Boiling and Freezing Points

Mix up a batch and see....

Hi,

Thanks for the advise. I was trying to see if sone one can calculate the value; not by experimenting. Experiments may not provide me with acurate results. Also I have to have materials and instruments. Can you suggest what should be done to get 200mm of Hg atmospheric pressure? Keep the pressure constant at 200mm while heating the sample in an open flask or are you going to tell me to conduct the experiment in a different planet where the atmospheric pressure is low?

Job

That's interesting.

If you don't have the materials and instruments to deduce this empirically then what sort of application will you being putting this knowledge to?

If you need to know then I figured you'd have an environment where it mattered and be able to find out through experimentation in the environment of interest.

If you really want to predict(approximate) then you will need to study cryoscopic and ebullioscopic constants.

To answer your question of how to experimentally measure the boiling point(s) at different pressures, the answer is simple. you need a vessel which can be completely sealed, with an insulating blanket and method of introducing a significant amount of the mixture to be tested (a vacuum-tight valve, e.g.), a controllable heating system, a thermometer and pressure gauge, a vacuum system, and a mixer (magnetic) in the vessel.

You should prepare the mixture or mixtures from degassed components. Evacuate the vessel, then add the test mixture. The free volume (the volume of the vessel not filled with the mixture) should be kept as small as possible. Record the pressure of the vacuum before adding the liquid. keep a record of the temperature and pressure of the system while heating, pausing the heating at different temperatures (or pressures) to mix the liquid and allow the apparatus to reach equilibrium. Note that the temperature can be lowered or raised. I will leave it up to you to answer the obvious questions, like how does it work, how do I get the numbers I want, etc.

Can you use this for the freezing point question?

That is correct. This solution will undergo fractional distillation, which is also the method used to obtain brandy from wine, whisky from malted barley, and vodka from potatoes. None of the components form azeotropes with any of the others, so the Methanol will boil off first, then the water, then the ethylene glycol. Since reducing the atmospheric pressure raises the partial pressure of the solvents in this solution, the boiling points will be depressed, but the order in which the three solvents boil off will not change.

The data you need are easily obtainable if you have access to a reasonably well-equipped High-School or University chemistry lab, and are willing to do a little work. If this combination behaves in a remarkable way, you can write a research paper. If not, there will be no grist for the editorial mill, but in either case you will have the information you need. Good luck.

Hi Bubbapeby,

Thanks for the answer. I have just one doubt. If this mixture is filled in a bellowed container, compressed the container to its shortest height and sealed. Now if the mixture is heated what will happen?

If there is insufficient space for the mixture to expand, the pressure will rise until the weakest part of the container ruptures.

Hi PWSlack,

I mentioned "Bellowed" container; indicating that container will expand as the liquid mix expands.

Job.

PWSlack already gave you your answer. The methanol component of your mixture will begin to boil before the other components. At 1 ATM the methanol will boil at approximately 158 degrees F. Any good moonshiner knows this !

OOPS...Meant Tornado gave the correct answer.

I think the working definition of "boiling point" is the temperature at which the sum of all the vapor pressures of the components of the mixture add up to equal the atmospheric pressure (or the container pressure.) The boiling point is taken at the defined mixture, not after fractionation has altered the component ratios, so there should only be one defined boiling point for a given described mixture for a specific pressure. Where it gets hairy is the variation in the fluid bonding energies caused by non-linear interactions in the mixture. So it turns out that for precise measurements, experimental testing is the best answer. Yeah, I know this sounds like BS, but that does not make it ugly.

Dear woodpower,

Thank you very much for your sensible advise. Boiling points of aqueous methanol and aqueous ethylene glycol of different composition are published by the manufacturers so it was clear that Tornado was not correct in this matter. For fractional distillation the temperature must me maintained slightly over the highest volatile liquid, in this case Methanol, in the mixture in order to extract most of the methanol. The condensate will contain traces of water and glycol.

Thanks once again

Job.

Not true. Tornado IS correct.

Fractional distillation is accomplished using a column distiller. The compounds in the boil pot are all brought to boil. The steam works up the column through copper packing. As the condensate is drawn off, the latent heat of vaporization self cools the top of the column, and maintains the condensate at the boiling point of the lowest boiling point component in the mix. Once the methanol component is boiled off, the temperature at the top of the column slowly rises to the boiling point of the next components boiling point. In this manner only can fractional distilling occur.

The issue here is not how fractional distillation takes place, but rather what the boiling point of the mixture is. And since you bring up the separate issue of how fractional distillation takes place, you should be careful to be accurate before making underlined didactic pronouncements. We can hope that students and interested readers visit this site, and some of them might actually believe that your description is accurate, which would be unfortunate. If you have actually run a fractional distillation, you would admit that the top of the column is not self-cooled, but requires a cooling system, which like the heating system at the bottom of the column, is a necessary component of the still. The plate rating of the column is a measure of the ability of the still to separate the various components, which under normal conditions is never theoretically complete. Would it not be more accurate to say that fractional distillation is a method of distillation which separates mixtures of volatile components into various containers or "fractions" which have the various original components in different proportions, with the usual goal of creating a functional separation of the components. The effectiveness of the fractionation is seen in the amount of the recovered component, the purity of the component, and the amount lost as impurities, and can be measured by the plate number at a given throughput. The actual process that takes place inside the column depends on the design of the column and is based on its physical characteristics, but it is essentially a counter-flow interaction of a rising stream of a mixture of gases with a falling stream of the same materials in the liquid phase. A well-designed column helps the two streams to thermally equillibrate.

That's a pretty good description of fractional column distillation. Yes, I have performed many of these. Although the top of the column IS cooled with a condenser, the latent heat of vaporization is key to fractional distillation. The counter flow you refer to is called a reflux and is a big part of the column design.

My initial description of this process may have been incomplete, but is was not inaccurate. I do not always find it useful on CR4 to provide answers with such detail that no further study by an interested party is necessary. Basic concepts are generally good enough, albeit akin to disarming sometimes.

Hi Tornado & Bubbapebi,

If my memory is correct, it is the temperature at which the sum of all partial vapours equals the atmospheric pressure as Woodpower stated.

Whether you gave me the correct answer or not, I appreciate the fact that you were not sarcastic and were serious in your comments.

I also appreciate that wjmfire corrected me with his post how fractional distelltion is done.

Thanks again,

Job

You supply us with the relevant graphs giving vapor pressure vs. composition of the ternary mixture and vapour pressure vs. temperature, then perhaps we can tell you the boiling point. Sounds like many hours of work to me. As for the freezing point, I don't think it can be calculated from the composition. The only equations I know of, work with non-volatile solutes (salts for instance) dissolved in a liquid solvent (water). Unless I'm mistaken, such properties have to be measured by experimentation.

If you have engineering library access try this reference, Perry's Chemical Engineers' Handbook.

In refrigeration different refrigerants are mixed to get a new refrigerant. Temperature - composition diagram, Raoults law concept are used to get the properties of new refrigerant. The temperature at which a liquid mixture begins to boil is called the bubble point temperature. Temperature glide is the difference in dew and bubble temperature for the compostion.Mixtures can be azeotropic when the difference in boiling point of the components is not very large and when the deviations from ideal behavior are very large. Any good chemical engineering handbook will give related calculation details for thermo physical properties.