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Superheating
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- 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 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.
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Encyclopedia
- 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 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. The term superheated water refers to water that exceeds 100°C under pressure, which is a different phenomenon. This article refers only to liquids above their actual boiling point in a metastable state.
Occurrence in the kitchen
Superheating can occur when a person heats an undisturbed cup of water in a microwave oven. When the container is removed, the water still appears to be below the boiling point. However, once the water is disturbed, some of it violently flashes to steam, spraying boiling water out of the container. The boiling can be triggered by jostling the cup, inserting a stirring device, or adding a substance like instant coffee or sugar. The chances of superheating are greater with smooth containers, because scratches or chips can house small pockets of air, which serve as nucleation points. Chances of superheating can increase with repeated heating and cooling cycles of an undisturbed container, like when a forgotten coffee cup is re-heated without being removed from a microwave oven. This is due to heating cycles progressively de-gassing the liquid. There are ways to prevent superheating in a microwave oven, such as putting a popsicle stick or plastic spoon in the glass, or using a scratched 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.
Myth
There is a common belief that superheating can occur only in pure substances. This is untrue, as superheating has been observed in coffee and other impure liquids. Impurities do prevent superheating if they introduce nucleation sites -- rough areas where gas is trapped. For example, sand will tend to suppress superheating in water. Dissolved gas can also provide nucleation sites when it comes out of solution and forms bubbles. But an impurity such as salt or sugar, dissolved in water to form a homogeneous solution, will not prevent superheating.
See also
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