Distillation

[MedPR]

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How would distillation mimic freeze-drying something?

Freeze-drying = freeze it, then remove the ice by reducing pressure and converting the ice to gas (sublimation).

I thought distillation was only for separating liquids, not a liquid from a solid?

 

[mcloaf]

How would distillation mimic freeze-drying something?

Freeze-drying = freeze it, then remove the ice by reducing pressure and converting the ice to gas (sublimation).

I thought distillation was only for separating liquids, not a liquid from a solid?

Well, freeze drying is just a specific application of separating two compounds based on sublimation point--one of them doesn't always have to be water. You just need to solidify them both and keep pressure and temperature such that one will sublime away from the other. In essence, this is pretty similar to distillation.

 

[milski]

In freeze drying you have two solids - one sublimates, the other does not (or at least they do it at very different rates) and you use that to separate them.
In destillation you have two liquids - one evaporates, the other does not (or at least they do it at very different rates) and you use that to separate them.

How does that sound?

 

[MedPR]

Well, freeze drying is just a specific application of separating two compounds based on sublimation point--one of them doesn't always have to be water. You just need to solidify them both and keep pressure and temperature such that one will sublime away from the other. In essence, this is pretty similar to distillation.

In freeze drying you have two solids - one sublimates, the other does not (or at least they do it at very different rates) and you use that to separate them.
In destillation you have two liquids - one evaporates, the other does not (or at least they do it at very different rates) and you use that to separate them.

How does that sound?

Eh, here's the question. From TBR Gen Chem Chapter 7.

An alternative to the freeze-drying procedure that would still yield dehydrated coffee is which of the following?

Answer: Distillation of the water to collect the residue.

I see how the freeze-drying process is similar to distillation in terms of separating one thing from another, but I don't understand how it works in this context.

Another one of the answer choices (the one I picked) is "Recrystallization using a water-ether mixed solvent"

Wouldn't this crystallize (solidify) the coffee while draining out the water-ether solvent?

 

[mcloaf]

Eh, here's the question. From TBR Gen Chem Chapter 7.

An alternative to the freeze-drying procedure that would still yield dehydrated coffee is which of the following?

Answer: Distillation of the water to collect the residue.

I see how the freeze-drying process is similar to distillation in terms of separating one thing from another, but I don't understand how it works in this context.

Another one of the answer choices (the one I picked) is "Recrystallization using a water-ether mixed solvent"

Wouldn't this crystallize (solidify) the coffee while draining out the water-ether solvent?

You're only going to recrystallize a compound if you're trying remove trace impurities from a solid. To do so you need to know the solubility of whatever it is you're trying to purify, because if the solubility of your compound in your solvent isn't exactly right (very low at low temp, very soluble at high temp) the whole procedure will fail. In this case, you aren't removing impurities but separating a two part mixture based on phase differences, and they haven't given you any info about the solubility of "dried" coffee (indeed, it the point of freeze dried coffee is that you can add it to water to get coffee, I don't think you'd have much success trying to recrystallize it in the solvents you've proposed).

Ultimately the only one of these choices that you know is going to remove the water and leave the other stuff (coffee residue behind) is distilling off the water.

 

[milski]

Eh, here's the question. From TBR Gen Chem Chapter 7.

An alternative to the freeze-drying procedure that would still yield dehydrated coffee is which of the following?

Answer: Distillation of the water to collect the residue.

I see how the freeze-drying process is similar to distillation in terms of separating one thing from another, but I don't understand how it works in this context.

Another one of the answer choices (the one I picked) is "Recrystallization using a water-ether mixed solvent"

Wouldn't this crystallize (solidify) the coffee while draining out the water-ether solvent?

Sort of guessing here, but I think in this case they mean that you already have the water solution and need to extract from that. Recrystallization would imply starting with a solid, dissolving and then crystalizing again. Seems like a very minor difference and I'm not sure what I would have answered, considering that extraction of caffeine is an example of organic/aqueous separation in my textbook.

 

[MedPR]

You're only going to recrystallize a compound if you're trying remove trace impurities from a solid. To do so you need to know the solubility of whatever it is you're trying to purify, because if the solubility of your compound in your solvent isn't exactly right (very low at low temp, very soluble at high temp) the whole procedure will fail. In this case, you aren't removing impurities but separating a two part mixture based on phase differences, and they haven't given you any info about the solubility of "dried" coffee (indeed, it the point of freeze dried coffee is that you can add it to water to get coffee, I don't think you'd have much success trying to recrystallize it in the solvents you've proposed).

Ultimately the only one of these choices that you know is going to remove the water and leave the other stuff (coffee residue behind) is distilling off the water.

Sort of guessing here, but I think in this case they mean that you already have the water solution and need to extract from that. Recrystallization would imply starting with a solid, dissolving and then crystalizing again. Seems like a very minor difference and I'm not sure what I would have answered, considering that extraction of caffeine is an example of organic/aqueous separation in my textbook.

Ok thank you both. I made a note to myself to read over common lab techniques. What you've both said makes sense though, and I think I would've been able to answer the question correctly if I was more familiar with what distillation and recrystallization were.

Another answer is "Filtration of the solute with sieves." I thought about that one too, but figured that there isn't any heat involved in filtration with sieves, so while you would get rid of a lot of the water, you couldn't completely dry the coffee.