Does water always boil at 100°C under normal atmospheric pressure?

Question

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?

Answer 1

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.

  
  ^{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.