The correct option is **B. lead (II) nitrate**. ### Detailed Explanation: 1. **Understanding the Properties of Solid X**: - The problem states that solid X, when heated, gives off a brown gas. This is a crucial clue. The brown gas is likely nitrogen dioxide (NO₂), which is produced when lead (II) nitrate decomposes upon heating. The reaction can be represented as: \[ 2 \text{Pb(NO}_3\text{)}_2 \rightarrow 2 \text{PbO} + 4 \text{NO}_2 + \text{O}_2 \] - This indicates that X could be lead (II) nitrate. 2. **Solubility in Sodium Hydroxide**: - The problem states that X is soluble in excess sodium hydroxide (NaOH). Lead (II) nitrate is soluble in NaOH, forming a soluble lead (II) hydroxide complex: \[ \text{Pb(NO}_3\text{)}_2 + 2 \text{NaOH} \rightarrow \text{Pb(OH)}_2 \downarrow + 2 \text{NaNO}_3 \] - When excess NaOH is added, lead (II) hydroxide can further react to form a soluble complex, such as lead (II) tetrahydroxide. 3. **Insolubility in Ammonium Hydroxide**: - The problem also states that X is insoluble in excess ammonium hydroxide (NH₄OH). Lead (II) hydroxide is indeed insoluble in excess NH₄OH, which differentiates it from other possible candidates. ### Evaluating Other Options: - **A. Basic Lead Carbonate**: - Basic lead carbonate (2PbCO₃·Pb(OH)₂) does not produce a brown gas upon heating; instead, it decomposes to lead oxide and carbon dioxide. It is also soluble in excess NaOH but would not be the correct answer due to the gas produced. - **C. Sodium Carbonate**: - Sodium carbonate (Na₂CO₃) does not produce a brown gas when heated. Instead, it decomposes to sodium oxide and carbon dioxide. It is also soluble in NaOH but does not fit the criteria of producing a brown gas. - **D. Zinc Nitrate**: - Zinc nitrate (Zn(NO₃)₂) does produce a brown gas (NO₂) when heated, but it is soluble in both NaOH and NH₄OH, which contradicts the problem statement. Therefore, it cannot be the correct answer. ### Summary of Key Points: - **Lead (II) nitrate** decomposes to produce nitrogen dioxide (brown gas) when heated. - It is soluble in excess sodium hydroxide but insoluble in excess ammonium hydroxide. - Other options either do not produce the brown gas or do not meet the solubility criteria. ### Revision Summary: - **Lead (II) nitrate** is the only compound that fits all criteria: produces brown gas upon heating, soluble in NaOH, and insoluble in NH₄OH. - Remember the decomposition reaction of lead (II) nitrate for gas identification. - Understand the solubility rules for hydroxides in sodium and ammonium hydroxides. - Differentiate between compounds based on their thermal decomposition products and solubility characteristics.