When heat is added to a liquid at its boiling point, with the pressure kept constant, the molecules of the liquid acquire enough energy to overcome the intermolecular forces that bind them together in the liquid state, and they escape as individual molecules of vapor until the vaporization is complete. Vaporization at the boiling point is known simply as boiling. The temperature of a boiling liquid remains constant until all of the liquid has been converted to a gas.
For each substance a certain specific amount of heat must be supplied to vaporize a given quantity of the substance. This amount of heat is known as the latent heat of vaporization of the substance. The quantity of heat applied for each gram (or each molecule) undergoing the change in state depends on the substance itself. For example, the amount of heat necessary to change one gram of water to steam at its boiling point at one atmosphere of pressure, i.e., the heat of vaporization of water, is approximately 540 calories. Other substances require other amounts.
Liquids can also change to gases at temperatures below their boiling points. Vaporization of a liquid below its boiling point is called evaporation, which occurs at any temperature when the surface of a liquid is exposed in an unconfined space. When, however, the surface is exposed in a confined space and the liquid is in excess of that needed to saturate the space with vapor, an equilibrium is quickly reached between the number of molecules of the substance going off from the surface and those returning to it. A change in temperature upsets this equilibrium; a rise in temperature, for example, increases the activity of the molecules at the surface and consequently increases the rate at which they fly off. When the temperature is maintained at the new point for a short time, a new equilibrium is soon established.
The pressure exerted by the vapor of a liquid in a confined space is called its vapor pressure. It differs for different substances at any given temperature, but each substance has a specific vapor pressure for each given temperature. At its boiling point the vapor pressure of a liquid is equal to atmospheric pressure. For example, the vapor pressure of water, measured in terms of the height of mercury in a barometer, is 4.58 mm at 0°C; and 760 mm at 100°C; (its boiling point).
Vaporization of an element or compound is a phase transition from the liquid phase to gas phase. There are two sorts of vaporization: evaporation and boiling. Evaporation is a phase transition from the liquid phase to gas phase that occurs at temperatures below the boiling temperature at a given pressure.
Boiling is a phase transition from the liquid phase to gas phase that occurs at or above the temperature the boiling temperature.
The term vaporization has also been used to refer to the physical destruction of an object upon exposure to great heat; this includes human bodies, as noted in discussions of the effects of nuclear attack, including the 1945 bombings of Hiroshima and Nagasaki and the 1952 "vaporization" of the Marshall Island of Elugelab by the Ivy Mike thermonuclear weapon test..