Superheating

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). Superheated liquids can be stable above their usual boiling point if the pressure is above atmospheric (see superheated water). This article refers only to liquids above their actual boiling point in a metastable state

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. 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. A foreign object added to the water prior to heating, whether it be a plastic spoon or a salt cube, greatly diminishes the chance of an explosion because it provides nucleation sites.

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.

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 can occur. 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.

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.