ANS:C - evaporation

Evaporation is a process by which a liquid substance undergoes a phase change from liquid to vapor state at temperatures below its boiling point. This transition occurs at the liquid's surface as molecules gain enough kinetic energy to escape the attractive forces of neighboring molecules and enter the vapor phase. Here's a detailed explanation of the process:

1. Molecular Kinetics: In a liquid, molecules are in constant motion due to thermal energy. At the liquid's surface, molecules experience an imbalance of attractive forces, as there are no molecules above them to exert an equal attractive force. This results in molecules near the surface having a higher probability of escaping into the vapor phase.
2. Energy Transfer: Molecules at the liquid's surface gain kinetic energy from the surroundings, typically through thermal energy from the environment. As molecules gain sufficient kinetic energy, they overcome the intermolecular forces holding them in the liquid phase and enter the vapor phase.
3. Vaporization: The process by which molecules transition from the liquid phase to the vapor phase is known as vaporization. Evaporation is a type of vaporization that occurs specifically at the liquid's surface.
4. Factors Affecting Evaporation:
   • Temperature: Higher temperatures generally increase the rate of evaporation by providing more thermal energy to liquid molecules, increasing their kinetic energy and the likelihood of escaping into the vapor phase.
   • Surface Area: A larger surface area exposed to the surrounding environment allows for more molecules to undergo evaporation simultaneously, thereby increasing the rate of evaporation.
   • Humidity: The presence of water vapor in the surrounding air affects evaporation. Higher humidity levels reduce the rate of evaporation, as the air already contains a significant amount of water vapor, limiting the capacity of the air to absorb additional moisture.
   • Airflow: Movement of air across the liquid's surface helps carry away vapor molecules, reducing the concentration of vapor near the liquid's surface and facilitating faster evaporation.
5. Applications of Evaporation:
   • Drying: Evaporation is commonly used for drying processes to remove moisture from solids or liquids.
   • Cooling: Evaporation of sweat from the skin helps regulate body temperature by dissipating heat through latent heat of vaporization.
   • Purification: Evaporation is utilized in various purification processes, such as the desalination of seawater to produce freshwater.

In summary, evaporation is a natural phenomenon and an essential process in various industrial, environmental, and biological contexts. It involves the transition of molecules from the liquid phase to the vapor phase and is influenced by factors such as temperature, surface area, humidity, and airflow.