I have got a question about distillation. In distillation as we know the mixture is boiled and the mechanism then goes like substances with high boiling points condense at the bottom and substances with lower boiling points condense on the way to the top. I don't know why, but I am having difficulty understanding that why should a substance with high boiling point go to the bottom and the substance with the low melting point get higher. This goes against my intuition. Basically what I think is that higher boiling point means greater energy to the system and thus higher output in form of reaching higher. However, according to the real observation, I am wrong.

So does the system behave like this? Higher boiling point means that after the system changes into gaseous state (most probably), it radiates energy faster that the substance with the lower melting point.

Is this explanation of the phenomena correct?

If yes then explain why does the substance with higher boiling point radiate energy faster?

If not then please mention the right mechanism .

• I think you are confusing centrifugation with distillation. In distillation, substances with lower boiling points will turn to gas before substances with higher boiling points. You can condense the gas while the lower temperature substance is boiling off to get a distilled substance. – Robert Stiffler Dec 23 '15 at 10:35
• Mr . Robert Stiffer, i am just saying why does , why does the liquid with lower boiling point reach higher the fractionating column , if the substance is embedded in a couple of other substances whose boiling point is higher please help – Faiz Iqbal Dec 23 '15 at 10:37
• A good place to start understanding distillation is Raoult's Law: chemguide.co.uk/physical/phaseeqia/idealpd.html – Gert Dec 23 '15 at 16:48
• Would Chemistry be a better home for this question? – Qmechanic Feb 25 '16 at 14:40

A fractionating column is designed so that the temperature inside it decreases with height i.e. it's very hot at the bottom of the column and gets colder as you go up the column. Any particular material will rise up the column until the temperature falls to its boiling point, and at that point the material will condense. So low boiling point materials will condense higher up the column than high boiling point materials.

• but i have heard that theres uniform trempreature throughout the fractionating column – Faiz Iqbal Dec 23 '15 at 13:04
• @FaizIqbal Then you have heard wrongly. Read the Wikipedia article I linked. This confirms that the temperature falls up the column. – John Rennie Dec 23 '15 at 13:06
• i think its addressing that the trempreature of the gas falls and not of the container or atmosphere – Faiz Iqbal Dec 23 '15 at 14:09
• Read carefully:  The vapor condenses on glass spurs (known as trays or plates) inside the column, and returns to the distilling flask, refluxing the rising distillate vapor. The hottest tray is at the bottom of the column and the coolest tray is at the top. – Bill N Dec 23 '15 at 18:09

The physics behind distillation is somewhat more complicated than stated. Mixtures of liquids do not boil at one specific temperature at a given pressure, they boil throughout a temperature range that is dependent on the liquid composition. For a simple batch still (e.g., what a moon shiner would use), the lowest-boiling-temperature components of the mixture tend to boil first as the temperature of the mixture is increased. This means that the vapor that is in equilibrium with the liquid is selectively enriched in those components.

A simplified view of a distillation column indicates that it can be considered to be multiple batch stills stacked on top of each other (the distillation column has trays in it). In addition, the liquid on each tray going up the column can be assumed to occur from condensing the vapor from the tray below it (this is a BIG simplifying assumption, but should illustrate the point). Given this assumption, it is easy to see that the lower-boiling-temperature components would selectively distribute themselves towards the top of the distillation column.

• Mixtures of liquids do not boil at one specific temperature at a given pressure. This is plain WRONG. Look at any vapour liquid equilibrium diagram: for each liquid composition there's a precise boiling point at a given pressure. e.g. chemguide.co.uk/physical/phaseeqia/bpcompn2.gif – Gert Dec 23 '15 at 16:39
• @Gert , re-read my statement. I stated that mixtures have a boiling range. – David White Dec 23 '15 at 16:45
• Your statement is ambiguous. Mixtures of liquids have boiling points that depend on mixture composition at a given pressure would be precise. I suggest strongly to edit. – Gert Dec 23 '15 at 16:53
• Fair enough. My statement was a bit ambiguous. For a batch still, there is an initial boiling point, and as more material is distilled, the boiling point increases until you reach the end point of the distillation, at which point, 100% has distilled. During the time that the distillation is taking place, the liquid remaining in the batch still is continuously changing composition, as is the vapor associated with that liquid. Note - based on the level of the original question, I DIDN'T want to get into a complicated discussion of vapor-liquid equilibrium, fugacity coefficients, etc. – David White Dec 23 '15 at 16:57