The correct option is **C. Acids produce hydrogen ions (H⁺) when dissolved in water.** ### Detailed Explanation: 1. **Understanding Acids:** - Acids are substances that can donate protons (H⁺ ions) when they dissolve in water. This is a fundamental property of acids and is part of the Brønsted-Lowry definition of acids and bases. - When an acid is added to water, it dissociates (breaks apart) to release H⁺ ions into the solution. For example, hydrochloric acid (HCl) dissociates as follows: \[ \text{HCl} \rightarrow \text{H}^+ + \text{Cl}^- \] - The presence of these H⁺ ions is what characterizes a solution as acidic. 2. **Why Option C is Correct:** - The statement in option C accurately describes the behavior of acids in aqueous solutions. When an acid dissolves in water, it increases the concentration of hydrogen ions (H⁺), which is the defining characteristic of an acidic solution. - This increase in H⁺ ions leads to a decrease in pH, making the solution more acidic. 3. **Why the Other Options are Incorrect:** - **Option A: Acids increase the concentration of hydroxide ions (OH⁻).** - This statement is incorrect because acids do not increase the concentration of hydroxide ions. In fact, when an acid is added to water, the concentration of hydroxide ions typically decreases. This is because the H⁺ ions from the acid can react with OH⁻ ions to form water (H₂O), thus reducing the concentration of hydroxide ions. - The reaction can be represented as: \[ \text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O} \] - **Option B: Acids decrease the concentration of hydrogen ions (H⁺).** - This statement is also incorrect. Acids, by definition, increase the concentration of hydrogen ions in solution. Therefore, this option contradicts the fundamental behavior of acids. - **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. For example, a strong acid like hydrochloric acid (HCl) can have a pH close to 0, while a weak acid like acetic acid (CH₃COOH) might have a pH around 4-5. ### Summary of Key Points: - Acids produce hydrogen ions (H⁺) when dissolved in water, which is the defining characteristic of acidic solutions. - The presence of H⁺ ions lowers the pH of the solution, making it acidic (pH < 7). - Acids do not increase hydroxide ion concentration; they actually decrease it by reacting with OH⁻ ions. - Understanding the behavior of acids in aqueous solutions is crucial for grasping fundamental concepts in chemistry, including acid-base reactions and pH calculations. This thorough understanding of acids will help you in various chemistry topics, including titrations, buffer solutions, and reaction mechanisms.