ANS:B - liquid film

In the context of mass transfer, a liquid film refers to a thin layer of liquid that surrounds a gas bubble or a solid surface. This liquid film plays a crucial role in facilitating the transfer of substances (such as gases or solutes) between the gas phase (for gas-liquid systems) or between the solid and liquid phases (for solid-liquid systems). Here's how the liquid film operates in various scenarios:

1. Gas-Liquid Systems:
   • In a gas-liquid system, such as the absorption of gases (like carbon dioxide in water), the liquid film forms around gas bubbles. The gas bubbles typically arise due to the introduction of gas into the liquid phase, such as during carbonation in cold drinks or aeration in wastewater treatment.
   • The gas bubble is surrounded by a thin layer of liquid, known as the liquid film. This film serves as the interface between the gas and liquid phases, where mass transfer occurs.
   • During absorption, for example, gas molecules from the bubble dissolve into the liquid film, diffusing through it to reach the bulk liquid. Conversely, in stripping processes, gas molecules may transfer from the liquid phase to the gas bubble through the liquid film.
2. Solid-Liquid Systems:
   • In a solid-liquid system, the liquid film forms around solid surfaces immersed in a liquid. This situation commonly arises in processes such as dissolution, precipitation, or chemical reactions occurring at solid surfaces.
   • The liquid film around the solid surface facilitates mass transfer between the solid and liquid phases. For instance, in dissolution processes, solute molecules from the solid surface dissolve into the liquid film before diffusing into the bulk liquid. Similarly, in precipitation reactions, reactants may adsorb onto the solid surface from the liquid film before undergoing reaction.

In summary, the liquid film is a thin layer of liquid that plays a critical role in mass transfer processes, both in gas-liquid and solid-liquid systems. It serves as the interface where substances can transfer between different phases, facilitating efficient mass transfer and chemical reactions. Understanding the dynamics of the liquid film is essential for optimizing industrial processes such as absorption, desorption, dissolution, and surface reactions.