In this experiment, you will investigate the cooling effect of evaporation. You wraps a thin piece of wet cotton cloth around the bulb of thermometer A. The...

(a) Thermometer B reading after 3 minutes

Thermometer B has a dry bulb and no evaporation occurs from it. It simply reads the air temperature of the room: 24 °C. [1]

(b) Total temperature drop on thermometer A

Thermometer A starts at 24 °C and falls to a steady 18 °C:

\(\text{Temperature drop} = 24 - 18 = \mathbf{6}\) °C [1]

(c) Why thermometer A reads lower than thermometer B

Water evaporates from the wet cloth wrapped around the bulb of thermometer A. [1]

Evaporation is a process in which the most energetic molecules escape from the liquid surface. To break free, these molecules require energy (latent heat of vaporisation), which is drawn from the thermometer bulb. This loss of thermal energy causes the temperature reading to fall. Thermometer B, having no water on it, does not lose energy this way and stays at room temperature. [1]

(d) Why thermometer A stops falling

A dynamic equilibrium is reached: the rate at which the wet bulb gains heat from the warmer surroundings (by conduction and convection from the 24 °C air) equals the rate at which it loses heat through evaporation. With no net energy change, the temperature remains constant at 18 °C. [1]

(e) Effect of faster fanning

The temperature on thermometer A would drop even lower. [1]

Faster air flow sweeps away the layer of saturated (water-vapour-rich) air close to the wet cloth more quickly. This maintains a steeper concentration gradient, increasing the rate of evaporation and therefore the rate at which latent heat is removed from the bulb. [1]

(f) Two other factors affecting the rate of evaporation

Any two from: [2]

• Temperature of the liquid or surroundings: higher temperature gives molecules more kinetic energy, so more can escape the surface.
• Humidity of the surrounding air: drier air allows faster evaporation because there is a greater concentration gradient.
• Surface area exposed to the air: a larger area gives more molecules the opportunity to escape per second.

(g) Practical application of evaporative cooling

Any one valid example: [1]

• Sweating cools the human body: perspiration evaporates from the skin, absorbing latent heat and lowering body temperature.
• Wrapping a wet cloth around a water bottle keeps the drink cool in hot weather.
• Evaporative coolers (desert coolers) blow air over wet pads to cool rooms in dry climates.