### Correct Option: D. The dissolution of metallic sodium is exothermic. #### Detailed Explanation: 1. **Understanding Dissolution**: - Dissolution is the process where a solute (in this case, sodium chloride or metallic sodium) dissolves in a solvent (water) to form a solution. This process can either absorb heat (endothermic) or release heat (exothermic). 2. **Dissolution of Sodium Chloride (NaCl)**: - When sodium chloride (table salt) is dissolved in water, the ionic bonds between the sodium ions (Na⁺) and chloride ions (Cl⁻) must be broken. This requires energy, which is absorbed from the surroundings, making the process endothermic. - However, the water molecules surround the ions and stabilize them, releasing energy in the form of heat. The overall process is slightly endothermic, as the energy required to break the ionic bonds is greater than the energy released during hydration. 3. **Dissolution of Metallic Sodium (Na)**: - When metallic sodium is added to water, it reacts vigorously rather than simply dissolving. The reaction can be represented as: \[ 2 \text{Na (s)} + 2 \text{H}_2\text{O (l)} \rightarrow 2 \text{NaOH (aq)} + \text{H}_2 (g) \] - This reaction is highly exothermic, meaning it releases a significant amount of heat. The heat released is due to the formation of sodium hydroxide (NaOH) and hydrogen gas (H₂). The energy released during the formation of new bonds in NaOH and the kinetic energy of the hydrogen gas contribute to the overall exothermic nature of the process. 4. **Why Other Options Are Incorrect**: - **Option A (Both processes are exothermic)**: This is incorrect because the dissolution of sodium chloride is endothermic, while the dissolution of metallic sodium is exothermic. - **Option B (Both processes are endothermic)**: This is also incorrect. As explained, the dissolution of metallic sodium is exothermic due to the heat released during the reaction with water. - **Option C (The dissolution of metallic sodium is endothermic)**: This is incorrect. The dissolution of metallic sodium is exothermic, as it releases heat during the reaction with water. 5. **Common Pitfalls**: - Students may confuse the dissolution of sodium chloride with the reaction of metallic sodium with water. It’s important to remember that sodium chloride simply dissolves, while metallic sodium reacts. - Misunderstanding the terms "endothermic" and "exothermic" can lead to incorrect conclusions. Remember that endothermic processes absorb heat, while exothermic processes release heat. ### Revision Summary: - The dissolution of sodium chloride in water is endothermic due to the energy required to break ionic bonds. - The dissolution of metallic sodium in water is exothermic because it involves a vigorous reaction that releases heat. - Always differentiate between simple dissolution and chemical reactions when discussing solutes in water. - Remember the definitions of endothermic and exothermic processes to avoid confusion in similar questions.