The boiling point of a substance is the temperature at which the liquid boils. This is the temperature at which the vapor pressure of the liquid is the same as that of the atmosphere. When the boiling point is reached the chemical changes from the liquid to the gas or vapor phase. This change does not occur instantly as all molecules have to gain sufficient energy to change from one state of matter to another.

Different liquids have different boiling points due to their chemical make up. Any liquid that has a high number of hydrogen bonds will have an elevated boiling point because extra energy must be added into the system to break the hydrogen bonds. For example, water (H₂O) has a boiling point of 212°F (100°C). Boiling points can also be altered by changes in pressure, the lower the pressure, the lower the boiling point, this is because the vapor pressure of the liquid at a lower atmospheric pressure does not have to be as high to become equal to that atmospheric pressure. Boiling points also can be altered by adding other chemicals to the liquid. The boiling point of water can be increased by several degrees by adding sodium chloride (common salt).