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Boiling is the rapid vaporization of a
liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...
, which occurs when a liquid is heated to its
boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envir ...
, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. There are two main types of boiling: nucleate boiling where small bubbles of vapour form at discrete points, and critical heat flux boiling where the boiling surface is heated above a certain critical temperature and a film of vapor forms on the surface. Transition boiling is an intermediate, unstable form of boiling with elements of both types. The boiling point of water is 100 °C or 212 °F but is lower with the decreased atmospheric pressure found at higher altitudes. Boiling water is used as a method of making it potable by killing microbes and viruses that may be present. The sensitivity of different micro-organisms to heat varies, but if water is held at for one minute, most micro-organisms and viruses are inactivated. Ten minutes at a temperature of 70 °C (158 °F) is also sufficient to inactivate most bacteria. Boiling water is also used in several
cooking Cooking, cookery, or culinary arts is the art, science and craft of using heat to Outline of food preparation, prepare food for consumption. Cooking techniques and ingredients vary widely, from grilling food over an open fire to using electric ...
methods including boiling,
steaming Steaming is a method of cooking using steam. This is often done with a food steamer, a kitchen appliance made specifically to cook food with steam, but food can also be steamed in a wok. In the American southwest, steam pits used for cooking ha ...
, and poaching.


Types


Free convection

The lowest heat flux seen in boiling is only sufficient to cause atural convection where the warmer fluid rises due to its slightly higher density. This condition occurs only when the superheat is very low, meaning that the hot surface near the fluid is nearly the same temperature as the boiling point.


Nucleate

'' Nucleate boiling'' is characterized by the growth of bubbles or pops on a heated surface (heterogeneous nucleation), which rises from discrete points on a surface, whose temperature is only slightly above the temperature of the liquid. In general, the number of
nucleation In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that deter ...
sites is increased by an increasing surface temperature. An irregular surface of the boiling vessel (i.e., increased surface roughness) or additives to the fluid (i.e., surfactants and/or nanoparticles) facilitate nucleate boiling over a broader temperature range, while an exceptionally smooth surface, such as plastic, lends itself to
superheating In thermodynamics, 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 boiling point, without boiling. This is a so-called ''metastable state ...
. Under these conditions, a heated liquid may show boiling delay and the temperature may go somewhat above the boiling point without boiling. Homogeneous nucleation, where the bubbles form from the surrounding liquid instead of on a surface, can occur if the liquid is warmer in its center, and cooler at the surfaces of the container. This can be done, for instance, in a microwave oven, which heats the water and not the container.


Critical heat flux

''Critical heat flux (CHF)'' describes the thermal limit of a phenomenon where a phase change occurs during heating (such as bubbles forming on a metal surface used to heat water), which suddenly decreases the efficiency of heat transfer, thus causing localised overheating of the heating surface. As the boiling surface is heated above a critical temperature, a film of vapor forms on the surface. Since this vapor film is much less capable of carrying heat away from the surface, the temperature rises very rapidly beyond this point into the transition boiling regime. The point at which this occurs is dependent on the characteristics of boiling fluid and the heating surface in question.


Transition

'' 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 Bubble, Bubbles or The Bubble may refer to: Common uses * Bubble (physics), a globule of one substance in another, usually gas in a liquid ** Soap bubble * Economic bubble, a situation where asset prices are much higher than underlying fundame ...
in a heated liquid is a complex physical process which often involves
cavitation Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, cal ...
and acoustic effects, such as the broad-spectrum hiss one hears in a kettle not yet heated to the point where bubbles boil to the surface.


Film

If a surface heating the liquid is significantly hotter than the liquid then film boiling will occur, where a thin layer of vapor, which has low thermal conductivity, insulates the surface. This condition of a vapor film insulating the surface from the liquid characterizes ''film boiling''.


Influence of Geometry


Pool boiling

"Pool boiling" refers to boiling where there is no forced convective flow. Instead, the flow occurs due to density gradients. It can experience any of the regimes mentioned above.


Flow boiling

"Flow boiling" occurs when the boiling fluid circulates, typically through pipes. It's movement can be powered by pumps, such as in power plants, or by density gradients, such as in a Thermosiphon or a heat pipe. Flows in flow boiling are often characterized by a void fraction parameter, which indicates the fraction of the volume in the system that is vapor. One can use this fraction and the densities to calculate the vapor quality, which refers to the mass fraction that is in the gas phase. Flow boiling can be very complex, with heavy influences of density, flow rates, and heat flux, as well as surface tension. The same system may have regions that are liquid, gas, and two-phase flow. Such two phase regimes can lead to some of the best heat transfer coefficients of any system.


Confined boiling

Confined boiling refers to boiling in confined geometries, typically characterized by a ond numberthat compares the gap spacing to the capillary length. Confined boiling regimes begin to play a major role when Bo < 0.5. This boiling regime is dominated by "vapor stem bubbles" left behind after vapor departs. These bubbles act as seeds for vapor growth. Confined boiling is particularly promising for electronics cooling.


Physics

The boiling point of an element at a given pressure is a characteristic attribute of the element. This is also true for many simple compounds including water and simple
alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...
s. Once boiling has started and provided that boiling remains stable and the pressure is constant, the temperature of the boiling liquid remains constant. This attribute led to the adoption of boiling points as the definition of 100°C.


Distillation

Mixtures of volatile liquids have a boiling point specific to that mixture producing vapour with a constant mix of components - the constant boiling mixture. This attribute allows mixtures of liquids to be separated or partly separated by boiling and is best known as a means of separating ethanol from water.


Uses


Refrigeration and air conditioning

Most types of refrigeration and some type of
air-conditioning Air conditioning, often abbreviated as A/C or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...
work by compressing a gas so that it becomes liquid and then allowing it to boil. This adsorbs heat from the surroundings cooling the fridge or freezer or cooling the air entering a building. Typical liquids include
propane Propane () is a three-carbon alkane with the molecular formula . It is a gas at standard temperature and pressure, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used a ...
, ammonia, carbon dioxide or nitrogen.


For making water potable

As a method of disinfecting water, bringing it to its boiling point at , is the oldest and most effective way since it does not affect the taste, it is effective despite contaminants or particles present in it, and is a single step process which eliminates most microbes responsible for causing
intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans ...
related diseases. The boiling point of water is at sea level and at normal barometric pressure. In places having a proper water purification system, it is recommended only as an emergency treatment method or for obtaining potable water in the wilderness or in rural areas, as it cannot remove chemical toxins or impurities. The elimination of micro-organisms by boiling follows first-order kinetics—at high temperatures, it is achieved in less time and at lower temperatures, in more time. The heat sensitivity of micro-organisms varies, at ,
Giardia ''Giardia'' ( or ) is a genus of anaerobic flagellated protozoan parasites of the phylum Metamonada that colonise and reproduce in the small intestines of several vertebrates, causing the disease giardiasis. Their life cycle alternates between ...
species (causes
Giardiasis Giardiasis is a parasitic disease caused by ''Giardia duodenalis'' (also known as ''G. lamblia'' and ''G. intestinalis''). Infected individuals who experience symptoms (about 10% have no symptoms) may have diarrhea, abdominal pain, and weight los ...
) can take ten minutes for complete inactivation, most intestine affecting microbes and ''
E. coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...
'' (
gastroenteritis Gastroenteritis, also known as infectious diarrhea and gastro, is an inflammation of the gastrointestinal tract including the stomach and intestine. Symptoms may include diarrhea, vomiting, and abdominal pain. Fever, lack of energy, and dehydra ...
) take less than a minute; at boiling point, '' Vibrio cholerae'' (
cholera Cholera is an infection of the small intestine by some strains of the bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts a few days. Vomiting and ...
) takes ten seconds and hepatitis A virus (causes the symptom of
jaundice Jaundice, also known as icterus, is a yellowish or greenish pigmentation of the skin and sclera due to high bilirubin levels. Jaundice in adults is typically a sign indicating the presence of underlying diseases involving abnormal heme meta ...
), one minute. Boiling does not ensure the elimination of all micro-organisms; the bacterial spores
Clostridium ''Clostridium'' is a genus of anaerobic, Gram-positive bacteria. Species of ''Clostridium'' inhabit soils and the intestinal tract of animals, including humans. This genus includes several significant human pathogens, including the causative ag ...
can survive at but are not water-borne or intestine affecting. Thus for human health, complete sterilization of water is not required. The traditional advice of boiling water for ten minutes is mainly for additional safety, since microbes start getting eliminated at temperatures greater than and bringing it to its boiling point is also a useful indication that can be seen without the help of a thermometer, and by this time, the water is disinfected. Though the boiling point decreases with increasing altitude, it is not enough to affect the disinfecting process.


In cooking

''Boiling'' is the method of
cooking Cooking, cookery, or culinary arts is the art, science and craft of using heat to Outline of food preparation, prepare food for consumption. Cooking techniques and ingredients vary widely, from grilling food over an open fire to using electric ...
food in boiling water or other water-based liquids such as
stock In finance, stock (also capital stock) consists of all the shares by which ownership of a corporation or company is divided.Longman Business English Dictionary: "stock - ''especially AmE'' one of the shares into which ownership of a company ...
or milk. Simmering is gentle boiling, while in poaching the cooking liquid moves but scarcely bubbles. The boiling point of water is typically considered to be , especially at sea level. Pressure and a change in the composition of the liquid may alter the boiling point of the liquid. High elevation cooking generally takes longer since boiling point is a function of atmospheric pressure. At an elevation of about , water boils at approximately . Depending on the type of food and the elevation, the boiling water may not be hot enough to cook the food properly. Similarly, increasing the pressure as in a
pressure cooker Pressure cooking is the process of cooking food under high pressure steam and water or a water-based cooking liquid, in a sealed vessel known as a ''pressure cooker''. High pressure limits boiling, and creates higher cooking temperatures which c ...
raises the temperature of the contents above the open air boiling point.


Boil-in-the-bag

Also known as "boil-in-bag", this involves heating or cooking ready-made foods sealed in a thick plastic bag. The bag containing the food, often frozen, is submerged in boiling water for a prescribed time. The resulting dishes can be prepared with greater convenience as no pots or pans are dirtied in the process. Such meals are available for camping as well as home dining.


Contrast with evaporation

At any given temperature, the molecules in a liquid have varying kinetic energies. Some high energy particles on the liquid surface may have enough energy to escape the intermolecular forces of attraction of the liquid and become a gas. This is called evaporation. Evaporation only happens on the surface while boiling happens throughout the liquid. When a liquid reaches its boiling point bubbles of gas form in it which rise into the surface and burst into the air. This process is called boiling. If the boiling liquid is heated more strongly the temperature does not rise but the liquid boils more quickly. This distinction is exclusive to the liquid-to-gas transition; any transition directly from solid to gas is always referred to as
sublimation Sublimation or sublimate may refer to: * ''Sublimation'' (album), by Canvas Solaris, 2004 * Sublimation (phase transition), directly from the solid to the gas phase * Sublimation (psychology), a mature type of defense mechanism * Sublimate of mer ...
regardless of whether it is at its boiling point or not.


See also

*
Phase diagram A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions (pressure, temperature, volume, etc.) at which thermodynamically distinct phases (such as solid, liquid or gaseous ...
* Phase transition * Explosive boiling or phase explosion *
Recovery time (culinary) Recovery time is the length of time it takes a cooking medium, such as fat or water, to return to the desired cooking temperature after the food is submerged in it. The term also pertains to the recovery time for ovens to return to their preset coo ...
* Enthalpy of vaporization


References

{{authority control Cooking techniques Phase transitions Heat transfer Gases