Boiling

Boiling is the rapid vaporization of a liquid, which typically occurs when a liquid is heated to a temperature such that its vapor pressure is above that of the surroundings. A liquid may also boil when the pressure of the surrounding atmosphere is sufficiently reduced, e.g. by use of a vacuum pump. Boiling occurs in three characteristic forms, which are nucleate, transition and film boiling.

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Nucleate boiling is the most common type of boiling and it is characterized by bubbles, which rise from discrete points on a surface, whose temperature is only slightly above the liquid?s saturation temperature. In general, the number of nucleation sites are increased by an increasing surface temperature. An irregular surface of the boiling vessel can create additional nucleation sites, while an exceptionally smooth surface (such as glass) lends itself to superheating.

Related Topics:
Nucleation - Superheating

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When the surface temperature reaches a maximum value, the critical superheat, vapor begins to form faster than liquid can reach the surface. Thus, the heated surface suddenly becomes covered with a vapor layer. Because of the vapor layer?s lower thermal conductivity, this vapor layer insulates the surface. This condition of a vapor film insulating the surface from the liquid characterizes film boiling.

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Transition boiling may be defined as the unstable boiling, which occurs at surface temperatures between the maximum attainable in nucleate and the minimum attainable in film boiling.

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:For a further discussion of the phenomenon of boiling in physics, see Boiling point.

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