Boiling

Types

At very low heat flux, only natural convection occurs, with the hot surface nearly at the boiling point. Nucleate boiling involves bubble growth at discrete nucleation sites on a heated surface, where the surface temperature is slightly above the liquid's temperature. Increasing surface temperature raises the number of nucleation sites. Surface roughness or additives like surfactants or nanoparticles broaden the temperature range for nucleate boiling, while a very smooth surface can cause superheating and boiling delay.

Homogeneous nucleation, where bubbles form within the liquid rather than on a surface, can happen when the liquid's center is warmer than the container walls, as in microwave heating. Critical heat flux (CHF) marks the thermal limit where a vapour film forms on the heating surface, drastically reducing heat transfer and causing rapid temperature rise into transition boiling. Transition boiling is an unstable intermediate regime between nucleate and film boiling.

Bubble formation involves complex processes including cavitation and acoustic effects, such as the hiss from a kettle before boiling.

Influence of geometry

Pool boiling occurs without forced convective flow, driven only by density gradients, and can exhibit any boiling regime. Flow boiling involves circulating fluid, often through pipes, moved by pumps or density gradients as in thermosiphons or heat pipes.

It is characterized by void fraction and vapor quality, and can be complex due to influences of density, flow rates, heat flux, and surface tension, sometimes yielding very high heat transfer coefficients. Confined boiling, relevant when the Bond number is below 0.5, is dominated by vapour stem bubbles left after vapour departure. It typically offers higher heat transfer coefficients but lower CHF than pool boiling, and is promising for electronics cooling.

Uses

Boiling water is an effective disinfection method, as bringing it to 100 °C kills most microbes responsible for intestinal diseases, without affecting taste or requiring removal of particles. At 70 °C, Giardia species require ten minutes for inactivation, while most intestinal microbes and E. coli take less than a minute; at boiling point, Vibrio cholerae is inactivated in ten seconds and hepatitis A virus in one minute. Bacterial spores like Clostridium can survive 100 °C but are not water-borne or intestinal threats.

The traditional advice to boil water for ten minutes provides extra safety, as disinfection begins above 60 °C and the visible boiling point serves as a useful indicator. Boiling is also used in cooking methods such as boiling, parboiling, blanching, steaming, and poaching. Many refrigeration and air-conditioning systems work by compressing a gas into liquid and then allowing it to boil, absorbing heat; common refrigerants include propane, ammonia, carbon dioxide, or nitrogen.

Contrast with evaporation

Evaporation occurs only at the liquid surface, where high-energy molecules escape intermolecular forces, while boiling happens throughout the liquid with bubbles forming and rising to the surface. When a liquid reaches its boiling point, further heating increases the activity of boiling but does not raise the temperature. The solid-to-gas transition is always called sublimation, regardless of boiling point.

Did You Know?

• The boiling point of an element or simple compound at a given pressure is a characteristic attribute, which led to the adoption of boiling points as the definition of 100 °C.
• Mixtures of volatile liquids can form constant boiling mixtures with a fixed vapour composition, allowing separation of components such as ethanol from water.
• Once boiling starts and remains stable at constant pressure, the temperature of the boiling liquid stays constant.