5
$\begingroup$

One of the questions in a recent past paper I completed was:

One way to make water from the reservoir suitable for drinking is by distillation.

Describe how water is distilled.

And I answered (perhaps too succinctly) that "The water is vaporised and the condensation is collected."

I would have thought my answer correct, in that it fully addresses the question, however, the mark scheme mentions two points I did not make:

  1. I should have mentioned heat.
  2. I should have mentioned cool (the mark scheme's words, not mine).

But I was wondering, are these two (common) aspects of distillation actually necessary to distill water?


Surely heating and cooling are not really necessary to distill water; couldn't pressure be reduced to evaporate the water, and increased again to condense it?

Or, in the context of the question, is the heating perhaps necessary to sterilise the water (for killing microbes)?

$\endgroup$
  • $\begingroup$ Welcome to chemistry.se! If you ask me, and by extension you are already doing this, your explanation is quite fine. That is the general process of distillation. In the case of water, however, reducing pressure might just not be enough to evaporate it. $\endgroup$ – Martin - マーチン May 27 '16 at 11:41
  • $\begingroup$ @Martin-マーチン According to this table (and my own - perhaps incorrect - presumptions about how vapour pressure works) pressure would need to be reduced to 0.0121 atm for water to boil at 10°C, which should be doable with even just vacuum pumps. $\endgroup$ – theonlygusti May 27 '16 at 11:50
  • $\begingroup$ Of course, 10°C being an amount chosen to represent the reservoir water temperature; a higher temperature means water can boil at a higher pressure. $\endgroup$ – theonlygusti May 27 '16 at 11:51
  • $\begingroup$ Why, of course you can make water boil and then condense by manipulating pressure alone. It would be slow and cumbersome, but very much possible. The problem is, as some water evaporates, the rest of it cools down, and continues to cool all the way down until it becomes ice, so you'll have to actually heat it to keep it at room temperature, and if you have the heater anyway, why bother with all that pressure stuff. $\endgroup$ – Ivan Neretin May 27 '16 at 11:52
  • $\begingroup$ You can but the instrumentation is far more complicated. I a laboratory I would prefer do not use a noise devise. Reach a pressure low enough is not a problem. In do not know which alternative is cheaper. $\endgroup$ – user1420303 May 27 '16 at 13:01
4
$\begingroup$

You obviously already have a general idea of what is going on in a distillation, so I will spare you the picture and everything.

Your question can be broken into two parts:

  1. Do I need heat for vapourisation?

  2. Do I need cooling for condensation?

I am going to address them one after the other.

Vapourisation

As you probably also know, reducing pressure is one means of evaporating liquids. This is used in rotavaps which use a $40~\mathrm{^\circ C}$ bath and reduce the pressure to make sure the solvent evaporates. You could think that just reducing the pressure should be enough to vapourise any liquid.

But if you vapourise a liquid only by reducing pressure, you will reduce the temperature of the liquid. This is the evaporation cooling that your body uses when sweating: If the water in sweat evaporates it cools the surface it evaporates from. Thus, after a certain time and especially if you attempt to distill large quantities of water, it will freeze and further evaporation will become exponentially harder. Therefore, a practical setup will always require heating.

Condensation

Effectively, this is the same discussion but in reverse. Note that if you boil water in a closed pot, the lid of the pot will get successively hotter due to condensing water releasing heat. After a certain time, the temperature of the surface you want to condense on will be so hot that the liquid will no longer condense.

This is especially a problem if you decided to reduce pressure for evaporation: If you have to reduce the pressure so far that the liquid would evaporate at $10~\mathrm{^\circ C}$, then there is no way that it will condense at a cooling trap of $25~\mathrm{^\circ C}$. Therefore, the condensation area must be cooled which is typically achieved with Liebig coolers or similar.

What about vapourisation followed by condensation in the same vessel with changing pressure?

Well, that is possible. It is not practical, though, since the inside of a single vessel can only have a certain pressure (within slight margins). And vapour spreads out uniformly. Your vessel would have to have an initial ‘bowl’ and a collecting one. You reduce pressure to evaporate out of the initial bowl and then re-raise pressure. The liquid will condense uniformly and you will lose a significant amount of it because it will drip back into the initial bowl. Your yield with a typical common setup is much better.

$\endgroup$
  • $\begingroup$ So you're saying that heating and cooling are necessary processes for distillation? $\endgroup$ – theonlygusti May 27 '16 at 21:13
  • $\begingroup$ @theonlygusti Necessary for a practical distillation setup, yeah. $\endgroup$ – Jan May 27 '16 at 21:20
  • $\begingroup$ but in terms of the question, is my answer technically fully correct? And so, the mark scheme wrong? $\endgroup$ – theonlygusti May 28 '16 at 0:09
  • $\begingroup$ @theonlygusti The question asks ‘describe how water is distilled’. Therefore, it is reasonable to require heat and cooling since the other setup is very impractical and I am not aware of it being used. $\endgroup$ – Jan May 28 '16 at 10:28
  • $\begingroup$ Ah, so you're saying I should have read it as "Describe how water is distilled" instead of "Describe how water can be distilled," therefore making my evaluation afterwards incorrect? $\endgroup$ – theonlygusti May 28 '16 at 22:17
1
$\begingroup$

The question is trying to explore whether you understand how distillation works. Yes, vapour is created and, yes, the condensate is collected. But your answer doesn't describe how that is done.

You are correct that you could use reduced pressure to create vapour, but you will still need some input of heat and some way to cool the vapour. Or you could have described the simpler process of applying heat at normal pressure and condensing the resulting vapour and the examiner would know you understood the basics.

It may be possible to do distillation purely with pressure swings, but the apparatus would be really complicated (how, for example, do you collect the low pressure vapour and then repressurize it to cause condensation?). Besides the reason why distillation used lowered pressure isn't to avoid the need for energy input but to avoid the need for heating the liquid to excessive temperatures to achieve energy input (some things will decompose at high temperatures). In a constant pressure vessel, the only way to condense the vapour is to cool it so you can't avoid the need for either cooling the vapour or heating the liquid.

Show you understand the basics and you will get full marks in the test. Don't overcomplicate.

$\endgroup$
  • $\begingroup$ Obviously, as I sat the test, I didn't think through the question in as much depth as I lay out in my post; if anything, I under-complicated my answer, in failing to mention how distillation can be achieved. I was simply a bit annoyed to find out that I had the "wrong" answer because I had failed to mention two processes which, in contemplation afterwards, don't even seem to be necessary for distillation. $\endgroup$ – theonlygusti May 28 '16 at 22:12
  • $\begingroup$ Anyway, I don't feel that this answer answers the question I've asked definitively enough; is heating/cooling necessary for distillation? $\endgroup$ – theonlygusti May 28 '16 at 22:13
  • $\begingroup$ @theonlygusti The only way you can possibly avoid heating and cooling is to have some immensely complicated apparatus that allows significant pressure swings. In practice and in real labs you will need heating or coolling or usually both. (BTW the answer does say this.) $\endgroup$ – matt_black May 28 '16 at 22:22
  • $\begingroup$ @theonlygusti And you have speculated that pressure alone could do the job: it can't (unless you have some very clever design of device, which you have not demonstrated). Low pressure makes the temperature differential required different, not non-existent (in a constant pressure apparatus.) $\endgroup$ – matt_black May 28 '16 at 22:26
1
$\begingroup$

You will need heat even if you play with pressure: this heat may come from a reservoir which stays at room temperature, yet heat will flow back and forth during evaporation and condensation.

And let us be honest: you gave a half-baked answer, which is half baked even if your teacher didn't write an essay about what is missing

$\endgroup$
0
$\begingroup$

If distilling means vaporization and then condensation of a liquid without applying work to the system, then the energy needed to transform the liquid to the gaseous state has to be supplied as heat.

This is called heat of vaporization, or latent heat of vaporization, since this does not necessarily mean that the temperature has to be raised.

$\endgroup$
  • $\begingroup$ I'm sorry, i don't really understand this answer; heat has to be applied, yet the temperature does not raise? $\endgroup$ – theonlygusti May 28 '16 at 8:27
  • $\begingroup$ Consider water at 100°C. You have liquid at 100°C and above it you have vapor at 100°C. If you now apply heat to the liquid water it would not raise the temperature. Instead the heat would be consumed to transform more liquid water to gaseous phase. This is the latent heat of vaporization. Follow the link to get additional information. $\endgroup$ – aventurin May 28 '16 at 11:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.