matter is made up of particles that are constantly moving . all particles have energy , but energy varies depend on temperature there are three states of matter solid , liquid , gas . the solid particle has least amount of energy and gaseous particle has high amount of energy . the temperature of substance is measure of average kinetic energy and change of phase occur when energy of particle has changed . there are spaces between particles , the average amount of empty space is in between molecules get progressively larger as sample of particle moves from solid to gas phase . these atrractive force between particles become stronger when particles are close to each other these attractive forces are called inter molecular forces.
for example :- WATER
when water is in solid phase ( ice ) water molecules have little energy it does not move away from each other the molecules held closely together called lattice. when ice heated , the energy in molecules increase , then some of water molecules able to overcome inter molecular forces that hold them together and molecules moves part in liquid phase and easily flow . if it is reheated liquid become water vapour which is gas . Gas particles has most energy and are on average at distances from each other which are much larger than size of atoms itself