The correct option is **C. Acids can donate protons (H⁺) to other substances in solution.** ### Detailed Explanation: 1. **Understanding Acids:** - Acids are substances that can donate protons (H⁺ ions) when dissolved in water. This is a fundamental characteristic of acids according to the Brønsted-Lowry acid-base theory. When an acid donates a proton, it increases the concentration of hydrogen ions in the solution. 2. **Why Option C is Correct:** - When an acid is added to water, it dissociates to release H⁺ ions. For example, hydrochloric acid (HCl) dissociates in water as follows: \[ \text{HCl} \rightarrow \text{H}^+ + \text{Cl}^- \] - The H⁺ ions can then interact with other substances in the solution, effectively donating protons. This ability to donate protons is what defines an acid in the Brønsted-Lowry framework. 3. **Why the Other Options are Incorrect:** - **Option A: Acids increase the concentration of hydroxide ions (OH⁻) in solution.** - This statement is incorrect because acids do not increase the concentration of hydroxide ions. Instead, they increase the concentration of hydrogen ions (H⁺). In an aqueous solution, the relationship between H⁺ and OH⁻ is governed by the water dissociation constant (Kw), which states that: \[ K_w = [\text{H}^+][\text{OH}^-] = 1.0 \times 10^{-14} \text{ at 25°C} \] - As the concentration of H⁺ increases due to the addition of an acid, the concentration of OH⁻ must decrease to maintain the constant value of Kw. - **Option B: Acids decrease the concentration of hydrogen ions (H⁺) in solution.** - This statement is also incorrect. Acids, by definition, increase the concentration of hydrogen ions in solution. When an acid is added to water, it dissociates to release H⁺ ions, thus increasing their concentration. - **Option D: Acids have a pH greater than 7.** - This statement is incorrect as well. The pH scale ranges from 0 to 14, where a pH of 7 is considered neutral (pure water). Acids have a pH less than 7. The lower the pH, the stronger the acid, indicating a higher concentration of H⁺ ions. ### Summary of Key Points: - Acids are defined by their ability to donate protons (H⁺) in solution. - The correct answer is C, as acids increase the concentration of H⁺ ions, not OH⁻. - Acids have a pH less than 7, contrary to what option D states. - Understanding the behavior of acids in aqueous solutions is crucial for grasping fundamental concepts in chemistry. ### Revision Summary: - Acids donate protons (H⁺) in solution (Correct: C). - Acids increase H⁺ concentration, not OH⁻ (Incorrect: A). - Acids do not decrease H⁺ concentration (Incorrect: B). - Acids have a pH less than 7 (Incorrect: D).