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 the case of ammonium chloride (NH4Cl), when it is heated, it does not melt into a liquid but instead transitions directly into gaseous ammonia (NH3) and hydrogen chloride (HCl) gases. This is a key characteristic of sublimation. 2. **Observation of Gases**: - As the ammonium chloride is heated, it decomposes into these two gases. The observation that "the solid gradually disappears" indicates that the solid is transitioning into gas, which is consistent with 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 gaseous ammonia with hydrogen chloride gas, forming solid ammonium chloride again. This is a classic example of a gas-to-solid transition, which can occur when the gases cool down and react, leading to the formation of a solid product. ### Why 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 create a liquid again. In this scenario, we are dealing with a solid that sublimates into gases, not a liquid being separated. 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, and the process is not about forming a solid from a liquid but rather a solid transitioning 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 and does not melt into a liquid before turning into gas, evaporation is not the correct term to describe what is happening in this experiment. ### Summary of Key Points - **Sublimation** is the process where a solid transitions directly to a gas without becoming a liquid. - Heating ammonium chloride produces gaseous ammonia and hydrogen chloride, which can react to form solid ammonium chloride again on cooler surfaces. - Distillation, precipitation, and evaporation do not accurately describe the process occurring with ammonium chloride in this scenario. ### Revision Summary - **Sublimation**: Solid to gas transition without a liquid phase. - **Gaseous Products**: NH3 and HCl are produced upon heating NH4Cl. - **Cloudy Deposit**: Result of gas reaction forming solid NH4Cl on cooler surfaces. - **Incorrect Options**: Distillation (liquid separation), precipitation (solid from solution), and evaporation (liquid to gas) do not apply.