The correct option for the question "blowing air over a liquid aids evaporation by" is **B. decreasing its vapour pressure**. ### Detailed Explanation **Understanding Evaporation:** Evaporation is the process by which molecules at the surface of a liquid gain enough energy to enter the gas phase. This process is influenced by several factors, including temperature, surface area, and vapor pressure. **Vapor Pressure:** Vapor pressure is the pressure exerted by the vapor in equilibrium with its liquid at a given temperature. It represents the tendency of particles to escape from the liquid into the vapor phase. When the vapor pressure of the liquid is high, fewer molecules will evaporate because the air above the liquid is already saturated with vapor. **How Blowing Air Affects Evaporation:** When air is blown over the surface of a liquid, it helps to remove the vapor molecules that have just evaporated. This action effectively reduces the concentration of vapor above the liquid surface, which in turn decreases the vapor pressure. As a result, the difference between the vapor pressure of the liquid and the surrounding air increases, promoting further evaporation. ### Why Option B is Correct: - **Decreasing Vapor Pressure:** Blowing air over the liquid decreases the vapor pressure above the liquid, allowing more molecules to escape into the vapor phase. This is the primary reason why blowing air aids evaporation. ### Why the Other Options are Incorrect: **A. Increasing its surface area:** - While increasing the surface area of a liquid does enhance evaporation (as more molecules are exposed to the air), blowing air does not directly increase the surface area of the liquid. The surface area is a fixed property of the liquid's container. Therefore, this option is not the best explanation for how blowing air aids evaporation. **C. Increasing its temperature:** - Blowing air over a liquid does not necessarily increase its temperature. In fact, if the air is cooler than the liquid, it could even lower the temperature. While higher temperatures do increase evaporation rates, the act of blowing air itself does not guarantee an increase in temperature. **D. Decreasing its density:** - The density of a liquid is primarily determined by its temperature and composition. Blowing air over a liquid does not significantly change its density. This option does not relate directly to the process of evaporation. ### Summary of Key Points: - **Evaporation Process:** Molecules at the surface of a liquid escape into the gas phase. - **Vapor Pressure:** The pressure exerted by vapor in equilibrium with its liquid; lower vapor pressure promotes evaporation. - **Effect of Blowing Air:** Blowing air removes vapor molecules, decreasing vapor pressure and enhancing evaporation. - **Incorrect Options:** Increasing surface area, temperature, or density do not accurately describe the effect of blowing air on evaporation. This understanding of evaporation and the role of vapor pressure is crucial for grasping the principles of thermodynamics and fluid dynamics in physics.