Editing Boiling point (section)

==Boiling point as a reference property of a pure compound==

As can be seen from the above plot of the logarithm of the vapor pressure vs. the temperature for any given pure [[chemical compound]], its normal boiling point can serve as an indication of that compound's overall [[Volatility (chemistry)|volatility]].  A given pure compound has only one normal boiling point, if any, and a compound's normal boiling point and [[melting point]] can serve as characteristic [[Physical property|physical properties]] for that compound, listed in reference books.  The higher a compound's normal boiling point, the less volatile that compound is overall, and conversely, the lower a compound's normal boiling point, the more volatile that compound is overall.  Some compounds decompose at higher temperatures before reaching their normal boiling point, or sometimes even their melting point.  For a stable compound, the boiling point ranges from its [[triple point]] to its [[Critical point (thermodynamics)|critical point]], depending on the external pressure.  Beyond its triple point, a compound's normal boiling point, if any, is higher than its melting point.  Beyond the critical point, a compound's liquid and vapor phases merge into one phase, which may be called a superheated gas.  At any given temperature, if a compound's normal boiling point is lower, then that compound will generally exist as a gas at atmospheric external pressure.  If the compound's normal boiling point is higher, then that compound can exist as a liquid or solid at that given temperature at atmospheric external pressure, and will so exist in equilibrium with its vapor (if volatile) if its vapors are contained. If a compound's vapors are not contained, then some volatile compounds can eventually evaporate away in spite of their higher boiling points.

[[File:Boiling point vs molar mass graph.png|thumb|350px|Boiling points of [[alkane]]s, [[alkene]]s, [[ether]]s, [[halogenoalkane]]s, [[aldehyde]]s, [[ketone]]s, [[Alcohol (chemistry)|alcohol]]s and [[carboxylic acid]]s as a function of molar mass]]
In general, compounds with [[ionic bond]]s have high normal boiling points, if they do not decompose before reaching such high temperatures.  Many [[metal]]s have high boiling points, but not all.  Very generally—with other factors being equal—in compounds with covalently bonded [[molecule]]s, as the size of the molecule (or [[molecular mass]]) increases, the normal boiling point increases.  When the molecular size becomes that of a [[macromolecule]], [[polymer]], or otherwise very large, the compound often decomposes at high temperature before the boiling point is reached.  Another factor that affects the normal boiling point of a compound is the [[Polarity (chemistry)|polarity]] of its molecules.  As the polarity of a compound's molecules increases, its normal boiling point increases, other factors being equal.  Closely related is the ability of a molecule to form [[hydrogen bond]]s (in the liquid state), which makes it harder for molecules to leave the liquid state and thus increases the normal boiling point of the compound.  Simple [[carboxylic acid]]s dimerize by forming hydrogen bonds between molecules.  A minor factor affecting boiling points is the shape of a molecule.  Making the shape of a molecule more compact tends to lower the normal boiling point slightly compared to an equivalent molecule with more surface area.

{| class="wikitable" style="text-align:center"
|+ Comparison of butane ({{nobold|C<sub>4</sub>H<sub>10</sub>}}) isomer boiling points
|- 
! Common name
| ''n''-[[butane]]
| [[isobutane]]
|-
! [[IUPAC nomenclature|IUPAC name]]
| butane
| 2-methylpropane
|- 
! Molecular<br/>form
| [[File:Butane-3D-balls.png|140px]]
| [[File:Isobutane-3D-balls.png|120px]]
|- 
! Boiling<br/>point (°C)
| −0.5
| −11.7
|}

{| class="wikitable" style="text-align:center"
|- 
|+ Comparison of pentane isomer boiling points
|- 
! Common name
| ''n''-[[pentane]]
| [[isopentane]]
| [[neopentane]]
|-
! [[IUPAC nomenclature|IUPAC name]]
| pentane
| 2-methylbutane
| 2,2-dimethylpropane
|- 
! Molecular<br/>form
| [[File:Pentane-3D-balls.png|160px]]
| [[File:Isopentane-3D-balls.png|140px]]
| [[File:Neopentane-3D-balls.png|120px]]
|- 
! Boiling<br/>point (°C)
| 36.0
| 27.7
| 9.5
|}

[[File:Binary Boiling Point Diagram new.svg|thumb|right|350px|'''Binary boiling point diagram''' of two hypothetical only weakly interacting components without an [[azeotrope]]]]
Most volatile compounds (anywhere near ambient temperatures) go through an intermediate liquid phase while warming up from a solid phase to eventually transform to a vapor phase.  By comparison to boiling, a [[Sublimation (phase transition)|sublimation]] is a physical transformation in which a solid turns directly into vapor, which happens in a few select cases such as with [[carbon dioxide]] at atmospheric pressure.  For such compounds, a [[sublimation point]] is a temperature at which a solid turning directly into vapor has a vapor pressure equal to the external pressure.