Boiling is the process by which a liquid turn into a vapor when it is heated to its boiling point. The change from a liquid phase to a gaseous phase occurs when the vapor pressure of the liquid is equal to the atmospheric pressure exerted on the liquid.
During the 1930s did we engineers begin to recognize the enormous potential of boiling for transferring heat under low temperature differences.
Molecules remain in the liquid phase until they gain sufficient kinetic energy (vibrational motion) to overcome the forces keeping them together. These forces include the attraction between molecules and the air pressure above the liquid. Adding heat to a liquid is an easy way to increase the kinetic energy of its particles.
At some particular temperature, the particles will have become energetic enough to disassociate themselves from their neighbors and become a vapor. This is called the boiling point.
All substances have specific boiling, melting, and freezing points. For example, the boiling point of pure water is 100°C.
At the boiling point, any heat added to the liquid is absorbed by the molecules and the liquid changes to the vapor phase. Bubbles of vaporized liquid (i.e., gas) form within the bulk liquid and then rise to the surface where they burst and release the gas. (At the boiling temperature the vapor inside a bubble has enough pressure to keep the bubble from collapsing.)
Once the water temperature reaches 100°C, it will begin to boil. This boiling point does not change whether in a large pot of water or a small droplet of water. Boiling, melting, and freezing points are constant, meaning they do not change unless the substance itself is changed.
Boiling water
