The correct option is **C. It increases the concentration of hydrogen ions (H⁺) in solution.** ### Detailed Explanation: 1. **Understanding Acids:** - An acid is defined as a substance that can donate protons (H⁺ ions) when dissolved in water. This is based on the Brønsted-Lowry definition of acids and bases. - Common examples of acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and acetic acid (CH₃COOH). 2. **Dissociation in Water:** - When an acid is dissolved in water, it dissociates into its constituent ions. For example, when hydrochloric acid (HCl) is added to water, it dissociates as follows: \[ \text{HCl} \rightarrow \text{H}^+ + \text{Cl}^- \] - This reaction shows that HCl donates a hydrogen ion (H⁺) to the solution, thereby increasing the concentration of hydrogen ions in the solution. 3. **Effect on pH:** - The pH scale measures the concentration of hydrogen ions in a solution. A lower pH indicates a higher concentration of H⁺ ions. Therefore, when an acid is added to water, the pH of the solution decreases, confirming that the concentration of H⁺ ions has increased. 4. **Why Option C is Correct:** - Since acids increase the concentration of hydrogen ions in solution, option C accurately describes the behavior of an acid when dissolved in water. ### Analysis of Other Options: - **Option A: It increases the concentration of hydroxide ions (OH⁻) in solution.** - This statement is incorrect because acids do not increase the concentration of hydroxide ions. Instead, they typically decrease the concentration of OH⁻ ions. When an acid donates H⁺ ions, it can react with OH⁻ ions to form water (H₂O), thus reducing the concentration of hydroxide ions. - **Option B: It decreases the concentration of hydrogen ions (H⁺) in solution.** - This statement is also incorrect. Acids are characterized by their ability to increase the concentration of hydrogen ions in solution. Therefore, this option contradicts the fundamental definition of an acid. - **Option D: It neutralizes all bases present in the solution.** - While acids can neutralize bases, this statement is misleading. Not all acids will neutralize all bases, and the extent of neutralization depends on the specific acid and base involved, as well as their concentrations. This option does not accurately describe the general behavior of acids in water. ### Summary of Key Points: - Acids increase the concentration of hydrogen ions (H⁺) when dissolved in water. - The dissociation of acids in water leads to a decrease in pH, indicating higher acidity. - Other options either misrepresent the behavior of acids or are too specific and not universally applicable. - Understanding the definitions and behaviors of acids and bases is crucial for mastering acid-base chemistry. This thorough understanding will help you in your studies and in answering similar questions in your exams.