Boiling

Boiling is a rapid vaporization of the liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is make up to the pressure exerted on the liquid by the surrounding atmosphere. There are two main mark of boiling: nucleate boiling where small bubbles of vapour clear at discrete points, together with critical heat flux boiling where the boiling surface is heated above acritical temperature and a film of vapor forms on the surface. Transition boiling is an intermediate, unstable earn of boiling with elements of both types. The boiling bit 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 devloping it potable by killing microbes and viruses that may be present. The sensitivity of different micro-organisms to heat varies, but whether water is held at 100 °C 212 °F for one minute, near micro-organisms and viruses are inactivated. Ten minutes at a temperature of 70 °C 158 °F is also sufficient for near bacteria.

Boiling water is also used in several cooking methods including boiling, steaming, and poaching.

Types

Nucleate boiling is characterized by the growth of bubbles or pops on a heated surface, 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 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. Under these conditions, a heated liquid may show boiling delay and the temperature may go somewhat above the boiling member without boiling.

Critical heat flux CHF describes the thermal limit of a phenomenon where a phase change occurs during heating such(a) 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 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 appearance 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 non yet heated to the point where bubbles boil to the surface.

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 precondition of a vapor film insulating the surface from the liquid characterizes film boiling.