Boiling Point and Atmospheric Pressure

What is the boiling point of a liquid?

The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the external pressure - the external pressure being atmospheric pressure if the flask is open to the air.

When you look up the boiling point of a compound in a textbook or online, it will give you the normal boiling point of the compound.

For example, the normal boiling point of water is 100 ^oC.

The normal boiling point is defined as the boiling point of a liquid at an external pressure of 1 atmosphere or 760 mm Hg. This is the boiling point of a liquid at sea level. Realize that the actual boiling point of a liquid changes depending on the atmospheric pressure. At higher elevations than sea level, atmospheric pressure decreases. Think of it as the higher up you are, the smaller the column of air that is sitting on your head. Less air on your head means lower pressure on your head.

For example, on top of Mount Everest, the highest point on Earth (8,848 meters above sea level), the atmospheric pressure is approximately 255 mm Hg; there is not a very big column of air above your head. On top of Mount Everest, water boils at approximately 70 ^oC. This is because if the atmospheric pressure is low, the vapor pressure will equal the atmospheric pressure at a lower temperature. The point of this discussion is that whenever you are performing a distillation, be sure to read the barometer and record the atmospheric pressure for that particular day.

Different atmospheric pressures will lead to different experimental results.