the molecules loosen up because it is going from a solid to a liquid, and the molecules in a solid are more close together. the molecules kind of "fall apart" and become less compact. therefore, making it a liquid. helpful?
In the same way solids seem 'solid', ice is solid. When ice ('solid' water) melts, it's internal structure loosens, yet it's chemical structure, H2O, remains. The molecules are more fluid and move about. If you boil water, the molecules will move more energetically, and eventually, you will get steam, with the molecules will be moving very rapidly about. Higher heat, more energy, fast molecular movement. If enough energy is applied, the chemical structure will break apart, and you will get a lot of atoms of some type. Much more energy and you will get subatomic particles.