boiling point

(Abbr. bp) The temperature at which a liquid boils at a fixed pressure, especially under standard atmospheric conditions.
Informal.
1. The point at which one loses one's temper.
2. The point of crisis; the turning point.

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boiling point

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Temperature at which a liquid is converted to vapour when heated. At the boiling point, addition of heat results in the transformation of the liquid into its vapour without an increase in temperature. A liquid's boiling point varies according to the liquid's characteristics and the applied pressure. Water at standard atmospheric pressure, or sea level, boils at 212 °F (100 °C), while ethanol boils at about 172 °F (78 °C). At higher altitudes, boiling points are lower and foods can take longer to cook; pressure cookers can be used to increase the pressure so that the boiling point is raised.

For more information on boiling point, visit Britannica.com.

McGraw-Hill Science & Technology Encyclopedia:

Boiling point

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The boiling point of a liquid is the temperature at which the liquid and vapor phases are in equilibrium with each other at a specified pressure. Therefore, the boiling point is the temperature at which the vapor pressure of the liquid is equal to the applied pressure on the liquid. The boiling point at a pressure of 1 atmosphere is called the normal boiling point.

For a pure substance at a particular pressure P, the stable phase is the vapor phase at temperatures immediately above the boiling point and is the liquid phase at temperatures immediately below the boiling point. The liquid-vapor equilibrium line on the phase diagram of a pure substance gives the boiling point as a function of pressure. Alternatively, this line gives the vapor pressure of the liquid as a function of temperature. The vapor pressure of water is 1 atm (101.325 kilopascals) at 100°C (212°F), the normal boiling point of water. The vapor pressure of water is 3.2 kPa (0.031 atm) at 25°C (77°F), so the boiling point of water at 3.2 kPa is 25°C. The liquid-vapor equilibrium line on the phase diagram of a pure substance begins at the triple point (where solid, liquid, and vapor coexist in equilibrium) and ends at the critical point, where the densities of the liquid and vapor phases have become equal. For pressures below the triple-point pressure or above the critical-point pressure, the boiling point is meaningless. Carbon dioxide has a triple-point pressure of 5.11 atm (518 kPa), so carbon dioxide has no normal boiling point. See also Triple point; Vapor pressure.

The normal boiling point is high for liquids with strong intermolecular attractions and low for liquids with weak intermolecular attractions. Helium has the lowest normal boiling point, 4.2 K (−268.9°C). Some other normal boiling points are 111.1 K (−162°C) for CH₄, 450°C (842°F) for n-C₃₀H₆₂, 1465°C (2669°F) for NaCl, and 5555°C (10031°F) for tungsten.

The rate of change of the boiling-point absolute temperature T_b of a pure substance with pressure is given by the equation below. ΔH_vap,m is the molar enthalpy (heat) of vaporization, and ΔV_vap,m is the molar volume change on vaporization. $\frac{dT_b}{dP} = \frac{T_b\Delta V_{\rm vap, m}}{\Delta H_{\rm vap, m}}$

The quantity ΔH_vap,m/T_b is ΔS_vap,m, the molar entropy of vaporization. The molar entropy of vaporization at the normal boiling point (nbp) is given approximately by Trouton's rule: ΔS_vap,m,nbp ≈ 87 J/mol K (21 cal/mol K). Trouton's rule fails for highly polar liquids (especially hydrogen-bonded liquids). It also fails for liquids boiling at very low or very high temperatures, because the molar volume of the vapor changes with temperature and the entropy of a gas depends on its volume.

When a pure liquid is boiled at fixed pressure, the temperature remains constant until all the liquid has vaporized. When a solution is boiled at fixed pressure, the composition of the vapor usually differs from that of the liquid, and the change in liquid composition during boiling changes the boiling point. Thus the boiling process occurs over a range of temperatures for a solution. An exception is an azeotrope, which is a solution that boils entirely at a constant temperature because the vapor in equilibrium with the solution has the same composition as the solution. In fractional distillation, the variation of boiling point with composition is used to separate liquid mixtures into their components. See also Azeotropic mixture; Distillation; Phase equilibrium.

American Heritage Dictionary of Idioms:

boiling point

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A climax or crisis; a high degree of fury, excitement, or outrage. For example, The union's disgust with management has reached the boiling point. This metaphoric term alludes to the temperature at which water boils. [Second half of 1700s]

Columbia Encyclopedia:

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 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.

Dictionary of Cultural Literacy: Science:

boiling point

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The temperature at which a given material changes from a liquid to a gas. The boiling point is the same temperature as the condensation point. (See phases of matter.)

Water boils at 212 degrees Fahrenheit or 100 degrees Celsius.

Wiley Dictionary of Flavors:

Boiling Point

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The temperature at which a substance's state changes from a liquid to a gas. See Boiling Point Elevation, Distillation, Condensation, Evaporation, Freezing Point Depression.

Saunders Veterinary Dictionary:

bp

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Base pair.

Mosby's Dental Dictionary:

boiling point

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The temperature at which the vapor pressure within a liquid equals atmospheric pressure.

Random House Word Menu:

categories related to 'boiling point'

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Random House Word Menu by Stephen Glazier

For a list of words related to boiling point, see:

Constants, Theories, and Values - boiling point: temperature at which the vapor pressure of a liquid equals the pressure of a gas above the liquid

Wikipedia on Answers.com:

Boiling point

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This article is about the boiling point of liquids. For other uses, see Boiling point (disambiguation).

The boiling point of 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 has a lower boiling point than when the liquid is at atmospheric pressure. A liquid at high-pressure has a higher boiling point than when the liquid is at atmospheric pressure. In other words, the boiling point of a liquid varies depending upon the surrounding environmental pressure (which tends to vary with elevation). Different liquids (at a given pressure) boil at different temperatures.

The normal boiling point (also called the atmospheric boiling point or the atmospheric pressure boiling 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 allow bubbles of vapor to form inside the bulk of the liquid. The standard boiling point is 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 of evaporation. Evaporation is a surface phenomenon in which molecules located near the liquid's edge, not contained by enough liquid pressure on that side, escape into the surroundings as vapor. On the other hand, boiling is a process in which molecules anywhere in the liquid escape, resulting in the formation of vapor bubbles within the liquid.

Contents

1 Saturation temperature and pressure
2 Relation between the normal boiling point and the vapor pressure of liquids
3 Properties of the elements
4 See also
5 References

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:

$T_B = \Bigg(\frac{\,R\,\ln(P_0)}{\Delta H_{vap}}+\frac{1}{T_0}\Bigg)^{-1}$

where:
$T B$	= the normal boiling point, K
$R$	= the ideal gas constant, 8.314 J · K⁻¹ · mol⁻¹
$P 0$	= is the vapor pressure at a given temperature, atm
$Δ H v a p$	= the heat of vaporization of the liquid, J/mol
$T 0$	= 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 100 °C (212 °F) at standard pressure. On top of Mount Everest, at 8,848 m (29,029 ft) elevation, the pressure is about 260 mbar (26.000 kPa) and the boiling point of water is 69 °C (156.2 °F). The boiling point decreases 1 °C every 285 m of elevation, or 1 °F every 500 ft.

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.^{[citation needed]}

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.^[6] 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.

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; because it is difficult to measure extreme temperatures precisely without bias, both have been cited in the literature as having the higher boiling point.^[7]

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.

Gas / Vapor

Low energy

Bose–Einstein condensate · Fermionic condensate · Quantum Hall · Rydberg matter · Strange matter · Superfluid · Supersolid

High energy

Degenerate matter · QCD matter · Quark–gluon plasma · Supercritical fluid

Other states

Colloid · Glass · Liquid crystal · Magnetically ordered (Antiferromagnet, Ferrimagnet, Ferromagnet) · String-net liquid · Superglass

Transitions

Boiling · Boiling point · Condensation · Critical line · Critical point · Crystallization · Deionization · Deposition · Evaporation · Flash evaporation · Freezing · Ionization · Lambda point · Melting · Melting point · Regelation · Saturated fluid · Sublimation · Supercooling · Triple point · Vaporization

Quantities

Enthalpy of fusion · Enthalpy of sublimation · Enthalpy of vaporization · Latent heat · Latent internal energy · Trouton's ratio · Volatility

Concepts

Binodal · Compressed fluid · Cooling curve · Equation of state · Leidenfrost effect · Mpemba effect · Order and disorder (physics) · Spinodal · Superconductivity · Superheated vapor · Superheating · Thermo-dielectric effect

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	b.p.
	bp (abbreviation)
	steam line (thermodynamics)