The Kinetic theory of matter states that all matter is made up of particles, and exists in one of three states, either solid, liquid or gas.
Their arrangements are often summarised as follows:
In the solid state the particles are packed together in an ordered, regular pattern with strong bonds holding the particles together.
When heat is applied and eventually breaks the strong bonds between the particles the solid melts.
When heat is applied to a solid, its temperature rises, but when the solid begins to melt, the heat supplied is used to break the bonds. Hence the temperature remains constant until all the solid has melted.
The liquid state consists of particles in constant motion, free to pass over one another.
Forces of attraction still exist, which prevent liquid particles from escaping, but these forces are much weaker than in a solid.
If the liquid is now supplied with heat, its temperature rises, but as the liquid boils the temperature remains constant as this heat is used to overcome the forces between the liquid particles allowing them to become widely separated.
This process is called vaporisation, and the substance is now in the gaseous state.
The particles are free to move randomly in any direction, and forces between particles are weak or negligible.
This steady temperature as a substance changes state can be observed for the reverse processes of liquefying and freezing. In these cases bonds are made, not broken during the change of state.