Changes of states

The kinetic theory model can be used to explain how a substance changes from one state to another.

Reason for a substance to change its state

In according to the kinetic particle theory, the particles of matter are in constant motion. Thus they have kinetic energy. Gas has the highest energy content than liquid. Solid has the least energy content.

Changes in states of matter on heating

When a matter is heated or cooled, the heat taken in or given out causes the change in kinetic energy of the particles. For this reason, a substance changes its state.

Melting and Melting Point

Melting is a process when a substance changes from a solid to a liquid. The temperature at which a solid becomes a liquid is called its melting point. Different substances have a different melting point. Oxygen, ice and iron have melting points at -219, 0 and 1535  ºC respectively.

When a solid is heated, its particles get more energy and vibrate more. This makes the solid expand. At the melting point, the particles vibrate so much and they break away from their positions. Finally, solid turns into a liquid. Things happen when the particles of a solid are heated until it melts-

Effect of heat on the particles of a solid

Boiling and Boiling Point

When a substance changes from a liquid to a gas at the boiling temperature, it is called boiling.

When a liquid is heated, its particles get more energy to move faster. They bump into each other more often and bounce further apart. This makes the liquid expand. At this point, the particles get enough energy to overcome the forces between them. They break away to form a gas. Things happen to the particles of a liquid that are heated until it boils-

Effect of boiling on the particles of a liquid

Evaporation

Sometimes a liquid can turn into a gas at a temperature below its boiling point. This is called evaporation. Below the boiling point, some particles of a liquid have enough energy to escape and form a gas. For this reason, the liquid evaporates.

A liquid that evaporates quickly at room temperature is called a volatile liquid. They have boiling point just above the room temperature. Example- petrol and perfume.

Difference between Boiling point and Evaporation:

Boiling Point	Evaporation
It occurs at boiling point	It occurs below the boiling point
It occurs rapidly	It occurs slowly
It occurs throughout the liquid	It occurs only at the surface of the liquid