boiling point

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boiling point

boiling 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. When heat is applied to a liquid, the temperature of the liquid rises until the vapor pressure of the liquid equals the pressure of the surrounding gases. At this point there is no further rise in temperature, and the additional heat energy supplied is absorbed as latent heat of vaporization to transform the liquid into gas. This transformation occurs not only at the surface of the liquid (as in the case of evaporation) but also throughout the volume of the liquid, where bubbles of gas are formed. The boiling point of a liquid is lowered if the pressure of the surrounding gases is decreased. For example, water will boil at a lower temperature at the top of a mountain, where the atmospheric pressure on the water is less, than it will at sea level, where the pressure is greater. In the laboratory, liquids can be made to boil at temperatures far below their normal boiling points by heating them in vacuum flasks under greatly reduced pressure. On the other hand, if the pressure is increased, the boiling point is raised. For this reason, it is customary when the boiling point of a substance is given to include the pressure at which it is observed, if that pressure is other than standard, i.e., 760 mm of mercury or 1 atmosphere (see STP). The boiling point of a solution is always higher than that of the pure solvent; this boiling-point elevation is one of the colligative properties common to all solutions.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Boiling Point

 

the temperature of the equilibrium transition of a liquid to vapor at constant external pressure. The abbreviations used for the term “boiling point” include bp and Tb.

Table 1. Boiling points of several substances
SubstanceBciling point (°C)
Hydrogen ...............–252.87
Nitrogen ...............–195.8
Argon ...............–185.7
Oxygen ...............–182.9
Acetone ...............56.5
Methyl alcohol ...............64.7
Ethyl alcohol ...............78.4
Nitric acid ...............83.3
Iodine ...............183.0
Glycerol ...............290.0
Sulfuric acid ...............330.0
Aluminum ...............2467
Copper ...............2567
Iron ...............2750
Osmium ...............5027 ± 100
Tantalum ...............5425 ± 100

At the boiling point, the saturated vapor pressure over the plane surface of the liquid becomes equal to the external pressure. As a result, bubbles of saturated vapor are formed throughout the volume of the liquid (see). The boiling point is a special case of a transition temperature for a first-order transition. Table 1 gives the boiling points of a number of substances at normal external pressure—that is, at a pressure of 760 mm Hg, or 101,325 newtons/m2.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

boiling point

[′bȯil·iŋ ‚pȯint]
(physical chemistry)
Abbreviated bp.
The temperature at which the transition from the liquid to the gaseous phase occurs in a pure substance at fixed pressure.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

boiling point

the temperature at which a liquid boils at a given pressure, usually atmospheric pressure at sea level; the temperature at which the vapour pressure of a liquid equals the external pressure
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005