boiling point

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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 pressurevapor pressure,
pressure exerted by a vapor that is in equilibrium with its liquid. A liquid standing in a sealed beaker is actually a dynamic system: some molecules of the liquid are evaporating to form vapor and some molecules of vapor are condensing to form liquid.
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 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 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
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 of vaporization to transform the liquid into gas. This transformation occurs not only at the surface of the liquid (as in the case of 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,
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) 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 solutionsolution,
in chemistry, homogeneous mixture of two or more substances. The dissolving medium is called the solvent, and the dissolved material is called the solute. A solution is distinct from a colloid or a suspension.
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 is always higher than that of the pure solvent; this boiling-point elevation is one of the colligative propertiescolligative properties,
properties of a solution that depend on the number of solute particles present but not on the chemical properties of the solute. Colligative properties of a solution include freezing point (see freezing), boiling point, osmotic pressure (see osmosis), and
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 common to all solutions.

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.

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.

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
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