How does matter change from one state to another?
Atoms/molecules gain enough energy to increase the mean atomic/molecular separation by overcoming inter-atomic/molecular forces.
Solid ➝ Liguid
Particles are initially held in an equilibrium position by inter-atomic/molecular bonds, and they vibrate about that equilibrium. As energy is transferred to the substance the atoms/molecules gain enough energy to overcome some inter-atomic/molecular bonds. The atoms/molecules become able to move past each other (translational motion). However, the mean separation of atoms/molecules does not increase significantly.
Liquid ➝ Gas
Particles are initially able to move past each other, but there is still a defined bulk of the substance. As energy is transferred to the substance atoms/molecules gain enough energy to break free from the bulk of the substance and transition into the gaseous phase. Their motion is entirely translational and restricted only by the walls of the container they are in (technically this is a description of an ideal gas, but it approximates well to a practical gas). The mean separation of atoms/molecules increases dramatically.