To change phases, a substance either gains or looses energy. If a substance gains energy, it will become more exited and the molecules will spread out. An example of this is water boiling into gas. If it looses energy, the molecules will become less exited and come closer together. For example, freezing water takes away energy from molecules, changing it from liquid to solid
When a gas reaches a gold surface, it looses much of the energy that it had, so the gas condenses from gas to liquid phase.
Hope this helped!! :D
