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.
Superheating is sometimes a concern with microwave ovens, as 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, however, 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 water shooting out. Obviously this presents a safety hazard. 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 sugar 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.