Superheating

See superheater for the device used in steam engines.

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 standard boiling point, without actually boiling. This can be caused by rapidly heating a homogeneous substance while leaving it undisturbed (in order to avoid the introduction of bubbles at nucleation sites).

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Mechanics

With the exception of superheated water below the Earth's crust, a superheated liquid is usually the result of artificial circumstances. Being such, it is metastable, and is disrupted once the circumstances abate, leading to the liquid boiling very suddenly and violently (a steam explosion). Superheating is sometimes a concern with microwave ovens, some of which can quickly heat water without physical disturbance. A person agitating a container full of superheated water by attempting to remove it from a microwave could easily be scalded.

Superheating is common when a person puts an undisturbed cup of water into the microwave and heats it. Once finished, the water appears to have not come to a boil. Once the water is disturbed, it violently comes to a boil. This can be simply from contact with the cup, or the addition of substances like instant coffee or sugar, which could result in hot scalding water shooting out. The chances of superheating are greater with smooth containers, like brand-new glassware that lacks any scratches (scratches can house small pockets of air, which can serve as a nucleation point).

Rotating dishes in modern microwave ovens can also provide enough perturbation to prevent superheating.

There have been some injuries by superheating water, like when a person makes instant coffee and adds the coffee to the superheated water^[1]. This sometimes results in an "explosion" of bubbles. There are some ways to prevent superheating in a microwave oven, like putting a popsicle stick in the glass, or having a scratched container to boil the water in. However this is very, very rare and can only happen under certain conditions. Foreign object added to the water prior to heating, whether it be a spoon or a salt cube, greatly diminishes the chance of an explosion because it provides nucleation sites.

Liquid would not be superheated if the liquid is heated via heated container (e.g. water in a pot on top of a stove) because the heated container surface that heats up the liquid provides nucleation sites for the liquid to boil off and cool down. This is in contrast to a microwave, where the water is directly heated via microwaves and not by the container.

Superheating also occurs in nuclear reactors and other types of high-temperature steam generators used for producing electricity, and is guarded against when it leads to corrosion or embrittlement of metal pipes.

Magnetrons, such as those used in microwave ovens, can also superheat steam in steam-power or steam-heating circuits, exponentially increasing steam thermal capacity. Advanced theories include powering the magnetron superheating circuit from electricity generated by the waste heat from the main steam circuit, resulting in additional heating BTUs for buildings at zero additional fuel cost or additional fossil fuel pollution.

Microwave superheating of seawater may be the explanation of current research into using microwave energy to ignite the elements in seawater: many studies have shown that passing superheated steam over heated metal or iron filings or within heated iron pipes decomposes the steam (H2O separates into hydrogen, the oxygen being reduced in the presence of red hot iron). The hydrogen-oxygen plasma ignites, which then consumes the sodium (55.04%), chloride (30.61%), sulfur (7.68%), and magnesium (3.69%) elements in seawater. A sodium vapor light uses the alkaline metal sodium in a similar way. In this case the focused radio frequency microwave energy creates a hot, glowing, gaseous plasma. Such an application by John Kanzius uses seawater as a fuel source.

Myth

A commonly mistaken belief is that superheating can only occur in pure substances. This is untrue because nucleation points for boiling do not include solid nucleation centres, but rather, seed-bubbles that occur due to the presence of solid nucleation centres. In other words, if there are solid nucleation centres in a substance (e.g. impure water) but without seed-bubbles (e.g. leaving impure water to stand or boiling it once to rid the water of the bubbles), superheating cannot occur^[2][3]. It is interesting to note however, that nucleation points for freezing include solid nucleation centres. That is to say, an impure substance cannot undergo supercooling^[2].

Scope restriction

Milk and water with starch content do not boil over because of superheating, but rather result in extreme foam buildup. This foam is stabilized by special substances in the liquids and therefore does not burst.

References

See also

• Supercooling
• Supersaturation
• Critical point
• Microwave oven
• Autoclave
• Boiling point