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Younes Shabany, professor in the Mechanical Engineering department at San Jose State University, writes in Heat Transfer: Thermal Management of Electronics:

For example, boiling temperature of water is a function of its pressure; water always boils at 100°C if the pressure is 101.42 kPa

Hasok Chang, professor at the University of Cambridge for History and Philosophy of Science, writes in The Myth of the Boiling Point:

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.

Is Hasok Chang account, that the standard ideas taught about the boiling point of water are flawed, true?

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2  
The part about removing dissolved air seems to be about superheating the water as described later on the page. That is a well-known phenomenen that can occur when you remove all possible nucleation sites. The short description you quoted seems to be pretty misleading. – Mad Scientist Feb 20 at 11:57
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"Myth" seems a little strong. Perhaps "idealised simplification" would be better. The point being that there are a lot of factors than can alter the boiling point that we usually ignore. Contaminants, deviations from good mixing, changes in the actual atmospheric pressure... Good teaching would admit these factors and encourage some real experimentation about their size. Then people would have a better appreciation of the difference between the ideal BP of theory and the real-world BP in practice. – matt_black Feb 20 at 15:37
    
This would be better asked on the Physics or Chemistry sites. – DJClayworth Feb 20 at 16:23
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@matt_black: Quite, the "myth" if any lies in the terminology. The temperature at which the vapour pressure of pure water equals standard pressure is 100°C. For fairly obvious reasons this is called the "boiling point", but then it's laziness and over-simplification to assume it will actually boil at the so-called "boiling point". If Chang's claim is taken to be, "we over-simplify boiling", then that's a different claim warranting different answers from if his claim is "nobody hears about super-heated water in school" (clearly false) or "it's possible to super-heat water" (clearly true). – Steve Jessop Feb 20 at 19:11
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So basically, which claim is supposed to be assessed here? "We all learn at school that pure water always boils at 100°C (212°F), under normal atmospheric pressure", or "There are actually all sorts of variations in the boiling temperature of water", or "this is [enough falsely-held belief to constitute] a myth"? It seems to me a matter of opinion whether the standard teaching is or is not flawed, even given that it's not precise. – Steve Jessop Feb 20 at 19:15
up vote 24 down vote accepted

Pure water at 1atm pressure boils at 100°C by definition (at least until 1954). However:

  • The new definition of centigrade is different, it depends on absolute zero degrees and the triple point of water.

    From 1744 until 1954, 0 °C was defined as the freezing point of water and 100 °C was defined as the boiling point of water, both at a pressure of one standard atmosphere with mercury being the working material. Although these defining correlations are commonly taught in schools today, by international agreement the unit "degree Celsius" and the Celsius scale are currently defined by two different temperatures: absolute zero, and the triple point of VSMOW (specially purified water). This definition also precisely relates the Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature with symbol K. Absolute zero, the lowest temperature possible, is defined as being precisely 0 K and −273.15 °C. The temperature of the triple point of water is defined as precisely 273.16 K and 0.01 °C

  • The temperature of water might not be uniform in the vessel. In fact hot water displays heat convection.

    The movement of steam or the motion of boiling water in a pot are also examples of convection

    How Cooking Works: Getting Food Hot

  • The pressure isn't normally exactly 1atm.

    pressure distribution
    In the picture, 1atm pressure is light pink

  • Everyday water contains impurities, such as limescale.

    Hard water is water that has high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone and chalk which are largely made up of calcium and magnesium carbonates.

  • Finally, water can be superheated. Superheated water will boil at more than 100°C.

    In physics, superheating (sometimes referred to as boiling retardation, or boiling delay) is the phenomenon in which a liquid is heated to a temperature higher than its boiling point, without boiling. Superheating is achieved by heating a homogeneous substance in a clean container, free of nucleation sites, while taking care not to disturb the liquid.

    This is a mythbusters video showing superheated water, including it not boiling at 100°C.

Outside the boundaries of the definition, water behaves in different ways. For example it might not even have a liquid phase.

triple point

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2  
Nice comprehensive answer, but you might want to also mention superheating due to lack of nucleation sites, as that seems to be a big part of what the linked article is (confused) about. – Ilmari Karonen Feb 20 at 14:22
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@IlmariKaronen done. – Sklivvz Feb 20 at 16:49
    
Also, the pressure maps are pretty obviously corrected to sea level pressure. Obviously, most places on land are not going to be at sea level. In my kitchen, water boils at ~95C; hiking in the nearby mountains, it might be 90C or less. – jamesqf Feb 20 at 19:29
    
The sentence "Pure water [..] boils at 100°C by definition (at least until 1954)." is a bit misleading. Pure water at normal pressure no longer boils at exactly 100 °C since 1948, when the unit kelvin of thermodynamic temperature was introduced, and the Celsius scale was defined in terms of the thermodynamic temperature. The International Practical Temperature Scale of 1948 (IPTS-48), p. 91 still used the boiling point of water as a fixed point (the International Temperature Scale is an approximation of the thermodynamic scale). – Massimo Ortolano Feb 20 at 19:35
    
Since 1990, when the International Temperature Scale of 1990 (ITS-90) was introduced, the boiling point of water is no longer a fixed point of the ITS, so that water no longer boils at 100 °C in any scale, neither in the thermodynamic scale nor in the international scale. – Massimo Ortolano Feb 20 at 19:38

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