Boiling (also called ebullition), a type of phase transition, is the rapid vaporization of a liquid, which typically occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure. Thus, a liquid may also boil when the pressure of the surrounding atmosphere is sufficiently reduced, such as the use of a vacuum pump or at high altitudes. Boiling occurs in three characteristic stages, which are nucleate, transition and film boiling. These stages generally take place from low to high surface temperatures, respectively.
Nuclear boiling is characterized by the incipience and growth of bubbles on a heated surface, 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 (i.e. increased surface roughness) can create additional nucleation sites, while an exceptionally smooth surface, such as glass, lends itself to superheating. Under special conditions, a heated liquid may show boiling delay when heated over its boiling point, by starting to 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.
The formation of bubbles in a heated liquid is a complex physical process which often involves cavitation and acoustic effects, such as the broad-spectrum hiss one hears in a kettle not yet heated to the point where bubbles roil the surface.
Boiling in cookery
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, amoeba, and other microbial pathogens. It thus can help prevent cholera, dysentery, and other diseases caused by microorganisms.
The temperature of a substance is constant as it undergoes a phase transition. Therefore, increasing the temperature of a liquid already boiling by increasing the rate of heat transfer is impossible: it will just boil more quickly. Once it has turned into steam, water will increase in temperature as heat is applied to it. 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. In Denver, Colorado, which is at an elevation of about one mile, water boils at approximately 95 C.  Depending on the type of food and the elevation, the boiling water may not be hot enough to cook the food properly. 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 a water soluble substance, such as salt or sugar also increases the boiling point. This is called boiling-point elevation. However, the effect is very small, and the boiling point will be increased by an insignificant amount. On the other hand, salt or ethylene glycol can cause significant freezing point depression. Due to variations in composition and pressure, the boiling point of water is almost never exactly 212 F / 100 C, but rather close enough for cooking.
Foods suitable for boiling include fish, vegetables, farinaceous foods such as pasta, eggs, meats, sauces, stocks and soups.
Boiling has several advantages. It is safe and simple, and it is appropriate for large-scale cookery. Older, tougher, cheaper cuts of meat and poultry can be made digestible. Nutritious, well flavoured stock is produced. Also, maximum color and nutritive value is retained when cooking green vegetables, provided boiling time is kept to the minimum.
On the other hand, there are several disadvantages. There is a loss of soluble vitamins from foods to the water, and boiled foods can look unattractive. Boiling can also be a slow method of cooking food.
Boiling can be done in two ways: The food can be placed into already rapidly boiling water and left to cook, the heat can be turned down and the food can be simmered; or the food can also be placed into the pot, and cold water may be added to the pot. This may then be boiled until the food is satisfactory.
Water on the outside of a pot, i.e. a wet pot, increases the time it takes the pot of water to boil. The pot will heat at a normal rate once all excess water on the outside of the pot evaporates.
Boiling for water purification
Boiling is used as a method of water purification. Boiling is commonly advocated as an emergency water treatment method, or as a method of potable water purification in rural or wilderness settings without access to a potable water infrastructure.
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