You learned in the last lesson that kinetic energy is the energy that an object in motion has. You also learned that the mass of an object and its speed determine its kinetic energy. In this topic, you will learn what kinetic energy is and how it can be used to explain some things.

Kinetic theory 

According to the kinetic theory, particles of matter have kinetic energy and are always moving. When the temperature of the matter goes up, the particles get more kinetic energy and can move faster.

In JSS1, you learned that the particles in a solid are close to each other. Since a solid has a set shape and size, it can’t flow. In a liquid, the particles are not as close together as they are in a solid. Because of this, a liquid can move. You also learned that the particles in a gas are loosely packed and move around randomly, so gas doesn’t have a clear shape or size.

This is a summary of the kinetic theory of gases:

1. The particles of gas have kinetic energy and move randomly in straight lines.

2. The molecules of gas collide with each other.

3. When molecules of gas collide with each other, they do not break.

4. Compared to the size of the container, the gas molecules don’t take up much space.

5. The average speed of the gas molecules is what determines the temperature of the gas.

Explanation of some phenomena using kinetic theory

Change of state

The state of a substance can change when heat energy is added or taken away. When a solid is heated, the bonds between its molecules break, and the molecules gain speed. Then the solid changes into a liquid. When the liquid is heated, the molecules gain kinetic energy and start moving around randomly, turning the liquid into gas. Melting is the process by which a solid turns into a liquid, and evaporation is the process by which a liquid turns into a vapour or gas.

Also, when a gas is cooled until it turns into a liquid, a process called condensation, the gas loses its kinetic energy. When the liquid is cooled, it loses its motion energy and becomes solid. This is called freezing or solidification.

A process called sublimation lets a gas turn back into a solid without going through the liquid state. For example, when water vapour is in air that is below freezing, it changes straight to ice without going through the liquid state first. This is how clouds get snow. A solid can also directly turn into a gas through a process called sublimation. Camphor is an example of a solid that can do this.


When heat is applied to a liquid, the molecules of the liquid gain kinetic energy, which makes them move to the surface of the liquid. The liquid’s saturated vapour pressure also goes up. When the liquid’s saturated pressure is just a little bit higher than the pressure of the air around it, the liquid starts to boil. When this happens is called the boiling point. The atmospheric pressure and the presence of impurities both affect the boiling point of a liquid. When there are impurities in a liquid, the temperature at which it boils rises. The liquid’s boiling point is higher when the air pressure is higher, and vice versa.


When a liquid is heated, the particles in the liquid gain kinetic energy and some of them move to the surface. As they get faster, they move away from the surface of the liquid. We call this “evaporation.” How fast a liquid evaporates depends on its temperature and what it is made of. The rate of evaporation goes up as the temperature goes up, and a liquid that is volatile will evaporate faster than one that is not volatile.

Differences between boiling and evaporation

1There is a certain temperature at which something boils.When liquid particles gain kinetic energy to move away from the surface of a liquid, this is called evaporation.
2The rate at which a liquid evaporates depends on how hot it is and what it is made of.The boiling point of a liquid is affected by the air pressure and the number of impurities in the liquid.
3The boiling occurs throughout the liquid.Evaporation happens at the liquid’s surface.

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