The melting point (or, rarely, liquefaction point) of a solid is the
temperature at which it changes state from solid to liquid at
atmospheric pressure. At the melting point the solid and liquid phase
exist in equilibrium. The melting point of a substance depends on
pressure and is usually specified at standard pressure. When
considered as the temperature of the reverse change from liquid to
solid, it is referred to as the freezing point or crystallization
point. Because of the ability of some substances to supercool, the
freezing point is not considered as a characteristic property of a
substance. When the "characteristic freezing point" of a substance is
determined, in fact the actual methodology is almost always "the
principle of observing the disappearance rather than the formation of
ice", that is, the melting point.
Melting point measurements
2.1 Techniques for refractory materials
4 Freezing-point depression
5 Carnelley's rule
6 Predicting the melting point of substances (Lindemann's criterion)
Melting point open data
8 See also
11 External links
Further information: List of elements by melting point
Melting points (in blue) and boiling points (in pink) of the first
eight carboxylic acids (°C)
For most substances, melting and freezing points are approximately
equal. For example, the melting point and freezing point of mercury is
234.32 kelvins (−38.83 °C or −37.89 °F). However,
certain substances possess differing solid-liquid transition
temperatures. For example, agar melts at 85 °C (185 °F)
and solidifies from 31 °C to 40 °C (89.6 °F to
104 °F); such direction dependence is known as hysteresis. The
melting point of ice at 1 atmosphere of pressure is very close  to
0 °C (32 °F, 273.15 K); this is also known as the ice
point. In the presence of nucleating substances, the freezing point of
water is not always the same as the melting point. In the absence of
nucleators water can exist as a supercooled liquid down to
−48.3 °C (−55°F, 224.8 K) before freezing. The
chemical element with the highest melting point is tungsten, at 3687 K
(3414 °C, 6177 °F); this property makes tungsten
excellent for use as filaments in light bulbs. The often-cited carbon
does not melt at ambient pressure but sublimes at about 4000 K; a
liquid phase only exists above pressures of 10 MPa and estimated
4300–4700 K (see carbon phase diagram). Tantalum hafnium
carbide (Ta4HfC5) is a refractory compound with a very high melting
point of 4215 K (3942 °C, 7128 °F). At the other
end of the scale, helium does not freeze at all at normal pressure
even at temperatures close to absolute zero; pressures greater than
twenty times normal atmospheric pressure are necessary.
Melting point measurements
Melting point apparatus
Kofler bench with samples for calibration
Many laboratory techniques exist for the determination of melting
Kofler bench is a metal strip with a temperature gradient
(range from room temperature to 300 °C). Any substance can be
placed on a section of the strip, revealing its thermal behaviour at
the temperature at that point.
Differential scanning calorimetry
Differential scanning calorimetry gives
information on melting point together with its enthalpy of fusion.
Automatic digital melting point meter
A basic melting point apparatus for the analysis of crystalline solids
consists of an oil bath with a transparent window (most basic design:
a Thiele tube) and a simple magnifier. The several grains of a solid
are placed in a thin glass tube and partially immersed in the oil
bath. The oil bath is heated (and stirred) and with the aid of the
magnifier (and external light source) melting of the individual
crystals at a certain temperature can be observed. In large/small
devices, the sample is placed in a heating block, and optical
detection is automated.
The measurement can also be made continuously with an operating
process. For instance, oil refineries measure the freeze point of
diesel fuel online, meaning that the sample is taken from the process
and measured automatically. This allows for more frequent measurements
as the sample does not have to be manually collected and taken to a
Techniques for refractory materials
For refractory materials (e.g. platinum, tungsten, tantalum, some
carbides and nitrides, etc.) the extremely high melting point
(typically considered to be above, say, 1800°C) may be determined by
heating the material in a black body furnace and measuring the
black-body temperature with an optical pyrometer. For the highest
melting materials, this may require extrapolation by several hundred
degrees. The spectral radiance from an incandescent body is known to
be a function of its temperature. An optical pyrometer matches the
radiance of a body under study to the radiance of a source that has
been previously calibrated as a function of temperature. In this way,
the measurement of the absolute magnitude of the intensity of
radiation is unnecessary. However, known temperatures must be used to
determine the calibration of the pyrometer. For temperatures above the
calibration range of the source, an extrapolation technique must be
employed. This extrapolation is accomplished by using
Planck's law of
radiation. The constants in this equation are not known with
sufficient accuracy, causing errors in the extrapolation to become
larger at higher temperatures. However, standard techniques have been
developed to perform this extrapolation.
Consider the case of using gold as the source (mp = 1063°C). In this
technique, the current through the filament of the pyrometer is
adjusted until the light intensity of the filament matches that of a
black-body at the melting point of gold. This establishes the primary
calibration temperature and can be expressed in terms of current
through the pyrometer lamp. With the same current setting, the
pyrometer is sighted on another black-body at a higher temperature. An
absorbing medium of known transmission is inserted between the
pyrometer and this black-body. The temperature of the black-body is
then adjusted until a match exists between its intensity and that of
the pyrometer filament. The true higher temperature of the black-body
is then determined from Planck's Law. The absorbing medium is then
removed and the current through the filament is adjusted to match the
filament intensity to that of the black-body. This establishes a
second calibration point for the pyrometer. This step is repeated to
carry the calibration to higher temperatures. Now, temperatures and
their corresponding pyrometer filament currents are known and a curve
of temperature versus current can be drawn. This curve can then be
extrapolated to very high temperatures.
In determining melting points of a refractory substance by this
method, it is necessary to either have black body conditions or to
know the emissivity of the material being measured. The containment of
the high melting material in the liquid state may introduce
Melting temperatures of some refractory
metals have thus been measured by observing the radiation from a black
body cavity in solid metal specimens that were much longer than they
were wide. To form such a cavity, a hole is drilled perpendicular to
the long axis at the center of a rod of the material. These rods are
then heated by passing a very large current through them, and the
radiation emitted from the hole is observed with an optical pyrometer.
The point of melting is indicated by the darkening of the hole when
the liquid phase appears, destroying the black body conditions. Today,
containerless laser heating techniques, combined with fast pyrometers
and spectro-pyrometers, are employed to allow for precise control of
the time for which the sample is kept at extreme temperatures. Such
experiments of sub-second duration address several of the challenges
associated with more traditional melting point measurements made at
very high temperatures, such as sample vaporization and reaction with
Pressure dependence of water melting point.
For a solid to melt, heat is required to raise its temperature to the
melting point. However, further heat needs to be supplied for the
melting to take place: this is called the heat of fusion, and is an
example of latent heat.
From a thermodynamics point of view, at the melting point the change
Gibbs free energy
Gibbs free energy (ΔG) of the material is zero, but the enthalpy
(H) and the entropy (S) of the material are increasing (ΔH, ΔS >
Melting phenomenon happens when the
Gibbs free energy
Gibbs free energy of the
liquid becomes lower than the solid for that material. At various
pressures this happens at a specific temperature. It can also be shown
displaystyle Delta S= frac Delta H T
Here T, ΔS and ΔH are respectively the temperature at the melting
point, change of entropy of melting and the change of enthalpy of
The melting point is sensitive to extremely large changes in pressure,
but generally this sensitivity is orders of magnitude less than that
for the boiling point, because the solid-liquid transition represents
only a small change in volume. If, as observed in most cases, a
substance is more dense in the solid than in the liquid state, the
melting point will increase with increases in pressure. Otherwise the
reverse behavior occurs. Notably, this is the case of water, as
illustrated graphically to the right, but also of Si, Ge, Ga, Bi. With
extremely large changes in pressure, substantial changes to the
melting point are observed. For example, the melting point of silicon
at ambient pressure (0.1 MPa) is 1415 °C, but at pressures in
excess of 10 GPa it decreases to 1000 °C.
Melting points are often used to characterize organic and inorganic
compounds and to ascertain their purity. The melting point of a pure
substance is always higher and has a smaller range than the melting
point of an impure substance or, more generally, of mixtures. The
higher the quantity of other components, the lower the melting point
and the broader will be the melting point range, often referred to as
the "pasty range". The temperature at which melting begins for a
mixture is known as the "solidus" while the temperature where melting
is complete is called the "liquidus". Eutectics are special types of
mixtures that behave like single phases. They melt sharply at a
constant temperature to form a liquid of the same composition.
Alternatively, on cooling a liquid with the eutectic composition will
solidify as uniformly dispersed, small (fine-grained) mixed crystals
with the same composition.
In contrast to crystalline solids, glasses do not possess a melting
point; on heating they undergo a smooth glass transition into a
viscous liquid. Upon further heating, they gradually soften, which can
be characterized by certain softening points.
Freezing-point depression and Supercooling
The freezing point of a solvent is depressed when another compound is
added, meaning that a solution has a lower freezing point than a pure
solvent. This phenomenon is used in technical applications to avoid
freezing, for instance by adding salt or ethylene glycol to water.
In organic chemistry, Carnelley's rule, established in 1882 by Thomas
Carnelley, states that high molecular symmetry is associated with high
melting point. Carnelley based his rule on examination of 15,000
chemical compounds. For example, for three structural isomers with
molecular formula C5H12 the melting point increases in the series
isopentane −160 °C (113 K) n-pentane −129.8 °C (143 K)
and neopentane −16.4 °C (256.8 K). Likewise in xylenes and
also dichlorobenzenes the melting point increases in the order meta,
ortho and then para.
Pyridine has a lower symmetry than benzene hence
its lower melting point but the melting point again increases with
diazine and triazines. Many cage-like compounds like adamantane and
cubane with high symmetry have relatively high melting points.
A high melting point results from a high heat of fusion, a low entropy
of fusion, or a combination of both. In highly symmetrical molecules
the crystal phase is densely packed with many efficient intermolecular
interactions resulting in a higher enthalpy change on melting.
Predicting the melting point of substances (Lindemann's
An attempt to predict the bulk melting point of crystalline materials
was first made in 1910 by Frederick Lindemann. The idea behind the
theory was the observation that the average amplitude of thermal
vibrations increases with increasing temperature.
when the amplitude of vibration becomes large enough for adjacent
atoms to partly occupy the same space. The Lindemann criterion states
that melting is expected when the vibration root mean square amplitude
exceeds a threshold value.
Assuming that all atoms in a crystal vibrate with the same frequency
ν, the average thermal energy can be estimated using the
equipartition theorem as
displaystyle E=4pi ^ 2 mnu ^ 2 ~u^ 2 =k_ B T
where m is the atomic mass, ν is the frequency, u is the average
vibration amplitude, kB is the Boltzmann constant, and T is the
absolute temperature. If the threshold value of u2 is c2a2 where c is
the Lindemann constant and a is the atomic spacing, then the melting
point is estimated as
displaystyle T_ m = cfrac 4pi ^ 2 mnu ^ 2 c^ 2 a^ 2 k_ B .
Several other expressions for the estimated melting temperature can be
obtained depending on the estimate of the average thermal energy.
Another commonly used expression for the Lindemann criterion is
displaystyle T_ m = cfrac 4pi ^ 2 mnu ^ 2 c^ 2 a^ 2 2k_ B .
From the expression for the
Debye frequency for ν, we have
displaystyle T_ m = cfrac 2pi mc^ 2 a^ 2 theta _ D ^ 2 k_ B h^
where θD is the
Debye temperature and h is the Planck constant.
Values of c range from 0.15–0.3 for most materials.
Melting point open data
In February 2011,
Alfa Aesar released over 10,000 melting points of
compounds from their catalog as open data. This dataset has been used
to create a random forest model for melting point prediction which
is now freely available. Open melting point data are also
available from Nature Precedings. High quality data mined from
patents and also models developed with these data were published
by Tetko et al.
List of elements by melting point
Melting points of the elements (data page)
Phases of matter
Slip melting point
^ Ramsay, J. A. (1949). "A new method of freezing-point determination
for small quantities" (PDF).
J. Exp. Biol. 26 (1): 57–64.
^ Haynes, p. 4.122.
^ The melting point of purified water has been measured as 0.002519 ±
0.000002 °C, see Feistel, R. & Wagner, W. (2006). "A New Equation
of State for H2O Ice Ih". J. Phys. Chem. Ref. Data. 35 (2):
1021–1047. Bibcode:2006JPCRD..35.1021F. doi:10.1063/1.2183324.
^ Haynes, p. 4.123.
^ Agte, C. & Alterthum, H. (1930). "Researches on Systems with
Carbides at High
Melting Point and Contributions to the Problem of
Carbon Fusion". Z. Tech. Phys. 11: 182–191.
^ The exact relationship is expressed in the Clausius–Clapeyron
^ "J10 Heat: Change of aggregate state of substances through change of
heat content: Change of aggregate state of substances and the equation
of Clapeyron-Clausius". Retrieved 19 February 2008.
^ Tonkov, E. Yu. and Ponyatovsky, E. G. (2005) Phase Transformations
of Elements Under High Pressure, CRC Press, Boca Raton, p. 98
^ Brown, R. J. C. & R. F. C. (2000). "
Melting Point and Molecular
Symmetry". Journal of Chemical Education. 77 (6): 724.
^ Haynes, pp. 6.153–155.
^ Lindemann FA (1910). "The calculation of molecular vibration
frequencies". Physik. Z. 11: 609–612.
^ Sorkin, S., (2003), Point defects, lattice structure, and melting,
Thesis, Technion, Israel.
^ Philip Hofmann (2008).
Solid state physics: an introduction.
Wiley-VCH. p. 67. ISBN 978-3-527-40861-0. Retrieved 13 March
^ Nelson, D. R., (2002), Defects and geometry in condensed matter
physics, Cambridge University Press, ISBN 0-521-00400-4
^ Bradley, J-C. and Lang A.S.I.D. (2011) Random Forest model for
melting point prediction. onschallenge.wikispaces.com
^ Predict melting point from SMILES. Qsardb.org. Retrieved on 13
^ ONS Open
Melting Point Collection. Precedings.nature.com. Retrieved
on 13 September 2013.
^ OCHEM melting point models. ochem.eu. Retrieved on 18 June 2016.
^ Tetko, Igor V; m. Lowe, Daniel; Williams, Antony J (2016). "The
development of models to predict melting and pyrolysis point data
associated with several hundred thousand compounds mined from
PATENTS". Journal of Cheminformatics. 8.
Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics
(92nd ed.). CRC Press. ISBN 1439855110.
Melting and boiling point tables vol. 1 by Thomas Carnelley (Harrison,
Melting and boiling point tables vol. 2 by Thomas Carnelley (Harrison,
Patent mined data Over 250,000 freely downloadable melting point data.
Also downloadable at figshare
States of matter (list)
Gas / Vapor
Quantum spin liquid
Enthalpy of fusion
Enthalpy of sublimation
Enthalpy of vaporization
Latent internal energy
Equation of state
Macroscopic quantum phenomena
Order and disorder (physics)