \([-OH]\). This is based on the relationship described by the ion product of water, where pure water at 25°C has equal concentrations of \([H^+]\) and \([-OH]\) (both approximately \(1 \times 10^{-7} \, M\)). In an acidic solution, the dominance of hydrogen ions leads to a lower pH (below 7), which is characteristic of acidic environments. Since acids release \([H^+]\) into the solution, the measurement of \([H^+]\) concentration exceeding that of \([-OH]\) confirms acidity. Conversely, environments with higher \([-OH]\) concentrations indicate a basic or alkaline solution, while equal concentrations would signify a neutral solution. Therefore, the correct condition indicating acidity is indeed when \([H^+]\) is greater than \([-OH]\).