Superheating
From Wikipedia, the free 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 standard boiling point, without actually boiling. This can be caused by rapidly heating a homogeneous substance while leaving it undisturbed (so as to avoid the introduction of bubbles at nucleation sites).
Because a superheated liquid is the result of artificial circumstances, it is metastable, and is disrupted as soon as 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 is more common with smooth containers, such as 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, such as 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, such as putting a popsicle stick in the glass, or having a scratched container to boil the water in.
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Milk and water with starch content do not boil over because of superheating, but because of extreme foam build up. This foam is stabilised by special substances in the liquids and therefore does not burst.
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[edit] External links
- Video of superheated water in a microwave explosively flash boiling, why it happens, and why it's dangerous.
- A series of superheated water with oil film experiments done in the microwave by Louis A. Bloomfield, physics professor at the University of Virginia. Experiment #13 proceeds with surprising violence.
