What happens to particles when you add heat energy?
The particles will move faster around in the available space.
If the temperature is increased, it means that the molecules will move (on average) with a higher speed. This means that the molecules have a higher kinetic energy:
Kinetic energy =
If we warm up a solution, we actually transfer energy into the molecules. This energy can be used as kinetic energy. Since the mass of the molecules doesn't change in this process, the velocity must increase. Therefore, the molecules will move faster around.
If a molecule has sufficiently enough speed, it could escape the solution and then it is called vapour or gas. This point where that happens is characteristic for a solution and called its boiling point.