ANS:B - sublimation

Sublimation is a process in which a substance transitions directly from a solid phase to a gas phase without passing through the intermediate liquid phase. Here's an explanation:

1. Phase Transition: Sublimation is a type of phase transition, similar to melting (solid to liquid) and vaporization (liquid to gas), but it occurs directly from solid to gas. This means that the substance skips the liquid phase entirely.
2. Endothermic Process: Sublimation is an endothermic process, meaning it requires the input of energy (heat) to overcome the intermolecular forces holding the solid together. As these forces are overcome, the solid particles gain enough energy to break free from the solid lattice and enter the gas phase.
3. Examples: Many substances undergo sublimation under specific conditions of temperature and pressure. Some common examples include:
   • Dry ice (solid carbon dioxide) sublimes at temperatures above -78.5°C (-109.3°F) at standard atmospheric pressure.
   • Iodine sublimes at room temperature and atmospheric pressure, producing a purple vapor.
   • Camphor, naphthalene (used in mothballs), and certain volatile essential oils also exhibit sublimation.
4. Applications: Sublimation has various practical applications, including:
   • Purification: Sublimation can be used to purify substances by separating them from impurities that do not sublime. This technique is called sublimation purification.
   • Freeze-drying: In freeze-drying (lyophilization), sublimation is used to remove moisture from food, pharmaceuticals, and biological samples while preserving their structure and properties.
   • Deposition: Sublimation is also used in vapor deposition techniques to deposit thin films of materials onto substrates for applications in electronics, coatings, and optics.
5. Conditions: Sublimation occurs under specific conditions of temperature and pressure. These conditions vary depending on the substance. For sublimation to occur, the surrounding environment must provide enough energy to overcome the substance's sublimation energy barrier.

In summary, sublimation is a phase transition in which a substance transitions directly from a solid phase to a gas phase without passing through the liquid phase. It is an endothermic process that occurs under specific conditions of temperature and pressure and has various practical applications in purification, freeze-drying, and deposition processes.