The correct option is **A. Sublimation**. ### Detailed Explanation: 1. **Understanding Sublimation**: - Sublimation is the process where a solid changes directly into a gas without passing through the liquid state. This occurs when the molecules of the solid gain enough energy to overcome the forces holding them together and escape into the air as gas. - Common examples of substances that undergo sublimation include dry ice (solid carbon dioxide), camphor, and naphthalene. When these substances are exposed to air, they gradually disappear as they transition from solid to gas. 2. **Why Sublimation is the Correct Answer**: - In the case of camphor, naphthalene, and dry ice, when they are left in an open environment, they do not melt into a liquid but instead turn directly into vapor. This is a hallmark of sublimation. - For instance, dry ice at room temperature will sublimate into carbon dioxide gas, which is why it seems to "disappear" when left out in the open. 3. **Why the Other Options are Incorrect**: - **B. Evaporation**: - Evaporation is the process where a liquid turns into a gas. This typically occurs at the surface of a liquid and requires the liquid to be present. Since the substances in question are solids, evaporation does not apply here. - **C. Diffusion**: - Diffusion refers to the movement of particles from an area of higher concentration to an area of lower concentration. While it is a process that can occur with gases and liquids, it does not describe the disappearance of a solid substance when exposed to air. - **D. Radiation**: - Radiation is the emission of energy as electromagnetic waves or as moving subatomic particles. It does not pertain to the physical state change of a substance like sublimation does. ### Example Calculation: While there are no calculations needed for this question, it is important to understand the energy aspect of sublimation. The energy required for sublimation can be quantified using the enthalpy of sublimation, which is the amount of energy needed to convert a mole of a solid directly into a gas at constant pressure. ### Common Pitfalls: - Confusing sublimation with evaporation: Remember that sublimation involves solids transitioning directly to gas, while evaporation involves liquids. - Misunderstanding the conditions under which sublimation occurs: Sublimation typically occurs at room temperature and pressure for certain substances, while others may require specific conditions. ### Revision Summary: - **Sublimation** is the direct transition of a solid to a gas without becoming a liquid. - **Examples** of substances that sublime include camphor, naphthalene, and dry ice. - **Evaporation** involves liquids, while **diffusion** and **radiation** do not describe the disappearance of solids. - Understanding the properties of substances and their phase changes is crucial for identifying processes like sublimation.