Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to a temperature such that its vapor pressure is above that of the surroundings. Boiling, however, occurs in three characteristic forms, which are nucleate, transition and film boiling.

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.

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.

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.

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

Boiling in cookery

In cookery, boiling is cooking food in boiling water, or other water-based liquid such as stock or milk. Simmering is gentle boiling, while in poaching the cooking liquid moves but scarcely bubbles.

Under special conditions, a liquid heated may show boiling delay when heated over its boiling point, by starting to boil suddenly and violently.

In places where the available water supply is contaminated with disease-causing bacteria, boiling water (and allowing it to cool) before drinking it is a valuable health measure. Boiling water for a few minutes kills most bacteria, amoebas, and other microbial pathogens. It thus can help prevent cholera, dysentery, and other diseases caused by these organisms.

Increasing the temperature of a liquid already boiling by adding heat is impossible. Pressure and a change in composition of the liquid may alter the boiling point of the liquid. For this reason, high elevation cooking generally takes longer since boiling point is a function of atmospheric pressure. The boiling point is defined as the temperature at which the vapor pressure of the substance equals the pressure above the substance. Increasing the pressure as in a pressure cooker raises the temperature of the contents above the open air boiling point. Adding salt also increases the boiling point. Due to variations in composition and pressure, the boiling point of water is almost never 212 F / 100 C, but rather close enough for cooking.