Discussion point 2: Why don't we all know about the variations?
The main reason that we do not easily observe the variations of the boiling point is that in everyday life we tend to boil water always in the same kind of way: in broad vessels with heating from the bottom, which requires intense heat sources because of the high rate at which heat is lost at the open surface and the sides. (In the graphite bath shown in Experiment 6, I could not make water boil at all in any wide-open vessels, despite urging the graphite temperature to over 250°C.) In order to see that the "standard" act of boiling could be different in a different sort of civilization, consider the following three situations, one fictitious and two real.
(a) Imagine a civilization with no access to open flames, but only to hot stones or sand; there, boiling would be done in narrow-necked flasks, routinely producing the kind of superheated boiling that I have observed in a volumetric flask heated with a graphite bath or a hotplate. Such people would not dream of drawing a sharp line between the "liquid" and "gas" regions in a phase diagram.
(b) Coming back to real life in the 20th/21st century, note widespread reports of the superheating of water in microwave ovens, which heat water directly and evenly, not by means of a hot solid surface. This phenomenon is often noticed because the superheated water is apt to boil over violently when instant coffee is dropped into it. (Try putting "superheated water" into Google; Joe Wolfe of the University of New South Wales has a particularly good web page: www.phys.unsw.edu.au/~jw/superheating.html). Some recent researchers on boiling have employed laser pulses as a heating mechanism.
(c) In the 19th century, the age of the steam engine, boiler explosions resulting from superheating created a real concern, and various methods of preventing superheating were devised successfully; I have not seen a good theoretical or historical account of this business, but I suspect in a boiler the boiling routinely happened without much air present, the space above the water being occupied by steam instead. In the 20th century, mechanical and chemical engineers encountered a wide variety of situations in which boiling takes place, and have been accumulating experimental and theoretical knowledge of the various different types of boiling that can take place in different situations.
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