What is the Boiling Point of Water?

[jimmydasaint]

I used to think that the temperature of water was 212 degrees Fahrenheit or 100 degrees Celsius, at normal atmospheric pressure, but it seems that the type of container and degassing of the water can considerably change the boiling point, sometimes up to 10 degrees Celsius.

 

Any comments on this quote and subsequent video demonstrations?

 

We all learn at school that pure water always boils at 100°C (212°F), under normal atmospheric pressure. Like surprisingly many things that "everybody knows", this is a myth. We ought to stop perpetuating this myth in schools and universities and in everyday life: not only is it incorrect, but it also conveys misleading ideas about the nature of scientific knowledge. And unlike some other myths, it does not serve sufficiently useful functions.

 

There are actually all sorts of variations in the boiling temperature of water. For example, there are differences of several degrees depending on the material of the container in which the boiling takes place. And removing dissolved air from water can easily raise its boiling temperature by about 10 degrees centigrade.

UC London

 

 

Link to Experiments

Edited April 24, 2011 by jimmydasaint

[insane_alien]

the boiling point of water at atmospheric pressure IS 100*C

 

it only changes if you make a solution (ie adding something to it.) this will tend to increase the boiling point for most things(such as salt or sugar) but will decrease for a certain few (ethanol)

 

what the article is talking about is super heating, a potentially dangerous meta-stable state where the water is above its boiling point but is still a liquid. in this state any nucleation point introduced will initiate vigerous boiling

 

once boiling occurs then a temperature of 100*C is maintained.

 

also, the experiment where they measured 99.7*C for the boiling point, that could have been many many things. thermometer error, local atmospheric pressure drop etc. etc.

[mississippichem]

it seems that the type of container and degassing of the water can considerably change the boiling point, sometimes up to 10 degrees Celsius.

 

Read about boiling point elevation and colligative properties.

[jimmydasaint]

Slight complication here guys. I'll let Chang speak for himself:

The experiments leave a puzzle regarding the very possibility of thermometry: if there were such unmanageable and ill-understood variations in the temperatures of boiling water, how could the boiling point have served as a fixed point of thermometry at all? It seems that the variations would have threatened the very notion of a definite "boiling point", but the very thermometers used for the investigation of the variations were graduated with sharp boiling points. The philosopher can only conjecture that there must have been an identifiable class of boiling phenomena with sufficiently stable and uniform temperatures, which allowed the calibration of thermometers with which scientists could then go on to study the more exotic instances

 

and

 

De Luc got as far as 112°C without boiling, but the highest temperature he recorded while the water was boiling was 103°C (this latter figure is also quite consistent with my own experimental results). Still, the latter is 3°C higher than the "normal" boiling temperature, and there was also Gay-Lussac's observation that the temperature of boiling water was over 101°C in glass vessels. Marcet (1842, 397 and 404) investigated this question with more care than anyone else at the time. In ordinary glass vessels, he observed the temperature of boiling water to range from 100.4° to 101.25°C. In glass treated with hot sulphuric acid, the temperature while boiling went easily up to 103° or 104°C, and was very unsteady in each case due to bumping. So, a further stabilizing factor was needed.

 

also

 

Gay-Lussac had shown how to prevent superheating in glass vessels by throwing in metal chippings or filings (or even powdered glass). Other investigators found other methods, such as the insertion of solid objects (especially porous things like charcoal and chalk), sudden localized heating, and mechanical shocks. But in many practical situations the prevention of superheating simply came down to not bothering too much. If one left naturally occurring water in its usual state full of dissolved air (rather than taking the trouble to purge air out of it), and if one left the container vessels just slightly dirty or rough (instead of cleaning and smoothing it off with something like hot sulphuric acid), and if one did not do anything else strange like isolating the water from solid surfaces, then common boiling did take place.

 

Link to article

[swansont]

In a broader sense, this is nothing new. There are myriad examples of statements that are true under a set of conditions but not true in general, and most of what science teaches is an approximation. It shouldn't be a surprise — nature is complex, and it's hard to comprehend, especially all at once, so we work from simple to more complex. You can spin the wheel and pick just about any other topic, and get all worked up like Chang did about this.

[jimmydasaint]

In a broader sense, this is nothing new. There are myriad examples of statements that are true under a set of conditions but not true in general, and most of what science teaches is an approximation. It shouldn't be a surprise — nature is complex, and it's hard to comprehend, especially all at once, so we work from simple to more complex. You can spin the wheel and pick just about any other topic, and get all worked up like Chang did about this.

 

I agree with you. There is a plethora of question marks in almost every field of enquiry. However, Chang mentioned that something that comes up in High School science should be taught with the complications added in. There are millions of school leavers who believe that electrons go round the nucleus in the 'Solar System' model and these misconceptions do stick in the mind for a long time unless corrected. Almost every day, I am forced to correct my half understood ideas about things. Many of my misconceptions are about geography- I chose history as a specialist subject.

 

So what do we do? Do we teach the basics and allow students to correct, or clarify their thoughts years later? Or do we warn them of the complications? I would opt for the latter approach.

Edited April 25, 2011 by jimmydasaint

[hypervalent_iodine]

This isn't really a phenomena limited to high school. You get told white lies an first and second year level university as well. It's just simpler. Going into more detail requires more time than a course often has and it is too much for people to comprehend at the end of semester.

 

In high school, I was taught a little but about things that affect boiling points of water in my final year. Same deal with electron orbitals. In fact, I don't know anyone who was taught the 'solar system' model in high school. To be honest, going into too much depth about something like boiling points seems a little pointless and mundane. There are other more critical things that need focussing on I think.

[TonyMcC]

I used to be a college lecturer. There is an old joke that says "Only half of what a lecturer says is true - the problem is even he/she doesn't know which half". The joke works because there is a lot of truth in it!

[SMF]

In my opinion, any college lecturer who hasn't planned the necessary simplifications to teach complicated information, and done so based on knowledge of the content, is not doing their job. SM

[TonyMcC]

In my opinion, any college lecturer who hasn't planned the necessary simplifications to teach complicated information, and done so based on knowledge of the content, is not doing their job. SM

The simple truth is that nobody knows absolutely everything about anything! I learnt a lot from my students and over the years I am not too proud to say that I tripped up sometimes. I believe that my willingness to consider my students thoughts and allowing them to influence my thoughts through open discussion engendered more respect than a more stilted response might have done. In any case, my students seemed to do well in their efforts to achieve their aims and qualifications!

[SMF]

TonyMcC. My statement expressed my concerned about a teacher who claims that only half of what they taught was true. Because scientific knowledge is always progressing, I mean "true" here to consist of what is known, not some absolute truth. The simplifications of complicated information that I have seen good teachers make are not falsehoods, they are clarifications designed for the educational level of the learner. I always tried to inform the students from time to time of my simplifications with a short bit of the very complicated material backing up what I said, or by suggesting that the last few minutes of lecture could easily provide enough material for a whole course. SM

[lemur]

I agree that the way phase-change is taught gives a false impression of how substances behave, or rather it causes students to assume that facts like the boiling point of water are truths that have a life of their own, which is why the OP expresses shock at the idea that the boiling point of water can change according to different factors. If people would just be taught why materials behave as solids, liquids, and gas and why they go from one type of behavior to the other, then the boiling points of various liquids would just be a convenient index for predicting when something was going to boil when you were trying to boil it and how hot it would be at that time..

Edited April 26, 2011 by lemur

[TonyMcC]

SMF - I deduce that you too have been a "teacher" with a responsibility to your students. I hope and sense that you will have done your best for your students, as indeed I did. The old joke says "half" and perhaps I should have said there is "some truth" rather than "a lot of truth". Strangely enough there were occasions during my working life when I worked with little or no margin of knowledge above that of my students. For a time I worked as a civilian for a Company manufacturing military products. My job was to teach customers the theory and practical maintenance of equipment I had never seen. That meant liaising with designers, gathering technical information, examining the equipment on the production line and planning the course I was to run. Later on in life I became a lecturer at a college of education and life became much more stable (and knowledge more certain) as I experienced the relative luxury of teaching basic and rarely changing principles! If, like me, you are retired then I wish you happiness and happy memories.