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Boiling point

From Wikipedia, the free encyclopedia

The boiling pointof an element or a substance is the temperature at which the vapor pressure of the

liquid equals the environmental pressure surrounding the liquid.[1][2]

A liquid in a vacuum environment has a lower boiling point than when the liquid is at atmosphericpressure. A liquid in a high pressure environment has a higher boiling point than when the liquid is atatmospheric pressure. In other words, the boiling point of liquids varies with and depends upon thesurrounding environmental pressure (which tends to vary with elevation). Different liquids (at a givenpressure) boil at different temperatures.

The normal boiling point(also called the atmospheric boiling pointor the atmospheric pressureboiling point) of a liquid is the special case in which the vapor pressure of the liquid equals the defined

atmospheric pressure at sea level, 1 atmosphere.[3][4]At that temperature, the vapor pressure of the liquid

becomes sufficient to overcome atmospheric pressure and lift the liquid to form bubbles inside the bulkof the liquid. The standard boiling pointis now (as of 1982) defined by IUPAC as the temperature at

which boiling occurs under a pressure of 1 bar.[5]

The heat of vaporization is the amount of energy required to convert or vaporize a saturated liquid (i.e.,a liquid at its boiling point) into a vapor.

Liquids may change to a vapor at temperatures below their boiling points through the process ofevaporation. Evaporation is a surface phenomenon in which molecules located near the vapor/liquidsurface escape into the vapor phase. On the other hand, boiling is a process in which moleculesanywhere in the liquid escape, resulting in the formation of vapor bubbles within the liquid.

Contents

1 Saturation temperature and pressure2 Relation between the normal boiling point and the vapor pressure of liquids3 Properties of the elements4 See also5 External links6 References

Saturation temperature and pressure

A saturated liquidcontains as much thermal energy as it can without boiling (or conversely a saturatedvaporcontains as little thermal energy as it can without condensing).

Saturation temperaturemeans boiling point. The saturation temperature is the temperature for acorresponding saturation pressure at which a liquid boils into its vapor phase. The liquid can be said tobe saturated with thermal energy. Any addition of thermal energy results in a phase transition.

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If the pressure in a system remains constant (isobaric), a vapor at saturation temperature will begin tocondense into its liquid phase as thermal energy (heat) is removed. Similarly, a liquid at saturationtemperature and pressure will boil into its vapor phase as additional thermal energy is applied.

The boiling point corresponds to the temperature at which the vapor pressure of the liquid equals thesurrounding environmental pressure. Thus, the boiling point is dependent on the pressure. Usually,boiling points are published with respect to atmospheric pressure (101.325 kilopascals or 1 atm). At

higher elevations, where the atmospheric pressure is much lower, the boiling point is also lower. Theboiling point increases with increased pressure up to the critical point, where the gas and liquidproperties become identical. The boiling point cannot be increased beyond the critical point. Likewise,the boiling point decreases with decreasing pressure until the triple point is reached. The boiling pointcannot be reduced below the triple point.

If the heat of vaporization and the vapor pressure of a liquid at a certain temperature is known, thenormal boiling point can be calculated by using the Clausius-Clapeyron equation thus:

where:

T = the normal boiling point, KR = the ideal gas constant, 8.314 J K-1 mol-1P = is the vapor pressure at a given temperature, atm

vap = the heat of vaporization of the liquid, J/molT = the given temperature, Kln = the natural logarithm to the base e

Saturation pressureis the pressure for a corresponding saturation temperature at which a liquid boilsinto its vapor phase. Saturation pressure and saturation temperature have a direct relationship: assaturation pressure is increased so is saturation temperature.

If the temperature in a system remains constant (an isothermal system), vapor at saturation pressure andtemperature will begin to condense into its liquid phase as the system pressure is increased. Similarly, aliquid at saturation pressure and temperature will tend to flash into its vapor phase as system pressure is

decreased.

The boiling point of water is 100 C (212 F) at standard pressure. On top of Mount Everest, at 8,848 melevation, the pressure is about 260 mbar (26.39 kPa) and the boiling point of water is 69 C. (156.2 F).The boiling point decreases 1C every 285 m of elevation.

For purists, the normal boiling point of wateris 99.97 degrees Celsius at a pressure of 1 atm (i.e.,101.325 kPa). Until 1982 this was also the standard boiling point of water, but the IUPAC nowrecommends a standard pressure of 1 bar (100 kPa). At this slightly reduced pressure, the standardboiling point of wateris 99.61 degrees Celsius.


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Trouton's constant

External links

References

^David.E. Goldberg (1988). 3,000 Solved Problems in Chemistry(1st ed.). McGraw-Hill. ISBN 0-07-023684-4. Section 17.43, page 321

1.

^Louis Theodore, R. Ryan Dupont and Kumar Ganesan (Editors) (1999). Pollution Prevention: The WasteManagement Approach to the 21st Century. CRC Press. ISBN 1-56670-495-2. Section 27, page 15

2.

^General Chemistry Glossary (http://www.chem.purdue.edu/gchelp/gloss/normalbp.html) Purdue Universitywebsite page

3.

^Kevin R. Reel, R. M. Fikar, P. E. Dumas, Jay M. Templin, and Patricia Van Arnum (2006).AP Chemistry(REA) - The Best Test Prep for the Advanced Placement Exam(9th ed.). Research & Education Association.ISBN 0-7386-0221-3. Section 71, page 224

4.

^Notation for States and Processes, Significance of the Word Standard in Chemical Thermodynamics, andRemarks on Commonly Tabulated Forms of Thermodynamic Functions(http://www.iupac.org/publications/pac/1982/pdf/5406x1239.pdf) See page 1274

5.

^Perry, R.H. and Green, D.W. (Editors) (1997). Perry's Chemical Engineers' Handbook(7th ed.). McGraw-Hill. ISBN 0-07-049841-5.6.

^Howard DeVoe (2000). Thermodynamics and Chemistry (1st ed.). Prentice-Hall. ISBN 0-02-328741-1.7.

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