Evaporation – SS2 Physics Lesson Note (Second Term)

Evaporation is the process by which liquid molecules break away from the surface of a liquid and remain as vapour, so the liquid changes to gas. It happens at the surface of the liquid at all temperatures. Boiling is different: it happens throughout the liquid at one fixed temperature, called the boiling point.

Behavioural objectives

By the end of the lesson, students should be able to:

• define evaporation;
• state the differences between evaporation and boiling;
• explain how pressure and impurities affect the boiling point of a liquid;
• define melting point and freezing point;
• describe how evaporation and pressure are used in the pressure cooker and the refrigerator.

What is evaporation?

Evaporation vector illustration. Labeled liquid surface substance change to gas state scheme. Educational explanation diagram with nature phenomenon when sun heats warm water and triggers rising vapor

Evaporation is defined as the process by which liquid molecules break away from the surface of the liquid to remain as vapour. It can also be simply defined as the process by which liquid changes to gas or vapour.

What is boiling point?

As heat is added to a liquid, its temperature increases steadily until it reaches a certain temperature. At this temperature, rapid evaporation occurs in every part of the liquid, with bubbles of vapour escaping to the surface. This process is known as boiling.

How does evaporation differ from boiling?

How does pressure affect boiling?

An increase in pressure at the surface of a liquid raises the boiling point of the liquid. Conversely, a decrease in pressure lowers the boiling point of the liquid.

How do impurities affect boiling?

The presence of impurities or dissolved substances in a liquid raises the boiling point of the liquid but lowers the freezing point.

What are melting point and freezing point?

The melting point is the temperature at which a solid substance has its bonds broken so that it flows as a liquid. It is also defined as the constant temperature at which a substance changes state from solid to liquid.

In the reverse direction, the constant temperature at which a substance changes state from liquid to solid is called the freezing point.

How is boiling used in the pressure cooker?

The fact that increased pressure raises the boiling point is put to useful application in the pressure cooker. The increased pressure of the trapped gas above the liquid raises the boiling point of the liquid inside the cooker. This provides the high cooking temperature needed to conserve fuel and save time.

How does a refrigerator use evaporation?

Refrigerators make use of the cooling effect of evaporation. A volatile liquid such as liquid ammonia or Freon evaporates inside the copper coil surrounding the freezing compartment, supported by an electric pump which reduces the pressure. As the volatile liquid evaporates in those coils, it absorbs heat from the surrounding air. This cools the inside of the refrigerator and its contents.

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 the refrigerator.

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

Evaluation

1. Define evaporation.
2. State two differences between evaporation and boiling.
3. What happens to the boiling point of a liquid when the pressure at its surface increases?
4. How do dissolved substances affect the boiling point and the freezing point of a liquid?
5. Explain how evaporation produces cooling in a refrigerator.

Assignment

1. Define melting point and freezing point.
2. Explain how the pressure cooker uses the effect of pressure on boiling point to save fuel and time.