Superheating

From Free net encyclopedia

See superheater for the device used in steam engines.

In physics, superheating (sometimes referred to as boiling retardation, boiling delay, or defervescence) 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 air bubbles at nucleation sites).

Because a superheated fluid 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 very dangerous situation. 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.

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More on microwaving

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; that is not to say that an old container automatically is safe). To prevent this, adding something to the water, such as a wooden spatula/popsicle stick, prior to heating it is suggested; rotating dishes in modern microwave ovens can also provide enough perturbation and prevent superheating.

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