# Fractional Distillation Question

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#### enTropeeeeeeeee

##### Full Member
10+ Year Member
So fractional distillation is used to separate 2 liquids that have a similar boiling point. Glass beads or any other inert material can be used to increase surface area in the distillation column to maximize condensation. Anyways, you apply heat to the distilling pot to begin vaporizing both liquids. The point is to vaporize the more voltatile liquid more. If I start with a 1:1 vapor ratio, the vapor inside the system will become 3:1 upon the first evaporation/condensation cycle because the more volatile liquid will want to go into gas phase much more. In fact, it goes into the gas phase 3x more readily than the less voltatile liquid. However, I can't figure out why it then becomes 9:1, 27:1, with each evaporation/condensation cycle.

So fractional distillation is used to separate 2 liquids that have a similar boiling point. Glass beads or any other inert material can be used to increase surface area in the distillation column to maximize condensation. Anyways, you apply heat to the distilling pot to begin vaporizing both liquids. The point is to vaporize the more voltatile liquid more. If I start with a 1:1 vapor ratio, the vapor inside the system will become 3:1 upon the first evaporation/condensation cycle because the more volatile liquid will want to go into gas phase much more. In fact, it goes into the gas phase 3x more readily than the less voltatile liquid. However, I can't figure out why it then becomes 9:1, 27:1, with each evaporation/condensation cycle.

if you look at the corresponding diagram in your BR book (although it might be in the chapter and not right next to the question), you'll see how it starts with a 1:1 mixture in solution. This is arbitrary of course, but makes for easy math. When it evaorates, it does so in a 3:1 preference, so the first vapor that forms is in a 3:1 ratio. This condenses on the walls and the beads, so that new solution is in a 3:1 ratio. This then reevaporates again with a 3:1 preference, so the the second wave of vapor is now (3x3) : (1x1) = 9:1. When the cycle repeats, it becomes (3x9) : (1x1) = 27:1.

Does this make sense? Hopefully the picture in the book makes sense in conjunction with this post.

if you look at the corresponding diagram in your BR book (although it might be in the chapter and not right next to the question), you'll see how it starts with a 1:1 mixture in solution. This is arbitrary of course, but makes for easy math. When it evaorates, it does so in a 3:1 preference, so the first vapor that forms is in a 3:1 ratio. This condenses on the walls and the beads, so that new solution is in a 3:1 ratio. This then reevaporates again with a 3:1 preference, so the the second wave of vapor is now (3x3) : (1x1) = 9:1. When the cycle repeats, it becomes (3x9) : (1x1) = 27:1.

Does this make sense? Hopefully the picture in the book makes sense in conjunction with this post.

This refers to the solution (which is 3 part high volatility liquid and 1 part low volatility liquid) that has condensed on the glass beads, in the distillation column?

This refers to the solution (which is 3 part high volatility liquid and 1 part low volatility liquid) that has condensed on the glass beads, in the distillation column?

Exactly. That first vapor condenses on the beads and then reevaporates, which is what fractional distillation is about. All the surface area allows this process to repeat several times.

Exactly. That first vapor condenses on the beads and then reevaporates, which is what fractional distillation is about. All the surface area allows this process to repeat several times.

That makes complete sense. I originally thought that the vapor condenses back into the original solution...which made no sense. Anyways, that explanation was both simple and clear. TYVM!