What happens to a water pool on a day when the humidity of the air is very low?

Analysis

The question asks about the effect of low atmospheric humidity on a body of water (a pool). To answer this, we need to understand the relationship between the air's moisture content and the process of evaporation.

Concept: Evaporation and Humidity

• Evaporation is the process where liquid molecules gain enough kinetic energy to escape into the air as vapor.
• Humidity refers to the amount of water vapor already present in the air.
• Relative Humidity is a measure of how "full" the air is with water vapor. When humidity is low, the air is "dry" and has a high capacity to hold more water vapor.
• The Relationship: Evaporation occurs most rapidly when the air is dry because there is a large concentration gradient between the water surface (saturated) and the surrounding air (unsaturated). The air "demands" more moisture, causing water molecules to leave the pool surface at a much faster rate.

Step-by-Step Solution

1. Identify the condition: The air has very low humidity, meaning it is very dry.
2. Evaluate the process: Because the air is dry, the space immediately above the water pool is not saturated with water vapor.
3. Determine the effect: Molecules from the pool will escape into the air more frequently to try and reach an equilibrium. This process is known as evaporation.
4. Conclusion: Since the air is dry (low humidity), the rate at which these molecules escape is significantly higher than on a humid day. Therefore, evaporation happens rapidly.

Why other options are incorrect:

• A & C: While evaporation can cause a slight cooling effect (latent heat of vaporization), the primary physical phenomenon described by the state of the air is the rate of the phase change, not the temperature change of the pool.
• B: Slow evaporation occurs when humidity is high (the air is already saturated and cannot easily accept more water vapor).

Exam Tip

Think of a dry sponge: if you place a dry sponge in a room with high humidity, it won't dry out quickly. If you place it in a room with low humidity (like a room with a dehumidifier), it dries out very fast. The air acts like a sponge for water vapor—the drier the air, the faster it "soaks up" the water from the pool.