Temperature of distilling water

[athon]

This is the initial spark that started my rant (see 'Science is...).

The students had an exam. In this exam, there was a question which showed a distillation set-up with a burner, a tripod, a round-bottomed flask with impure water in it, a thermometer inserted in the very top of the flask (not touching the liquid), a condensing tube and a beaker at the end.

The kids had to suggest the temperature the thermometer would read during the distillation.

The answer we had to mark as correct was not the answer I thought. Worse yet, the kids (as we had done this experiment already, and they had recorded the actual temperature) gave the answer I would have agreed with. The science advisor for the borough stuck to her guns, refusing to budge and change the answer. Worse yet, other members of the department said I was wrong and refused to settle it by observing the experiment again with me.

Grrr!

So, any suggestions as to what the approxiamate temperature should have been?

Athon

 

[Doubt]

Been a while since I had to worry about temperatures that low.

I would guess 100 degrees C, taking into account energy consumed/released in transformation.

Do I win a prize?

 

[Rob Lister]

Been a while since I had to worry about temperatures that low.

I would guess 100 degrees C, taking into account energy consumed/released in transformation.

Do I win a prize?

100c would be my guess too. Maybe something ever so slightly more if the distillation tube is long and/or narrow.

ETA: I suppose too it depends on your altitude. You don't live in Denver, do you?

 

[Diamond]

100c would be my guess too. Maybe something ever so slightly more if the distillation tube is long and/or narrow.

ETA: I suppose too it depends on your altitude. You don't live in Denver, do you?

Less than 100C. It cools as it reaches the top, otherwise it wouldn't condense.

 

[Rob Lister]

Less than 100C. It cools as it reaches the top, otherwise it wouldn't condense.

If it's the setup I'm thinking of, the top is closed by a rubber stopper with a themometer sticking through it. All condensation would be through the condensation tube out the side.

 

[davefoc]

If the answer isn't about 100 C it seems like a trick question. Many small factors happening causing temperature to be not exactly 100 C.

1. Even without boiling space above heated water would equalize at termperature very close to water temperature because of radiation and convection from water. Slight cooling from walls would reduce temperature somewhat.

2. With boiling pressure increases small amount so boiling water is slightly higher than 100 C thereby raising temperature of vapor slightly above 100 C.

3.Any water condensing on sides chamber where vapor are would reduce temperature of vapor somewhat.

4. The thermometer could add a measurement error by conducting heat through thermometer glass in cooler area to area in chamber where vapor is. I'd have more confidence in termperature taken with thermocouple.

As to people not wanting to look at your experimental setup: Jeez, I wouldn't want to work with people like that but I have.

 

[Chris Haynes]

Also, depending on what the impurities are... the temperature of boiling could be different. I actually do not know the answer, but I do know that when you add a certain amount of sugar to water it boils at a higher temperature. As the water evaporates the temperature will go higher. This means that it can be very dangerous when cooking candy, as the steam is very very hot and the spoon. It is advisable to keep a bowl of ice water nearby.

See:
http://www.exploratorium.edu/cooking/candy/sugar-stages.html

 

[davefoc]

Hmm, I've thought about this a little bit more.

I think the dominant effect would be measurement error. Apparently one end of the thermometer is in ambient air and other end is immersed in water vapor. I suspect the specific heat of water vapor is much closer to air than liquid water so the thermometer bulb reaches an equilibrium temperture based on amount of its surface expose to room temperature and amount of its surface exposed to water vapor and the conductivity of the glass.

When a thermometer is used to measure the temperature of a liquid the specific heat of liquid is so much higher than specific heat of air that the temperature of air can be ignored. In this experiment this is not the case. The thermometer is immersed in two different gases at different temperatures. So the amount of the surface area of thermometer immersed in each of the fluids is very significant.

 

[69dodge]

Ok, so we have all the possibilities here: some say 100 °C, some say less, some say more. (I don't really know, but I thought of the impurities raising the boiling point too, like Hydrogen Cyanide.)

athon, what did the experiment say? What is the "official" answer?

 

[Doubt]

I will chime in again.

The one thing some of you are missing is that the conversion process where water becomes a gas absorbs energy without raising the temperature. This is an endothermic reaction. So what happens is the water reaches 100 C and begins to turn to steam. The steam itself should still be around 100 degrees C unless it is trapped or restricted and absorbs more heat.

In short, there is little or no temperature change happening while the state transformation takes place.

Anything condensing inside the heating chamber wold still be at 100 C just after condensing. That is the reverse exothermic reaction, which would take place during condensation.

The state transformation should act as a temperature regulator with all the excess energy, (heat) being carried into the condensation chamber.

But as Rob said, if the tube were long, it could be different since it could act as a restriction on the gas flow.

 

[Beausoleil]

You don't have to boil to distill, so the question is unanswerable. When making ultra-pure reagents by distillation boiling is avoided.

 

[Jyera]

1. Perhaps the exam question should include another setup.
A similar setup, except that it uses pure water instead of water with impurity.
The temperature of this setup should be given to the student thereby giving a reference for the impure ones.

2. Differences in atmospheric pressure, ambient temperature, accuracy of themometer should then be normalised.

3. What is the concept behind this exam questions?

4. The answer should be consistent with the concept taught.

 

[athon]

Wow, I thought this would be an easy one.

HC wins the prize; even taking experimental error into account, it boils higher than 100.C. Typically (depending on how impure) at around 105 - 110.C. Although I'm not 100% sure on all water-based solutions, adding salt or sugar alters the melting points and boiling points of water. The melting point decreases (hence why you add salt to snow to melt it at a lower temperature) and the boiling point increases (why you add some salt to boiling water when boiling rice or pasta).

It's pretty much text book science, and something I've been showing in experiments for years (boiling water and then throwing in a handful of salt; the solution's temperature increases by as much as 5 or 6 degrees C). Even if the thermometer is above the solution, the gas immediately above is only slightly cooler than the liquid. For the water to condense, you need the chilled condensation tube.

The thing is, I don't mind people disagreeing, but when you can demonstrate this practically, shouldn't this be the basis of an argument, and not your assumptions? Kids should be referring to first-hand experimental data rather than 'what the teacher says'.

Athon

 

[athon]

1. Perhaps the exam question should include another setup.
A similar setup, except that it uses pure water instead of water with impurity.
The temperature of this setup should be given to the student thereby giving a reference for the impure ones.

2. Differences in atmospheric pressure, ambient temperature, accuracy of themometer should then be normalised.

3. What is the concept behind this exam questions?

4. The answer should be consistent with the concept taught.

The concept addresses separation of a solution, so pure water (while you could teach that) sort of makes it irrelevant.

You also have to keep in mind that these are 11 year old kids, who often have enough trouble grasping scientific concepts. There is an endemic problem with students simply being spoon fed information; I think learning first hand from experiments should be encouraged at every step.

The problem is that borough science advisors are less concerned with scientific thinking than they are with kids simply being able to parrot back facts and figures.

Athon

 

[Doubt]

The concept addresses separation of a solution, so pure water (while you could teach that) sort of makes it irrelevant.

You also have to keep in mind that these are 11 year old kids, who often have enough trouble grasping scientific concepts. There is an endemic problem with students simply being spoon fed information; I think learning first hand from experiments should be encouraged at every step.

The problem is that borough science advisors are less concerned with scientific thinking than they are with kids simply being able to parrot back facts and figures.

Athon

So what was the answer they regarded as correct?

 

[Soapy Sam]

Athon. This is daft. The temperature at the top of the flask depends on the position of the bunsen burner and the amount of gas going through it. Remove the flask, water, distillation tube.
Now hold the thermometer over the bunsen in the same spatial coordinates. What temperature is it now?

If the thermometer is not in contact with the water, it tells you precisely nothing of interest. And the water (if boiling and at STP is at 100C for reasons others have pointed out).

And if the kids ran the experiment and got a different answer, then the lesson taught by "science" is that we need to get all the ducks in a row when we do an experiment.
And never believe textbooks.

 

[Chris Haynes]

Wow, I thought this would be an easy one.

HC wins the prize; even taking experimental error into account, it boils higher than 100.C. ...

Oh thank you!!! Who knew knowing how to cook would lead to this prize!!! Though I have been known to teach kids and cub scouts some basic science in the kitchen.

 

[athon]

Athon. This is daft. The temperature at the top of the flask depends on the position of the bunsen burner and the amount of gas going through it. Remove the flask, water, distillation tube.
Now hold the thermometer over the bunsen in the same spatial coordinates. What temperature is it now?

Unless I've missed something (completely possible)...

Here's the simple, high school answer. The true, complex physics answer is pretty cool but more long winded (and something I don't touch in year 7)

Temperature and heat are different things. Heat is the energy, temperature is the measurement of heat energy a particle possesses. It's a bit like money and value; money is the stuff, value is the measure of money a single thing can be said to have.

In a system like this one, heat energy enters the water (we'll assume pure water for the moment) via the burning gas. At the very surface of the water, molecules have enough heat energy at room temperature to 'ignore' the hydrogen bonds they have with their neighbour and spread out (evaporation).

As more heat energy is added to the water, the molecules continue to move and spread out. As a molecule that is completely encircled by other water molecules reaches about 100.C, it has enough energy to escape its hydrogen bonds, hence the mass becomes a gas.

Adding more heat energy does not increase temperature, but contributes to increased kinetic energy of the molecules, hence you can't boil the water past 100.C no matter how hot the flame is. By making it more difficult to allow the water molecules to leave, however (pressure, or adding an impurity) you increase the temperature before the state changes.

If the thermometer is not in contact with the water, it tells you precisely nothing of interest. And the water (if boiling and at STP is at 100C for reasons others have pointed out).

If you want completely accurate measurements of the boiling point of the liquid, perhaps. But the difference of a few percent of a degree (the amount of difference the distance between the liquid and the thermometer and the cooler temperature of the apparatus will influence the result by) is hardly much to worry about in a year 7 science lesson.

And if the kids ran the experiment and got a different answer, then the lesson taught by "science" is that we need to get all the ducks in a row when we do an experiment.
And never believe textbooks.

It's not that we should never believe textbooks. It's that there are no absolutes; we trust information based not solely on the authority, but on a range of factors.

And ducks should never be in a line. It makes it easier for the crocodiles to eat them.

Poor ducks.

Athon

 

[Soapy Sam]

Temperature and heat are different things. Heat is the energy, temperature is the measurement of heat energy a particle possesses. It's a bit like money and value; money is the stuff, value is the measure of money a single thing can be said to have.

They taught us that with sparklers. High temperature, low heat content. Don't grab the stem.

If you want completely accurate measurements of the boiling point of the liquid, perhaps. But the difference of a few percent of a degree (the amount of difference the distance between the liquid and the thermometer and the cooler temperature of the apparatus will influence the result by) is hardly much to worry about in a year 7 science lesson

What I meant was that you don't know the boiling point of the water. It's impure remember; - the thermometer in that position doesn't tell you the water temperature. It tells you the temperature of the thermometer. That might have more to do with the position of the burner and the design of the tripod than anything else.
You said it was a round bottom flask, so I assume it is sitting in an open tripod rather than on an asbestos / ceramic coated one. If so, the flame and the corona of hot air around it will heat up the flask itself. I know I burned my pinkie-poos more than once that way.

 

[69dodge]

Originally posted by Soapy Sam
Athon. This is daft. The temperature at the top of the flask depends on the position of the bunsen burner and the amount of gas going through it. Remove the flask, water, distillation tube.
Now hold the thermometer over the bunsen in the same spatial coordinates. What temperature is it now?

If the thermometer is not in contact with the water, it tells you precisely nothing of interest. And the water (if boiling and at STP is at 100C for reasons others have pointed out).

Removing the flask and water entirely is a very different experiment.

To check whether it's the impurities' raising of the boiling point that is causing the measured temperature to be higher than 100 °C, run the same experiment with pure water and compare.

A bit of Googling leads me to believe that a 5 °C -- 10 °C increase in boiling point is about right for a very concentrated salt solution. athon, how much salt was in the water when you ran your experiment? (I guess the exam question didn't specify the concentration?)