Question

An air cooler will work most efficiently under which condition?

• A. Cooler facing window, no salt in water
• B. Cooler facing away from window, no salt
• C. Cooler facing window, salt added in water
• D. Cooler facing away from window, salt added

Hint:

To maximize air cooler performance, think about maximizing the evaporation rate. Two key requirements are: 1. Dry Air Supply: Always ensure good ventilation by placing the cooler at an open window. 2. High Evaporation Rate of Water: Use clean, pure water. Adding impurities like salt hinders evaporation.

Answer 1

The Correct Option is A

Step 1: Understanding the Question: We need to identify the set of conditions from the options that maximizes the cooling efficiency of an air cooler. This involves two factors: cooler placement (ventilation) and water composition (evaporation rate).

Step 2: Detailed Explanation: Factor 1: Cooler Placement and Ventilation An air cooler's efficiency depends on the rate of evaporation. Evaporation is faster when the air is hot and has low humidity.
• Facing window (i.e., at an open window): This setup allows the cooler to draw in a continuous supply of fresh, hot, dry air from outside. This is the ideal condition for maximum evaporation and cooling.
• Facing away from window (i.e., inside a closed room): This setup recirculates the indoor air. The cooler adds moisture to this air, quickly raising the room's humidity. High humidity slows down evaporation, making the cooler inefficient. Conclusion: The cooler must be placed at a window for ventilation. This eliminates options (B) and (D). Factor 2: Water Composition (Adding Salt) The cooling process relies on the evaporation of water. We need to consider how adding salt affects the evaporation rate.
• Adding a non-volatile solute like salt to water reduces the water's vapor pressure. This phenomenon is known as vapor pressure lowering.
• The rate of evaporation is directly proportional to the vapor pressure of the liquid. A lower vapor pressure means that water molecules escape into the air less readily.
• Therefore, adding salt to the water will decrease the rate of evaporation and, consequently, reduce the cooling efficiency of the air cooler. Conclusion: No salt should be added to the water. This eliminates option (C).

Step 3: Final Answer: Combining both factors, the most efficient condition is to have the cooler placed at a window to draw in fresh air, and to use pure water with no salt. This corresponds to option (A).