Definition of Evaporation

Evaporation is a change of state from the liquid to the gaseous state. It takes place at all temperatures and from the surface of the liquid. Volatile liquids such as Freon, ether, alcohol, vaporize easily. Evaporation causes cooling because latent heat is taken from the body to change the liquid to the gaseous state. Example the human utilizes the process of evaporation in cooling after exercise.

Factors Affecting the Rate of Evaporation

1. Temperature: The rate of evaporation increases as temperature increase.
2. Pressure: The rate of evaporation decreases with increase in pressure.
3. Area of liquid surface exposed: The greater the surface area of liquid exposed, the more rapid will be the evaporation.
4. The nature of the liquid: The lower the boiling point of the liquid, the greater will be the rate of evaporation.
5. Wind and dryness of the air: Dryness of the air around the body causes rapid evaporation from the body. Wind blows away liquid vapour and causes rapid evaporation.

Application of the Cooling Effect of Evaporation and Latent Heat

1. Refrigerator
2. Air conditioner

Application in Refrigerators

Refrigerators make use of the cooling effect of evaporation. The volatile liquid such as liquid ammonia or freon evaporates inside copper coil surrounding the freezing compartment, supported by electric pump which reduced the pressure.  As the volatile liquid evaporates in those coils, it absorbs heat from the surround air, consequently cooling the inside of the refrigerator and its content.

The vapour produced is pumped off into the condenser, where it is compressed by the pump and condenses back to liquid.  The latent heat given out during this condensation is quickly dissipated by an arrangement of cooling fins at the back of refrigerator.

Heat is eliminated by convection and radiation to the surroundings and by conduction into fins.  The liquid is again passed into the evaporator coil and thus the level of cooling is regulated by a thermostat connected to the switch.

EVALUATION – Post answers at the bottom

1. What is evaporation?
2. Mention and explain that affects the rate of evaluation.
3. Explain the mode of operation of the refrigerator.