Boiling point
Boiling point
Boiling point
Saturation temperature and pressure
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 condensing).
Saturation temperature means boiling point. The saturation temperature is the temperature for a corresponding saturation pressure at which a liquid boils into its 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), 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. 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. The boiling point increases with increased pressure up to the 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 is known, the normal boiling point can be calculated by using the Clausius-Clapeyron equation thus:
where:
TB
= the normal boiling point, K
R
= the ideal gas constant, 8.314 J K-1 mol-1
P0
= is the vapor pressure at a given temperature, atm
Hvap
= the heat of vaporization of the liquid, J/mol
T0
= the given temperature, K
ln
= the natural logarithm to the base e
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 condense into its liquid phase as the system pressure is increased. Similarly, a liquid at saturation pressure and temperature will tend to flash into its vapor phase as system pressure is decreased.
The boiling point of water is 100C (212F) at standard pressure. On top of Mount Everest the pressure is about 260 mbar (26.39 kPa) so the boiling point of water is 69C. (156.2F).
For purists, the normal boiling point of water is 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 now recommends a standard pressure of 1 bar (100 kPa). At this slightly reduced pressure, the standard boiling point of water is 99.61 degrees Celsius.
Relation between the normal boiling point and the vapor pressure of liquids
A typical vapor pressure chart for various liquids
The higher the vapor pressure of a liquid at a given temperature, the lower the normal boiling point (i.e., the boiling point at atmospheric pressure) of the liquid.
The vapor pressure chart to the right has graphs of the vapor pressures versus temperatures for a variety of liquids. As can be seen in the chart, the liquids with the highest vapor pressures have the lowest normal boiling points.
For example, at any given temperature, propane has the highest vapor pressure of any of the liquids in the chart. It also has the lowest normal boiling point(-42.1 C), which is where the vapor pressure curve of propane (the purple line) intersects the horizontal pressure line of one atmosphere (atm) of absolute vapor pressure.
In terms of intermolecular interactions, the boiling point represents the point at which the liquid molecules possess enough thermal energy to overcome the various intermolecular attractions binding the molecules as liquid (e.g. dipole-dipole attraction, instantaneous-dipole induced-dipole attractions, and hydrogen bonds) and therefore incur a phase change into the next phase (gas). Therefore the boiling point of a liquid is also an indicator of the strength of weak attractive forces between the liquid's molecules.
Properties of the elements
Further information: List of elements by boiling point
The element with the lowest boiling point is helium. Both the boiling points of rhenium and tungsten exceed 5000 K at standard pressure. Due to the experimental difficulty of precisely measuring extreme temperatures without bias, there is some discrepancy in the literature as to whether tungsten or rhenium has the higher boiling point.
See also
Superheating
Boiling-point elevation
Critical temperature
Joback method (Estimation of normal boiling points from molecular structure)
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
^ Louis Theodore, R. Ryan Dupont and Kumar Ganesan (Editors) (1999). Pollution Prevention: The Waste Management Approach to the 21st Century. CRC Press. ISBN 1-56670-495-2. Section 27, page 15
^ General Chemistry Glossary Purdue University website page
^ 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
^ Notation for States and Processes, Significance of the Word Standard in Chemical Thermodynamics, and Remarks on Commonly Tabulated Forms of Thermodynamic Functions See page 1274
^ Perry, R.H. and Green, D.W. (Editors) (1997). Perry's Chemical Engineers' Handbook (7th ed.). McGraw-Hill. ISBN 0-07-049841-5.
^ Howard DeVoe (2000). Thermodynamics and Chemistry (1st ed.). Prentice-Hall. ISBN 0-02-328741-1.
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States of matter
Solid
Liquid
Gas
Plasma
Low-temperature
Boseinstein condensate Fermionic condensate Superfluid Supersolid
High-energy
Degenerate matter Quarkluon plasma Strange matter Supercritical fluid
Other
Colloid Superconductivity Supercooling Superglass Superheating
Concepts
Boiling point Cooling curve Critical point Equation of state Melting point Phase transition Triple point
Lists
List of states of matter
Categories: Thermodynamics | Fundamental physics concepts | Temperature
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