what happens on the molecular level to cause phase change?
Let's see on the molecule of water as example.& The two H's and the O are held together by intramolecular forces (forces within a single molecule) called covalent bonds. These covalent bonds are very strong and are not broken by the energy required for phase change, meaning the H's and O stay bound together during phase change. What is happening when heat is supplied is the breaking of relatively weak intermolecular forces (forces between several molecules) called hydrogen bonds. For more information on hydrogen bonding wikipedia probably has something. Anyway, the reason for water's relatively high boiling point is strong forces between the oxygen of one water molecule and the hydrogen of another water molecule (hydrogen bonding). The closer these forces bring the molecules the closer the properties are to a solid. When heat is supplied the molecules move around freely and the hydrogen bonding does not hold the molecules in a closely packed state. This state of low density is called gas. While the molecules are not being pulled together by the hydrogen bonding the individual covalent bonds holding the two H's to the one O are not affected, as it would take much more energy than that to break covalent bonds, thus water is water whether it be gas, liquid, or solid.