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vaporization,change of a liquid or solid substance to a gas or vapor. There is fundamentally no difference between the terms gas and vapor, but gas is used commonly to describe a substance that appears in the gaseous state under standard conditions of pressure and temperature, and vapor to describe the gaseous state of a substance that appears ordinarily as a liquid or solid. Although most substances undergo changes of state in the order of solid to liquid to gas as the temperature is raised, a few change directly from solid to gas in a process known as sublimationsublimation
, change of a solid substance directly to a vapor without first passing through the liquid state. The term is also used to describe the reverse process of the gas changing directly to the solid again upon cooling.
..... Click the link for more information. .
The Boiling Point and Latent Heat of Vaporization
When heat is added to a liquid at its boiling pointboiling point,
temperature at which a substance changes its state from liquid to gas. A stricter definition of boiling point is the temperature at which the liquid and vapor (gas) phases of a substance can exist in equilibrium.
..... Click the link for more information. , with the pressure kept constant, the molecules of the liquid acquire enough energy to overcome the intermolecular forcesintermolecular forces,
forces that are exerted by molecules on each other and that, in general, affect the macroscopic properties of the material of which the molecules are a part. Such forces may be either attractive or repulsive in nature.
..... Click the link for more information. 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 heatlatent heat,
heat change associated with a change of state or phase (see states of matter). Latent heat, also called heat of transformation, is the heat given up or absorbed by a unit mass of a substance as it changes from a solid to a liquid, from a liquid to a gas, or the
..... Click the link for more information. 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.
Evaporation and Vapor Pressure
Liquids can also change to gases at temperatures below their boiling points. Vaporization of a liquid below its boiling point is called evaporationevaporation,
change of a liquid into vapor at any temperature below its boiling point. For example, water, when placed in a shallow open container exposed to air, gradually disappears, evaporating at a rate that depends on the amount of surface exposed, the humidity of the air,
..... Click the link for more information. , 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).
the passage of a substance from the condensed phase (liquid or solid) to the gas phase (first-order phase transition). Vaporization includes evaporation and boiling. Evaporation is vaporization at the free surface of the condensed phase and includes sublimation in the case of solids. Boiling is vaporization characterized by bubbles of saturated vapor that appear on the heated surface and grow within the fluid.