An element or compound will boil after passing its respective boiling point, but there are more aspects to this process than just temperature. The system must be open to the atmosphere. If the system is completely enclosed, the liquid will reach its critical point. After a liquid has been heated past it's boiling point, the molecules will start to behave differently than in the liquid phase. They will take on a less organized structure, and the molecules will begin to enter the atmosphere in the form of a gas.