When you apply heat energy to molecules what happens?
Depends on the type of molecule.
Heat basically provides molecules with more energy.
In solids, the particles vibrate about fixed positions. When supplied with heat the vibrate faster and the inter-molecular space between them increases. At a certain temperature (melting point), the particles have vibrated enough and have moved apart to melt into a liquid.
In liquids, the particles slide over each other. That's their way of 'vibrating'. So when supplied with heat, the vibrate faster, and slide more vigorously. The same phenomena takes place; at a certain temperature the particles have moved apart enough and have gained enough energy to vibrate to an extent where they are now gases.
In gases, only more random movement occurs when heat is applied.