i need help with the fractional distillation method of a couple of substances at least more than two substances , just like air actually what am i looking for is that why do substances in air with a low boiling point like nitrogen reach higher the fractionating column even if it looks like that substances with higher boiling point will absorb more energy and thus eventually result in greater kinetic energy (at least to me). please help . heres how i came to know that nitrogen rises higher the fractionating column


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    – Jan
    Dec 25, 2015 at 0:39

2 Answers 2


Suppose you have a mixture of two compounds, one with a low boiling point and another with a higher boiling point in a flask.Prepare a distillation setup connected to a collector. As you start the distillation process, the lower boiling compound would start condensing (through the condensor to the collector). If you had only two compounds, you are left with the compound with the high boiling point in the flask. In case you have a mixture left, you could distill out the another compound on increasing the temperature.


The boiling point is in relation to the strength of the intermolecular forces. If the forces between the molecules are strong, the molecules require a greater average kinetic energy to overcome those forces sufficiently well to separate and become a gas.

The boiling points, in Kelvin, for the these gasses are:

  Gas      K
Nitrogen  77.4
Argon     84.2
Oxygen    90.2

In the distillation column the temperature increases and eventually nitrogen boils off at 77.4K, argon and oxygen are still liquid. Then as it gets hotter the nitrogen molecules get more energy and the gas becomes less dense and rises up the column. By the time it gets to 84.2K at the bottom (where the heat is), argon boils but oxygen is still liquid.

Imagine it is 84.2K for all the gases, they all have the same average kinetic energy. The argon atoms only just have enough kinetic energy to become a gas, the forces between them are present pulling them together. If it loses a small amount of energy it will condense, and as a gas it is relatively dense. The nitrogen has enough kinetic energy to easily overcome these forces and expands, rising above the argon. The oxygen molecules have stronger intermolecular forces so even at this temperature, they do not have enough kinetic energy to overcome the forces and become a gas, they are still a liquid in the bottom.

Usually a fractional distillation column is hot at the bottom and gets cooler as you go up away from the heat source. Things become a gas and rise until they get to the place in the column where the temperature makes them condense into a liquid again onto some collectors, but in this case you would probably pump the nitrogen out as a gas out the top and drain the liquid oxygen off the bottom.


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