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سطر 17:
(Cf. DeVoe, Howard, Thermodynamics and Chemistry. Prentice-Hall, 2001)
==Saturation temperature and pressure==
[[File:11. Температурата и вриењето на течност.ogv|thumb|right|280px|Demonstration of the lower boiling point of water at lower pressure, achieved by using a [[vacuum pump]].]]
{{Main|Vapor–liquid equilibrium}}
A ''saturated liquid'' contains as much thermal energy as it can without boiling (or conversely a ''saturated vapor'' contains as little thermal energy as it can without [[Condensation|condensing]]).
'''Saturation temperature''' means ''boiling point''. The saturation temperature is the temperature for a corresponding saturation pressure at which a liquid boils into its [[Gas|vapor phase]]. The liquid can be said to be saturated with [[thermal energy]]. Any addition of thermal energy results in a [[phase transition]].
If the pressure in a system remains constant ([[Isobaric process|isobaric]]), a vapor at saturation temperature will begin to condense into its liquid phase as thermal energy ([[heat]]) is removed. Similarly, a liquid at saturation temperature 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 the surrounding environmental pressure. Thus, the boiling point is dependent on the pressure. Boiling points may be published with respect to the [[National Institute of Standards and Technology|NIST, USA]] [[Standard conditions for temperature and pressure|standard pressure]] of 101.325 [[kilopascal|kPa]] (or 1 [[atmospheric pressure|atm]]), or the [[International Union of Pure and Applied Chemistry|IUPAC]] standard pressure of 100.000 kPa. At higher elevations, where the atmospheric pressure is much lower, the boiling point is also lower. The boiling point increases with increased pressure up to the [[critical point (thermodynamics)|critical point]], where the gas and liquid properties 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 point cannot be reduced below the triple point.
If the heat of vaporization and the vapor pressure of a liquid at a certain temperature are known, the boiling point can be calculated by using the [[Clausius–Clapeyron equation]], thus:
:<math>T_\text{B} = \Bigg(\frac{1}{T_0} - \frac{R\,\ln \frac{P}{P_0}}{\Delta H_\text{vap}}\Bigg)^{-1},</math>
where:
:<math>T_B</math> is the boiling point at the pressure of interest,
:<math>R</math> is the [[ideal gas constant]],
:<math>P</math> is the vapour pressure of the liquid at the pressure of interest,
:<math>P_0</math> is some pressure where the corresponding <math>T_0</math> is known (usually data available at 1 atm or 100 kPa),
:<math>\Delta H_\text{vap}</math> is the [[heat of vaporization]] of the liquid,
:<math>T_0</math> is the boiling temperature,
:<math>\ln</math> is the [[natural logarithm]].
'''Saturation pressure''' is the pressure for a corresponding saturation temperature at which a liquid boils into its vapor phase. Saturation pressure and saturation temperature have a direct relationship: as saturation pressure is increased, so is saturation temperature.
If the temperature in a [[system]] remains constant (an ''[[isothermal]]'' system), vapor at saturation pressure and temperature will begin to [[condensation|condense]] into its liquid phase as the system pressure is increased. Similarly, a liquid at saturation pressure and temperature will tend to [[Flash evaporation|flash]] into its vapor phase as system pressure is decreased.
There are two conventions regarding the ''standard boiling point of water'': The ''normal boiling point'' is {{convert|99.97|°C|°F|1|lk=on}} at a pressure of 1 atm (i.e., 101.325 kPa). The IUPAC recommended ''standard boiling point of water'' at a standard pressure of 100 kPa (1 bar)<ref>[http://goldbook.iupac.org/S05921.html Standard Pressure] IUPAC defines the "standard pressure" as being 10<sup>5</sup> Pa (which amounts to 1 bar).</ref> is {{convert|99.61|°C|°F|1|lk=on}}.<ref name=iupac/><ref>[http://highered.mcgraw-hill.com/sites/dl/free/0073529214/395307/appdxs1_2.pdf Appendix 1: Property Tables and Charts (SI Units)], Scroll down to Table A-5 and read the temperature value of 99.61 °C at a pressure of 100 kPa (1 bar). Obtained from McGraw-Hill's Higher Education website.</ref> For comparison, on top of [[Mount Everest]], at {{convert|8848|m|ft|abbr=on}} elevation, the pressure is about {{convert|34|kPa|torr|0|lk=on|abbr=on}}<ref>{{cite journal |author=West, J. B. |date=1999 |title=Barometric pressures on Mt. Everest: New data and physiological significance |journal=Journal of Applied Physiology |volume=86 |issue=3 |pages=1062–6 |pmid=10066724}}</ref> and the boiling point of water is {{convert|71|°C|°F|0|lk=on}}.
The Celsius temperature scale was defined until 1954 by two points: 0 °C being defined by the water freezing point and 100 °C being defined by the water boiling point at standard atmospheric pressure.
== شاهد أيضا ==
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