Temperature of distilling water
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Soapy Sam said:
I agree, but the temperature of the thermometer will give you a fairly accurate reading of the temperature of the gas that's escaped the liquid. And this gas will have the same temperature as the liquid (as it possesses the same energy as it did at the moment it separated from the solution). The difference will be due to the thermometer being slightly cooler, and perhaps (I'd need to look this one up) the volume of the flask above the liquid. But these are negligible when exploring a 5 - 10.C increase in boiling point of an impure solution. As I said, we are talking an error margin at the most of a degree or two here.
As for those hot tripods, no matter how many times you tell kids not to touch them, you'll always get one or two...
Athon
Rob Lister
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athon said:I agree, but the temperature of the thermometer will give you a fairly accurate reading of the temperature of the gas that's escaped the liquid. And this gas will have the same temperature as the liquid (as it possesses the same energy as it did at the moment it separated from the solution). The difference will be due to the thermometer being slightly cooler, and perhaps (I'd need to look this one up) the volume of the flask above the liquid. But these are negligible when exploring a 5 - 10.C increase in boiling point of an impure solution. As I said, we are talking an error margin at the most of a degree or two here.
As for those hot tripods, no matter how many times you tell kids not to touch them, you'll always get one or two...
Athon
I'm confused in a confusing sort of way. If the energy from the heat source is enough to break the weak H-H bonds, and the solution is at something less than saturation, meaning that most of the bonds were of the H-H type, then even if the water is not physically boiling it should still distill.
IOW, at what point did the distillation process began?
/novice
Rob Lister said:
You're right. Distillation does not require the liquid to boil; it only requires evaporation to occur. It began as soon as the surface molecules had enough energy to escape the surface tension created by the H-H bonds with surrounding molecules. I don't know exactly what this energy is for water without looking it up, however.
Athon
Kilted_Canuck
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Do the science advisors know that by 'impure water' you mean water with sugar in it?
We did fractional distillation in my 2nd year organic chem course this year and my water boiled at around 98 degrees here, that was after boiling off the organic solvents but before boiling off other dissolved solids.
I guess altitude makes quite a difference.
We did fractional distillation in my 2nd year organic chem course this year and my water boiled at around 98 degrees here, that was after boiling off the organic solvents but before boiling off other dissolved solids.
I guess altitude makes quite a difference.
Altitude will make a difference.
I am a bit troubled by the issue of BPE here, when the water evaporates, it no longer contains any of the impurity, so the water may be at 110C, so the steam will be a bit superheated when it comes off the water, BUT it's going to spend a lot of that on the glass, etc, as well as the thermometer, so there will be a huge quantity of energy available at 100C to prevent cooling, and a some more above that from the heat capacity of superheated steam.
So I think this is a badly designed question, it's something that requires knowing the heat losses from everything, as well as the exact restriction of the downstream apparatus (which will determine the boiling point of the water as well, perhaps to an extent larger than the dissolved solids, depending on the restriction) and of course the back pressure from the restriction will depend on the heat input to the water after it's reached its STP boiling point.
I vote "bad question", the only intuition I'd have would be that it would be somewhat over 100C.
And if you try this in Rocky Mountain National Park up at the ranger station, you'll get another result entirely.
I am a bit troubled by the issue of BPE here, when the water evaporates, it no longer contains any of the impurity, so the water may be at 110C, so the steam will be a bit superheated when it comes off the water, BUT it's going to spend a lot of that on the glass, etc, as well as the thermometer, so there will be a huge quantity of energy available at 100C to prevent cooling, and a some more above that from the heat capacity of superheated steam.
So I think this is a badly designed question, it's something that requires knowing the heat losses from everything, as well as the exact restriction of the downstream apparatus (which will determine the boiling point of the water as well, perhaps to an extent larger than the dissolved solids, depending on the restriction) and of course the back pressure from the restriction will depend on the heat input to the water after it's reached its STP boiling point.
I vote "bad question", the only intuition I'd have would be that it would be somewhat over 100C.
And if you try this in Rocky Mountain National Park up at the ranger station, you'll get another result entirely.
davefoc said:
You are right, but thermometers may be calibrated with this in mind. They are either "total immersion" or "partial immersion". Total immersion types are calibrated with the entire thermometer immersed in the fluid being measured. Partial immersion thermometers will have a line around them that indicates the depth to which it should be immersed in the fluid. When I was in school, many years ago, the thermometers we used were mostly 76 mm partial immersion type, which means they were calibrated with 76 mm immersed in the fluid and the rest of the thermometer in ambient air.
Kilted_Canuck said:
It was not relevant. It just said 'impure water'; as far as I'm aware, I can't think of any impurities that decrease the boiling point. I'd be overjoyed to be shown wrong on that one.
The pressure would of course alter results, but I do have to remind people here that this is not a high-level physics question that requires detail answers. My argument is that all evidence points to impure water boiled in a distillation apparatus makes the thermometer read a figure higher than 100.C. The 'right' answer (according to the mark scheme) was arrived at on the assumption that water always boils at that temperature. The answer my kids gave was arrived at after doing the experiment and discussing what they actually saw.
It was a bad question for a number of reasons; it was seeking a fact the kids should know (BP of water) using incorrect information; it was suggesting an answer based on an assumption and not on actual evidence; it was veiled in an experiment that was not a unit requirement (I used it simply to demonstrate distillation, yet there was nothing in the scheme of work to demand I do this).
Two things; firstly, you're trying to derail a thread with off-topic discussion. If you have something to add to a discussion about boiling points OR assessing education using assumed knowledge, then please comment.
Secondly; science is about evaluating the information presented to you based on established data and supported models. This means all information presented is open to close scrutiny and critical attack. Only when it survives this will it be of any real use.
Something I'm trying to teach my students, and something that is made difficult by present exam assessments.
Athon
athon said:
Yes, but the answer to this question depends in huge part on exactly what the details are, hence my objection.
Suppose that the fluid being distilled was 50% ethyl alcohol, 50% water. Now what temperature do you get?
(and, careful with that vapor there, guys, put out that match)
Suppose the solute was 30% ethylene glycol. Now what?
Olaf/QII said:
You lose more credibility every time you make a point, Olaf. Is this a serious question, or did you simply need to post something to attempt to save face? Sometimes simply nodding and learning from something can be a good thing, you know.
Here's what my kids -- 11 year olds -- were capable of understanding;
You have established data (water boils at 100.C). You observe a novel situation (distillation of impure water) and attempt to apply a supported model (particle model explaining evaporation due to heat energy) to the situation in order to predict a result (thermometer will read 100.C and stop). The actual observation conflicts with expected results hypothesised based on the model and established data (thermometer reaches 110.C).
Now...why?
The experiment does not answer this. It can't. But by comparing it with other experiments that use pure water, some theories can be proposed. The revised model predicts that impure water will have a higher boiling point than pure water, a prediction that stands with repeated experimentation.
Kids are asked in the exam question what they would predict the thermometer would read. Based on these experiments...they can expect a reading of over 100.C.
They can reach this conclusion without knowing previous data, but to have a full grasp of the concept they should know it.
Where have I contradicted myself?
Athon
Doubt
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Since I had the wrong answer, I need to correct the situation. I have to brainwash Hydrogen Cyanide and Athon into believing I was right. Or I have to blackmail the people running the board to editing my initial response.
Or just deal with reality.
So what we have here is a group looking for a fixed answer without taking into account what really happens. Unless there was a definition problem. I would have taken tap water for impure water rather than something with salt or sugar added to it. That makes for a badly worded question on the test, even if the contents of the water were known.
Or just deal with reality.
So what we have here is a group looking for a fixed answer without taking into account what really happens. Unless there was a definition problem. I would have taken tap water for impure water rather than something with salt or sugar added to it. That makes for a badly worded question on the test, even if the contents of the water were known.
Chris Haynes
Perfectly Poisonous Person
Doubt said:
I had some practical experience. Now you could all try this practical experience and learn how to work with sugar syrup. The changes in sugar syrup with increasing temperature as the water evaporates are used when making jams, jellies, candy and the very hot sugar syrup that cooks the beaten egg-whites in meringe icings. You have to be careful because a drop of hot sugar syrup on your hand can bring up blisters!
Also there is the use of salt to lower the temp of ice in old-fashioned ice-cream makers. My brother would have to turn turn turn and turn the crank while we put in layers of ice and rock salt.
Chris Haynes
Perfectly Poisonous Person
Hmmm... when I think about it ---a most likely type of water someone would want to distill may be sea water. I think this is a good excuse to go to the beach, get a bottle of water from there and then see what temperature it boils at (I am at about 30 meters above sea level). Also, to try and boil it down until the salt is concentrated --- perhaps it may require a couple of liters to get enough salt to be worthwhile.
I read somewhere that when sea-water freezes the salt is pushed out of the water, and the ice is similar to fresh-water ice. That also sounds like something that could be tested.
I just had confirmation from my 11 year old daughter that it would be interesting to try. We can get a jump on next year's 6th grade science fair project.
I read somewhere that when sea-water freezes the salt is pushed out of the water, and the ice is similar to fresh-water ice. That also sounds like something that could be tested.
I just had confirmation from my 11 year old daughter that it would be interesting to try. We can get a jump on next year's 6th grade science fair project.
What was the problem, Athon? The "correct" answer on the test material was 100C?
You should point out to the borough science advisor. Email her a few links (one, for example):
http://dbhs.wvusd.k12.ca.us/webdocs/Solutions/BP-Elev-and-FP-Lower.html
If the object of the experiment was to boil already distilled water, which should boil at 100C, you could use this as a platform to discuss with your kids why their experiment yielded different results. Were their impurities? How would atmospheric pressure affect the boiling point?
This may be a little "deep" for 11-year-old kids to understand, and you may have unwittingly undermined the experiment (if it was really intended to demonstrate the boiling point of distilled water). But, if you have some smarties, you could make it a nice platform for on the scientific process.
Good luck.
-TT
You should point out to the borough science advisor. Email her a few links (one, for example):
http://dbhs.wvusd.k12.ca.us/webdocs/Solutions/BP-Elev-and-FP-Lower.html
If the object of the experiment was to boil already distilled water, which should boil at 100C, you could use this as a platform to discuss with your kids why their experiment yielded different results. Were their impurities? How would atmospheric pressure affect the boiling point?
This may be a little "deep" for 11-year-old kids to understand, and you may have unwittingly undermined the experiment (if it was really intended to demonstrate the boiling point of distilled water). But, if you have some smarties, you could make it a nice platform for on the scientific process.
Good luck.
-TT
ThirdTwin said:
The problem is that no matter what I said, the advisor was unconcerned with changing it. It was as if the correct answer - or the process to reach that answer - are irrelevant. So no links are going to make a difference. I could fight it, but at the expense of then making myself seem like a trouble-maker (and making my job more difficult) I don't see why a one mark question really matters.
The added variables would work in some classes, but most year 7s struggle with the basics. I'm more concerned with them applying observation and analysis skills than remembering a number.
Athon
Beausoleil
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athon said:
In real life you don't start to distill water by throwing salt into it. Surely "impure" just means "not yet distilled" in this context.
What's the boiling point of tap water where you live?
Beausoleil said:
I'm not sure what you mean by this exactly. I know when we simply boil normal tap water in the lab, the results are typically just over 100.C. Kids get confused by this, as they are taught it should stop at 100.C exactly. We then discuss the purity of tap water, how clean the beakers are, etc.
Athon