The correct option is **C. NO2**. ### Detailed Explanation: To determine which of the given nitrogen derivatives results in an aqueous solution with a pH of less than 7, we need to analyze the behavior of each compound in water and how they affect the pH. 1. **Understanding pH**: - The pH scale measures the acidity or basicity of a solution. A pH of 7 is neutral, below 7 is acidic, and above 7 is basic. - Acids release hydrogen ions (H⁺) in solution, which lower the pH. 2. **Analyzing Each Option**: - **A. N2O (Nitrous oxide)**: - N2O is a neutral gas and does not react with water to produce H⁺ ions. Therefore, it does not affect the pH significantly and remains close to neutral (pH ≈ 7). - **B. NO (Nitric oxide)**: - NO is also a neutral gas and does not form acids or bases when dissolved in water. It does not contribute to the acidity of the solution, so it also maintains a pH around 7. - **C. NO2 (Nitrogen dioxide)**: - NO2 is a significant contributor to acid formation. When NO2 dissolves in water, it reacts to form nitric acid (HNO3) and nitrous acid (HNO2): \[ 2 NO_2 + H_2O \rightarrow HNO_3 + HNO_2 \] - Both HNO3 and HNO2 are acids, which release H⁺ ions into the solution, thus lowering the pH below 7, making the solution acidic. - **D. NaNO3 (Sodium nitrate)**: - NaNO3 is a salt formed from a strong acid (HNO3) and a strong base (NaOH). When dissolved in water, it dissociates into Na⁺ and NO3⁻ ions. Neither of these ions contributes to acidity or basicity, so the solution remains neutral (pH ≈ 7). ### Why Other Options Are Incorrect: - **A. N2O**: Does not produce H⁺ ions in solution; remains neutral. - **B. NO**: Does not react to form acids; remains neutral. - **D. NaNO3**: Dissociates into neutral ions; does not affect pH. ### Summary of Key Points: - **NO2** reacts with water to form acids, lowering the pH. - **N2O** and **NO** do not affect pH significantly; they remain neutral. - **NaNO3** is a neutral salt that does not contribute to acidity. - The presence of H⁺ ions in a solution is what determines if the pH is below 7 (acidic). ### Revision Summary: - **pH < 7** indicates an acidic solution. - **NO2** forms acids in water, lowering pH. - **N2O**, **NO**, and **NaNO3** do not lower pH; they remain neutral. - Understanding the behavior of compounds in water is crucial for predicting pH changes.