How do molecules behave in different phases of matter?
The behaviour of molecules in different phases of matter represents a balance between the kinetic energies of the molecules and the attractive forces between them.
All molecules are attracted to each other. At the same time, all molecules are in constant motion, and their kinetic energies increase with the temperature.
At very low temperatures, the kinetic energy of the molecules is low. The molecules cannot move fast enough to get away from the attractions of their neighbours. They are “glued” in place in a crystalline arrangement. The molecules are in the solid state.
At higher temperatures, the kinetic energy of the molecules is higher. The molecules will have enough energy to slide past their neighbours, but not enough energy to escape their attractions. The system is in the liquid state.
At very high temperatures, the kinetic energy of the molecules is so high that the molecules will be able to escape the attractions of their neighbours and move at will within the container. The system is in the gas state.
You can find an excellent simulation of this process (requires Java!) at