Temperature of distilling water

[Soapy Sam]

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.-jj

I agree, but as Athon says, we may be seeing complications 11 year olds will not, because they see the question specifically in the context of an exam on classwork.

Beausoleil- He's in London.
a) At sea level
b) Probably in an Atlantic Depression- (It's close to Wimbledon time)
and
c) With very hard water- probably somewhere in the several hundred ppm [Ca++] range.

Not to mention the raw sewage.
Nobody drinks tap water in that cesspool of depravity.

 

[Rob Lister]

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

I should have greatly liked to have been a kid in your class. In fact, I seem to be one right now.

 

[Orangutan]

Athon,

This is an interesting topic.

I'm afraid I only did chemistry until I was 13 but I did physics until I was 19.

here's how I thought about the problem, could you let me know how I did.

Assumtions.
We are at sea level, i.e pure H20 boils at 100C.
The impure water is a salt solution, as in your class demo.
The water in the flask is boiling.
There is no pressure buildup inside the apparatus.

Now let's imagine i'm a molecule of H20 that's just escaped the surface of the water, at the moment I'm 100C+.

If I condense on the thermometer and I will start to raise the temperature of the thermometer. Eventually the thermometer will reach 100C as more molecule reach the thermometer.

I'm still happy. Im a water molecule swiming about with all my friends at 100C in a film of water covering the thermometer.

now another molecule comes along from the solution at 100C+ he warms us all up a bit.

The thermometer gets the +,

But the termometer also tries to pass the + back to us, we arn't happy, one of us takes the + and evaporates as steam, cooling the rest of us, and the thermometer, to and avarage of 100C.

Infact every now and again one of us will get hot enough to evaporate on our own cooling us to below 100C, luckily there's a constant stream of 100+ molecules coming from the water to maintain our equilibruium.

So, my answer would be 100C.

How did I do?

O.

 

[athon]

I agree, but as Athon says, we may be seeing complications 11 year olds will not, because they see the question specifically in the context of an exam on classwork.

Beausoleil- He's in London.
a) At sea level
b) Probably in an Atlantic Depression- (It's close to Wimbledon time)
and
c) With very hard water- probably somewhere in the several hundred ppm [Ca++] range.

Not to mention the raw sewage.
Nobody drinks tap water in that cesspool of depravity.

Thanks for that, Soapy. It is definately depressing at the moment.

I should have greatly liked to have been a kid in your class. In fact, I seem to be one right now.

Well, Rob, I want you to stand up and get that gum from your mouth. Cud's for cows, mate. You call that a tie? You're mother's not here to dress you now, sunshine, so make yourself look presentable. Now sit and get out your pen.

What do you mean you didn't bring a pen? No, I don't have one; do I look like a stationery store? Did you see 'McRae's Stationery' above the door on the way in? No. I didn't think so.

Maybe you should go see the head for a talk, hm?



Athon

 

[athon]

Athon,

This is an interesting topic.

I'm afraid I only did chemistry until I was 13 but I did physics until I was 19.

here's how I thought about the problem, could you let me know how I did.

Ok. Cool.

Assumtions.
We are at sea level, i.e pure H20 boils at 100C.
The impure water is a salt solution, as in your class demo.
The water in the flask is boiling.
There is no pressure buildup inside the apparatus.

We'll make it a saturated solution for argument's sake. And you mispelt 'assumptions'. That's one mark off. teehee

Now let's imagine i'm a molecule of H20 that's just escaped the surface of the water, at the moment I'm 100C+.

No, you'd have slightly more heat energy, as the impurity changed the polarity of your molecule, making the H bonds a little stronger. Therefore you'd typically need more than 100.C to pull free.

At the very surface, however, you have less molecules surrounding you. You only need to overcome surface tension. This can be accomplished with a lot less than 100.C. The exact amount, I'd have to look up.

I'm curious now to know what that might be.

If I condense on the thermometer and I will start to raise the temperature of the thermometer. Eventually the thermometer will reach 100C as more molecule reach the thermometer.

Not exactly, although I see what you're saying. The thermometer will reach a temperature between its old temperature and the temperature of the molecule. The water doesn't give all of its heat energy over, don't forget. You read '100.C' (or whatever) because the thermometer is calibrated to read that at an equivalent temperature.

It's confusing when you have to consider so many variables, like the overall temperature of the environment and the individual temperatures of each molecule, etc. I'm probably complicating it uneccessarily.

(I hope I haven't misunderstood something, or mistated it. I'm happy to be corrected here).

I'm still happy. Im a water molecule swiming about with all my friends at 100C in a film of water covering the thermometer.

now another molecule comes along from the solution at 100C+ he warms us all up a bit.

The thermometer gets the +,

But the termometer also tries to pass the + back to us, we arn't happy, one of us takes the + and evaporates as steam, cooling the rest of us, and the thermometer, to and avarage of 100C.

Infact every now and again one of us will get hot enough to evaporate on our own cooling us to below 100C, luckily there's a constant stream of 100+ molecules coming from the water to maintain our equilibruium.

So, my answer would be 100C.

How did I do?

O.

Your scientific thinking gets top marks. I'd still argue some of the finer details, but overall the explanation of temperature equilibrium is a good one.

You get a smilie-face sticker.



Athon

 

[clusterm2]

Are we talking multiple choice answers here?

 

[Orangutan]

Originally posted by athon
You get a smilie-face sticker.



Athon

YAY!.


I'm sorry, at the start I meant 100C+ to mean 100 degrees celcius + some extra as I'm from an unpure source, I sould have made that more obvious.

Having re-thought it through. I think I'm saying you can't condense water from an unpure source any higher than 100C because once the aparatus get's that hot the condensate will re-evaporate. In fact if you allow your condensate to get that hot you are losing alot of water as steam from the apparatus.

A temprature change from the thermometer to the end of the spout would allow the thermometer to read more that 100C and allow condensate to form along the length of the spout.

When you measured it in the lab was there condensate on the thermometer or was it dry?

I agree that it's a complicated question, with many variables left out. Any kid who answered around 100C should get a mark. As it would be possible to build the same apparatus that measures 100- 100 and 100+ doing the same job (with varing efficeincy).

Sometimes I think it would be fun to be a science teacher. But then I remember I like getting paid.


Thanks again for an interesting topic.

 

[patnray]

YAY!.


I'm sorry, at the start I meant 100C+ to mean 100 degrees celcius + some extra as I'm from an unpure source, I sould have made that more obvious.

Having re-thought it through. I think I'm saying you can't condense water from an unpure source any higher than 100C because once the aparatus get's that hot the condensate will re-evaporate. In fact if you allow your condensate to get that hot you are losing alot of water as steam from the apparatus.

All the below assumes atmospheric pressure at sea level: Steam can be hotter than 100 deg. C even at atmospheric pressure. (Any gas can be heated above its boiling point. Steam cannot, however, be at equilibruium with pure water at anything other than 100 deg.) Now, we agree that the steam, as it leaves the boiling salt water, is somewhat hotter than 100 deg because the salt raises the boiling point. Some steam may condense on the thermometer, but it is now pure water and will reevaporate as it is exposed to more steam at greater than 100 deg. Eventually we reach a steady state with the boiling water, the steam above that, and the thermometer all at the same temperature (asuming the steam escapes and does not raise the pressure). Since the steam, and thermometer, are at greater than 100 deg, there should be no (or very little) condensation on the thermometer at this point. And the thermometer should read > 100 deg (if immersed the correct depth)...

If measured before reaching steady state, with lots of condensate on the thermometer, then it will probably read 100 deg...