The correct option is **B. sublimation**. ### Explanation of the Correct Answer 1. **Understanding Sublimation**: - Sublimation is the process where a solid changes directly into a gas without passing through the liquid phase. In this case, when ammonium chloride (NH4Cl) is heated, it does not melt into a liquid but instead transitions directly into gaseous ammonia (NH3) and hydrogen chloride (HCl) gases. This is characteristic of certain solids, including ammonium chloride. 2. **Observation of Gases**: - As the solid NH4Cl is heated, it decomposes and releases gases. The two gases produced are ammonia (NH3) and hydrogen chloride (HCl). The reaction can be represented as: \[ \text{NH}_4\text{Cl (s)} \rightarrow \text{NH}_3\text{ (g)} + \text{HCl (g)} \] - The gradual disappearance of the solid indicates that it is transitioning into gas, which is a hallmark of sublimation. 3. **Formation of White Cloudy Deposit**: - The white cloudy deposit observed on the cooler part of the test tube is likely due to the reaction of the ammonia gas with hydrogen chloride gas, forming ammonium chloride again in the form of tiny solid particles. This is a classic example of a gas-solid reaction where the gases cool and recombine to form a solid, which appears as a white cloud. ### Why the Other Options are Incorrect - **A. Distillation**: - Distillation is a separation technique that involves heating a liquid to create vapor and then cooling the vapor to collect the liquid. In this scenario, we are dealing with a solid that sublimates into gases, not a liquid being separated from a mixture. Therefore, distillation does not apply here. - **C. Precipitation**: - Precipitation refers to the formation of a solid from a solution, typically when two soluble reactants combine to form an insoluble product. In this case, there is no solution involved; instead, we are observing the direct transition of a solid to gas. Thus, precipitation is not relevant. - **D. Evaporation**: - Evaporation is the process where a liquid turns into vapor. Since ammonium chloride is a solid in this experiment and does not pass through a liquid state, evaporation is not the correct term to describe what is happening. ### Summary of Key Points - **Sublimation** is the process where a solid transitions directly to a gas without becoming a liquid. - Heating ammonium chloride produces ammonia and hydrogen chloride gases, which can recombine to form a solid deposit. - Distillation, precipitation, and evaporation do not accurately describe the process occurring with ammonium chloride in this scenario. - Observing the formation of a white cloud indicates a gas-solid reaction, typical of sublimation processes. This thorough understanding of sublimation and the behavior of ammonium chloride under heat will help you in your chemistry studies and exams.