Boiling point

phase diagram one „usual “material and the water

the boiling point (abbreviation: Sdp) or also Kochpunkt (ABC: Kp) of a pure material is a pair in its phase diagram and consists of two sizes: The saturation temperature (particularly also boiling temperature) andthe Sättigungsdampfdruck (particularly also simmering pressure) at the phase boundary line between gas and liquid. It consists thus of the two variables of state pressure and temperature with the transition of a material of the liquid to the gaseous state of aggregation .

The boiling point places thoseConditions, which are present during the phase transition of a material of the liquid into the gaseous phase, which one calls simmering or evaporation. Besides it is identical for the reverse procedure of the condensation, however only with pure materials, to the point of condensation. With the evaporation of a material mixture it to a changed simmering behavior and one come observe boiling range instead of an individual boiling point. During a phase transition of the liquid into the gaseous phase below the boiling point, one speaks of an evaporation.

InThe boiling temperatures are indicated to tables for normal print, thus with 1013,25 hPa. This boiling point becomes as normal boiling point, which calls indicated boiling temperature normal boiling temperature (T simmer). The term boiling point is used thereby frequently as short form for the normal boiling temperatureand therefore generally linguistic usage represents usually their synonym , which would reduce however the point of simmering to only one pair and therefore is formally inkorrekt.
With a Schnellkochtopf one makes for example too uses oneself that the boiling temperature and thatSimmering pressure from each other detach. By an increase in pressure from usually bar (1000hPa) one reaches in this way an increase of the boiling temperature of the water of 100 °C on approximately 120 °C. Both temperatures represent boiling temperatures, however is only the value of100 °C also the boiling temperature under normal print and thus the normal boiling temperature. A mixture of both terms is nonspecific, by no means natural therefore and should be avoided.

Table of contents

simmering procedure

Temperaturänderung mit der Zeit beim Erwärmen eines flüssigen Reinstoffes
change of temperature with the time when warming up a liquid pure material

major item: Evaporate, heat of vaporization and Siedeverzug

below and above the boiling point lead a heating up of the liquid and/or. the gas only for an increase of the temperature. The supplied energy is converted into kinetic energy of the particles. During the phase transition of the liquid toGas however remains the temperature constantly, if also the pressure remains constant. All supplied thermal energy is invested into the change in status.

If the boiling point is reached, then with further supply from energy the chemico-physical reciprocal effects between the particles are solved -the particles cross into the gaseous phase. The temperature of the liquid stagnates, since the supplied thermal energy is used completely for the solution of the between-molecular connections. One designates the energy, which is needed for this with a mol of the material, alsoas evaporation enthalpy and their not amount of material-referred counterpart as heat of vaporization. Only if all particles are in the gaseous phase, the temperature of the system rises.

Water, hydrogen peroxide or caustic solutions (for example caustic soda solution) without dust particle or gas vesicle leave themselves inwarm up to pure containers also beyond the boiling temperature, without it comes to simmering. Smallest disturbances, as for example vibrations, which draw a mixing, can lead to a separation like an explosion of the liquid from the vapor phase, which one asSiedeverzug designates. Due to its one causes Siedesteinchen so mentioned from clay/tone or pumice stone to liquids, which are gefärdet of a Siedeverzug , in chemistry, which are not attacked by the chemical, but by its porous structure the education of small blistersfacilitate, so that it does not come to the Siedeverzug.

See also : Boiling point curve phase diagram of carbon dioxide all pairs of temperature pressures at the phase boundary line

gas liquid in

a phase diagram

together taken those

result in evaporation, gases , evaporation, transpiration , Pictet Trouton rule [work on] Boiling point curve, whereby on it a thermodynamic equilibrium prevails. One calls the boiling point curve here frequently also simmering curve, boiling-point line, simmering pressure curve or boiling point curve. This curve is limited thereby by two points:

  • Tripelpunkt: The pair of pressure temperature levels is lower thanthe Tripel temperature and/or the Tripel pressure, then is only a transition between solid and gaseous state, thus a sublimation and/or. Resublimation possible.
  • Critical point: If the pair of pressure temperature levels is higher than the critical temperature and/or the critical pressure, then exists betweenthe density of the liquid and that of the gaseous condition no more difference, why one does not separate it also more by a phase boundary line and the material therefore in this condition than überkritsches fluid designation.

The equilibrium of the boiling point curve is in dynamic equilibrium. From a liquid steadily particles step into the gaseous phase over it evaporate. On the other hand these particles step it also again into the liquid phase in condense. The numerical ratio of the particles withdrawing from the liquid phase andparticles occurring the again it is here both on the temperature, and on the pressure dependent: The higher the temperature is, the more particles evaporates due to its higher speed (see Maxwell Boltzmann distribution). The more particles evaporate, thethe steam pressure becomes in addition, higher, and the more particles condense also again. An equilibrium adjusts itself if exactly the same into the gaseous phase cross much particle, as withdraw again into the liquid phase. There in this conditionthe gaseous phase is satisfied, speaks one then also of the Sättigungsdampfdruck. One calls the thermodynamic regularity, from which the boiling point curve is derived quantitatively, Clausius Clapeyron equation. For water this connection between Sättigungsdampfdruck and saturation temperature leaves itself also over the approximation equationsdetermine of the type of the liking US formula.

change of equilibrium by the example of the water

Gleichgewichtsänderung am Beispiel des Wassers
change of equilibrium by the example of the water

starting point: Water is in the equilibrium with its gaseous phase with the boiling point Sdp (74 °C, 333 hPa):

H 2 O (liquid)< math> \; \ overrightarrow {\ leftarrow}< /math> H2 O (gaseous)

the reactions of the system to the changes of individual variables of state come down to a shift of the equilibrium position: It runs off that phase transition strengthened, which cancels the disturbance again (see principle of the smallest obligation).

  1. Becomes the systemon 53 °C cooled down, then the steam pressure of the gaseous phase is too high, and water vapour condenses so long, until the steam pressure exhibits the new equilibrium value of 143 hPa or no gaseous water is more remaining.
  2. Becomes the system on 95 °Cwarms up, then the steam pressure of the gaseous phase is too low and water evaporates so long, until the steam pressure exhibits the new equilibrium value of 845 hPa or no liquid water is more remaining.
  3. Becomes the pressure at constant temperature of 333 up560 hPa increases, then the steam pressure of the gaseous phase is too high, and gaseous water condenses so long, until the steam pressure exhibits the old equilibrium value of 333 hPa or no water vapour is more remaining.
  4. Becomes the pressure at constant temperature of333 to 65 hPa degrades, then the steam pressure of the gaseous phase is too low, and water evaporates so long, until the steam pressure exhibits the old equilibrium value of 333 hPa or no liquid water is more remaining.

dependent on materialness of theBoiling point

Siedepunkte einiger Wasserstoffverbindungen
of boiling points of some Wasserstoffverbindungen
  1. the boiling point is from the molecular mass and/or. Molecule mass of the material dependently. It applies: The larger the molecular mass is, the more highly is the boiling point. One compares for example the row HCl (36 g/mol) - HBr (81g/mol) - REAR ONE (128 g/mol) on the dark-blue line, then one can recognize this connection well. Explanation: The larger the mass of a particle is, the more kinetic energy needs it over into the gaseous phase to cross to be able.
  2. The boiling point is besides dependent on the strength of the binding forces between the smallest particles of the liquid phase: The stronger the binding forces are, the more highly are the boiling point, since these would have to be first overcome. This becomes clear, if one compares for example HF and HCl:In the liquid hydrogen fluoride the molecules train hydrogen bonds , while in liquid hydrogen chloride the weaker dipole dipole reciprocal effects prevail. Same applies to the comparatively very high boiling point of the water, which becomes clear, if one compares these with carbon dioxide and the influence of the mol masseswith considers.
The observation that materials have a higher boiling point than similar materials with higher molecular measures, is called anomaly of boiling point.
Than dipole dipole reciprocal effects van that Waals reciprocal effects are still weaker. For this reason all Wasserstoffverbindungen of the elements IV. have with the comparison. Main group the lowest boiling points.
The strength of the between-molecular binding forces depends also on geometry of the molecules. See in addition the boiling points of the homologous number of the hydrocarbons or the alcohols.

examples of normal boiling points of pure materials

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Chemical elements

  • the lowest normal boiling temperature of all elements also - °C helium has 269, although it has a larger molecular mass than hydrogen with normal boiling temperature of -253 °C. This lies in the fact justified to polarize that the hydrogen molecule somewhat more easilyis as helium and therefore also somewhat stronger van that Waals reciprocal effects trains.
  • The highest normal boiling temperature has tungsten with 5555 °C.
  • A group comparison of noble gases, nonmetals, metalloids and metals shows that metals have a clearly higher boiling point than nonmetals, there those Metallic binding (apart from the ion and atomic bond) the strongest connection represents. Exceptions:
Mercury has a normal boiling temperature carbon unusually low for metals with 357
°C exhibits a point of simmering of 4827, extremely high for nonmetals, °C.
Minimum maximum average graphic illustration
of noble gasescarbon mono oxide has -269
-62 -170.5 nonmetals of -253
4830 414.1 metalloids of 335
3900 1741.5 metals 357
5930 2755.9 [

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] connections one of the lowest normal boiling temperatures also - 191.6 °C, highest wise metal carbide such as titanium (IV) - carbide (TIC, 4820 °C) andTungsten (IV) - carbide (WC, 6000 °C on).

A characteristic is present with a modification of sulfur trioxide (SO 3): here the normal boiling temperature with 44,8 is lower °C than the normal fusing temperature with 62,3 °C.

The critical pressure is under the normal print,so no normal boiling temperature can be indicated. In order to bring the liquid nevertheless to simmering, this must happen under lower pressure. In this case also the simmering pressure must be indicated for the indication of the boiling temperature, what a further reason for it is,to separate the terms normal boiling temperature and boiling point strictly.

If the pressure of the Tripelpunktes lies over the normal print, then instead of the normal boiling temperature the normal sublimation temperature or a boiling temperature is indicated for higher simmering pressure. Example: Sulfur hexafluoride SF 6 sublimates 63 under normal print - °C.

Many, above all macromolecular, organic compounds decompose with heating up before reaching the boiling point. Their between-molecular connections are stronger than the connections within the molecule. Here one cannot indicate a boiling temperature, but only the decomposition temperature. Example: Sulfuric acid decomposes340 °C, before the simmering procedure begins.- Wrong! Sulfuric acid simmers with 310°C and completely without decomposition!

homogeneous multicomponent systems

the boiling points of homogeneous mixtures such as alloys, gas mixtures or aqueous solutions point changed boiling points and a changed opposite the pure materialsSimmering behavior up.

boiling point increase

is dissolved in a solvent a material, then the boiling point of the mixture in the comparison to the pure solvent , one increases speaks regarding the Sättigungsdampfdruck of the solution effect. This is simplified because of it,that the particles of the solved material the transition of the solvent particles into the gaseous phase obstruct. After the Raoult law of François Marie Raoult (1830 - 1901) is proportional this increase ΔT Sdp to the amount of material of the solved material:

Solvent ebullioskopische constant
in K · kg/mol
water 0.51
phenol 3.04
acetic acid 3.07
benzene 2.53
carbon disulfide 2.37
Kohlenstofftetrachlorid 4.95
Naphthalin 5.8
< math> \ delta T_ {Sdp} = K_e \ cdot b = K \ cdot n< /math>

Here the individual symbols stand for the following sizes:

  • ΔT Sdp - Siedepunktserhöhung
  • K e - ebullioskopische constant
  • of b - Molalität of the solved material
  • K - molecular Siedepunktserhöhung
  • n - amount of material

the porportionality factor is stated either the ebullioskopische constant ( also boiling point constant of K S), thus like the changethe boiling point of a kilogram of the solution opposite the pure solvent, whereby the amount of material of the solved material amounts to a mol or the molecular Siedepunktserhöhung, which is less more common and no statement meets to the mass.
Thus for example the boiling point risesfrom a kilogram water around 0,51 °C on 100,51 °C, if one dissolves exactly one mol of any other material therein, presupposed the material separates in water and is not not even heavyvolatile. One solves two mol in a kilogram waterup, then the water simmers only with (100 + 2 · 0,51 °C) = 101.02 °C.

It is to be noted with the fact that salts in what winners dissociate solution. Common salt (NaCl) disintegrates for example into the ions well + and to Cl-. The boiling point increase is expected therefore (in diluted solutions) twice as highly like first.

A practical example: Noodle water has a typical common salt content of 10 g/kg. With a mol mass of 58,4 g/mol this, together with doubling mentioned above, corresponds to 0,34mol/kg ions. As a result of the Salzgehalt thus a boiling point increase of only about 0.17 K arises.

The Raoult law applies only to „ideal solution “, which is solutions, with which a material is only physically loosened. With „not-ideal “solutions step during of theMixing energetic features (heating up or cooling) up, which are caused in the training by hydrogen bonds or by Protolysen. Thus deviations from the Raoult law result. Only in very strong dilution the formula applies also with „not-ideal “solutions in approximation, whyone in case of the ideal solution also of an infinitely diluted solution speaks. The Siedepunktserhöhung is besides a kolligative characteristic and depends therefore on the particle number of the solved material, not however on its kind. Over a conversion thatthe Siedepunktserhöhung can serve above formula also for the mol mass regulation, which one calls Ebullioskopie.

Likewise dependent on the concentration of the solved materials is the melting point, why one speaks also of a melting point degradation. A cause for these effects is likewise oneIncrease of the chemical Potenzials. If one combines boiling point increase and melting point degradation, then altogether an expansion of the thermodynamic condition range of the liquid shows up zulasten the other states of aggregation.

boiling range

Siedediagramm für Stickstoff-Sauerstoff-Gemische
boiling-point diagram for nitrogen oxygen mixtures

becomes a mixture (= by the entropy increasedescribing uniform distribution procedure) heats up, then it begins to simmer, if the temperature has the boiling temperature of that component, which exhibits the lowest boiling point. When simmering now the particles of this component step increased into the gaseous phase over. Thus however those changesComposition of the mixture, and its boiling point change thereby continuously. This temperature rise ends only then if the boiling point of that component with the highest boiling temperature is reached. One speaks therefore also in this case of a boiling range (also simmering interval, simmering border)the mixture and no more of a boiling point. The dependence of the aggregate condition and the composition of mixtures of the temperature is represented in T-x-boiling-point diagrams:

Example: If a liquid mixture contains in equal parts nitrogen and oxygen, then that is appropriate for boiling rangebetween - 191.5 °C and - 183 °C.

With azeotropen material mixtures the boiling temperature of the material mixture is higher or lower than the boiling temperature of the two pure material components with a certain amount of material relationship. With this mixing proportion a boiling point and no boiling range are present.

Siedediagramm azeotroper Gemische
  • Sdp1: Boiling point thatPure material component 1
  • Sdp2: Boiling point of the pure material component 2
  • x: Amount of material portion of component of 2 in the azeotropen mixture


  • Water (Sdp 100 °C) and ethanol (Sdp 78.3 °C) - azeotropes mixture with 96% ethanol: Sdp 78.2 °C
  • water (Sdp 100 °C) and HCl (Sdp 83 °C) - azeotropes mixture with 20,2% HCl: Sdp 108.6 °C

meaning for the organisms

the simmering behavior of the water leads under the physical conditions on earth to the fact that water in large quantities asLiquid exists. This is one of the fundamental conditions for the development of organisms.

At a lower air pressure or higher temperatures of the water would certainly different be this and to the fact would lead that waters evaporate within shortest time and thus alsoan important condition for the life at all, i.e. liquid water, to find substantially more rarely would be. With a higher air pressure and/or. however less and less water for a lower temperature could evaporate, and thus the condition for precipitation became, i.e. gaseous water inthe atmosphere, ever more rarely, which would draw for example a restriction of the fresh water occurrences.


  • chemical analytics: The boiling point is a specific material property. Thus pure material can be characterized due to their boiling point.
  • Schnellkochtopf: Becomesthe water in above the hermetically locked pot on over 100 °C heated up, increases the boiling point and the simmering pressure of the water. Thus it comes to a faster refining.
  • The autoclave functions according to the principle of the Schnellkochtopfes. It becomes sterilizationof laboratory instruments assigned.
  • The Ebullioskopie (lat. bulla = simmering blister, gr. skopein =) is a method regards for the determination of the molecular masses by Siedepunktserhöhung. Since Siedepunktserhöhungen fail smaller than freezing point depressions, the cryoskopy is usually preferred. With both methodsa special thermometer applies its, which 1888 by Ernst Beckmann (1853 - 1923) was developed: the Beckmann thermometer. It exactly read off a scale, which covers only approximately 6°, can however also on 0,01 degreesbecome. The zero point of the scale can be stopped to the temperature wished in each case.
  • Distillation
  • fractionating distillation (for example Erdöldestillation)
  • water demineralization
  • steam engines
  • levelling: Since the air pressure with increasing height sinks, also the boiling point sinks. As rule of thumb is considered: The boiling pointper 300 m around approximately a degree is lowered. Thus the respective height over the middle sea level (Normalnull) can be measured by the determination of the boiling temperature of pure water.

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