Boiling

Boiling or ebullition is the rapid phase transition from liquid to gas or vapour; the reverse of boiling is condensation. Boiling occurs when a liquid is heated to its boiling point, so that the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Boiling and evaporation are the two main forms of liquid vapourization.

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 vapour 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 100 °C (212 °F) 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 methods including boiling, blanching, steaming, and poaching.

The lowest heat flux seen in boiling is only sufficient to cause natural convection, where the warmer fluid rises due to its slightly lower 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 boiling is characterised 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 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 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.