Under what condition does the body melt; hardens?

Atoms and molecules of solids and liquids are held near each other by powerful forces of mutual attraction. In order for a solid to begin to melt, it is necessary to heat the body to a special temperature – the melting point. In the process of heating, the oscillatory motions of the particles that make up the body intensify, the distance between them increases, and the volume of the body increases. Further heating leads to a weakening of the forces of attraction of the particles, crystal bonds are broken and the body passes into a liquid state. When hardening, the whole process goes in the opposite direction. Cooling down, the body, which is at the melting temperature, goes into a state where the activity of molecules decreases, the distance between them decreases and the forces of mutual attraction increase, which leads to a transition to a solid state.