**Correct Option: B** ### Detailed Explanation To understand why option B is correct and the others are not, we need to delve into the definitions and behaviors of acids and bases in aqueous solutions. #### Understanding Acids and Bases 1. **Acids**: Substances that donate protons (H⁺ ions) when dissolved in water. This increases the concentration of hydrogen ions (H⁺) in the solution, leading to a lower pH (typically less than 7). 2. **Bases**: Substances that accept protons or donate hydroxide ions (OH⁻) in solution. This results in a decrease in the concentration of hydrogen ions (H⁺), leading to a higher pH (typically greater than 7). ### Analysis of Each Option **A. Acids increase the concentration of hydroxide ions (OH-) in solution.** - **Why it's wrong**: Acids do not increase the concentration of hydroxide ions. Instead, they increase the concentration of hydrogen ions (H⁺). When an acid dissolves in water, it dissociates to release H⁺ ions, which can lead to a decrease in OH⁻ concentration due to the water equilibrium (H₂O ⇌ H⁺ + OH⁻). Therefore, this statement is incorrect. **B. Bases decrease the concentration of hydrogen ions (H+) in solution.** - **Why it's correct**: When a base is added to an aqueous solution, it either dissociates to release OH⁻ ions or reacts with H⁺ ions to form water. This effectively reduces the concentration of H⁺ ions in the solution, which raises the pH. For example, sodium hydroxide (NaOH) dissociates in water to give Na⁺ and OH⁻. The OH⁻ ions can react with H⁺ ions to form water, thus decreasing the H⁺ concentration. This is the correct description of the behavior of bases. **C. Acids react with metals to produce hydroxides and hydrogen gas.** - **Why it's wrong**: While acids do react with some metals to produce hydrogen gas, they typically produce a salt and hydrogen gas, not hydroxides. For example, when hydrochloric acid (HCl) reacts with zinc (Zn), it produces zinc chloride (ZnCl₂) and hydrogen gas (H₂). The formation of hydroxides is characteristic of base reactions, not acids. **D. Bases have a pH less than 7.** - **Why it's wrong**: Bases actually have a pH greater than 7. The pH scale ranges from 0 to 14, where a pH of 7 is neutral (pure water). A pH less than 7 indicates an acidic solution, while a pH greater than 7 indicates a basic (alkaline) solution. Therefore, this statement is incorrect. ### Summary of Key Concepts - **Acids** increase H⁺ concentration and lower pH (less than 7). - **Bases** decrease H⁺ concentration and raise pH (greater than 7). - Acids react with metals to produce salts and hydrogen gas, not hydroxides. - The pH scale is a measure of acidity or basicity, with 7 being neutral. ### Revision Summary - Acids donate H⁺ ions, increasing their concentration in solution. - Bases accept H⁺ ions or provide OH⁻ ions, decreasing H⁺ concentration. - Acids react with metals to produce salts and hydrogen gas, not hydroxides. - Bases have a pH greater than 7, while acids have a pH less than 7.