The
melting point of a solid is the temperature range at which it changes state from solid to
liquidLiquid is one of the principal states of matter. A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material. The surface is a free surface where the liquid is not constrained by a container....
. At the melting point the solid and liquid phase exist in equilibrium. When considered as the temperature of the reverse change from liquid to solid, it is referred to as the
freezing point. Because of the ability of some substances to supercool, the freezing point is not considered to be 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",
i.e. the melting point.
Fundamentals
For most substances,
meltingMelting is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a solid substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid ordering...
and
freezingIn physical science, freezing or solidification is the process in which a liquid turns into a solid when cold enough. The freezing point is the temperature at which this happens. Melting, the process of turning a solid to a liquid, is almost the exact opposite of freezing...
points are approximately equal. For example, the melting point
and freezing point of the
elementA chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. The term is also used to refer to a pure chemical substance composed of atoms with the same number of protons.Common examples of elements...
mercuryMercury , also called quicksilver or hydrargyrum , is a chemical element with the symbol Hg and atomic number 80...
is 234.32
kelvinThe kelvin is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero kelvin...
(−38.83
°CCelsius is a temperature scale that is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
or −37.89
°FFahrenheit is the temperature scale proposed in 1724 by, and named after, the physicist Daniel Gabriel Fahrenheit . Today, the scale has been replaced by the Celsius scale in most countries; it is still in use for non-scientific purposes in the United States and a few other nations, such as...
). However, certain substances possess differing solid-liquid transition temperatures. For example,
agarAgar or agar agar is a gelatinous substance derived from seaweed. Historically and in a modern context, it is chiefly used as an ingredient in desserts throughout Japan, but in the past century has found extensive use as a solid substrate to contain culture medium for microbiological work...
melts at 85 °C (185 °F) and solidifies from 31 °C to 40 °C (89.6 °F to 104 °F); this process is known as
hysteresisA system with hysteresis has memory. Such a system is said to exhibit path-dependence, or "rate-independent memory". .In a deterministic system with no dynamics or hysteresis, it is possible to predict the system's output at an instant in time, given only its input at that instant in time...
.
Certain materials, such as
glassIn general Glass refers to a solid, brittle, transparent material, commonly used for windows, bottles, or eyewear. Examples of glassy materials include, but are not limited to, soda-lime glass, borosilicate glass, acrylic glass, sugar glass, Muscovy-glass, or aluminium oxynitride. The term glass...
, may harden without crystallizing; these are called
amorphous solidAn "amorphous solid" is a solid in which there is no long-range order of the positions of the atoms. . Most classes of solid materials can be found or prepared in an amorphous form...
s. Amorphous materials as well as some polymers do not have a true melting point as there is no abrupt phase change at any specific temperature. Instead, there is a gradual change in their
viscoelasticViscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like honey, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just...
properties over a range of temperatures. Such materials are characterized by a
glass transitionGlass transition or vitrification refer to the transformation of a glass-forming liquid into a glass, which usually occurs upon rapid cooling. It is a dynamic phenomenon occurring between two distinct states of matter , each with different physical properties...
which occurs at a glass transition temperature which may be roughly defined as the "knee" point of the material's density vs. temperature graph.
The melting point of
iceIce is a solid phase, usually crystalline, of a non-metallic substance that is liquid or gas at room temperature, such as carbon dioxide ice , ammonia ice, or methane ice. However, the predominant use of the term ice is for water ice, technically restricted to one of the 15 known crystalline phases...
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 substancesNucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form via nucleation in liquids are gaseous bubbles, crystals, or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...
the freezing point of water is the same as the melting point, but in the absence of nucleators water can
supercoolSupercooling is the process of lowering the temperature of a liquid or a gas below its freezing point, without it becoming a solid.A liquid below its standard freezing point will crystallize in the presence of a seed crystal or nucleus around which a crystal structure can form...
to −42 °C (−43.6 °F, 231 K) before freezing.
Unlike the
boiling pointThe 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....
, the melting point is relatively insensitive to
pressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
because the solid/liquid transition represents only a small change in volume.
Melting points are often used to characterize organic and inorganic compounds and to ascertain their
purityPurity is the absence of impurity in a substance.Purity may also refer to:* in Buddhism, purity refers to a spiritual purity of character or essence.* Abstinence from vices and/or abundance of virtue....
. The melting point of a pure substance is always higher and has a smaller range than the melting point of an impure substance. The more impurity is present, the lower the melting point and the broader the range. Eventually, a minimum melting point will be reached. The mixing ratio that results in the lowest possible melting point is known as the
eutectic pointThe melting point of a mixture of two or more solids depends on the relative proportions of its ingredients. A eutectic or eutectic mixture is a mixture at such proportions that the melting point is a local temperature minimum, which means that all the constituents crystallize simultaneously at...
.
The
chemical elementA chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. The term is also used to refer to a pure chemical substance composed of atoms with the same number of protons.Common examples of elements...
with the highest melting point is
tungstenTungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A steel-gray metal, tungsten is found in several ores, including wolframite and scheelite...
, at 3683 K (3410 °C, 6170 °F) making it excellent for use as filaments in light bulbs. The often-cited
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
does not melt at ambient pressure but
sublimesSublimation of an element or compound is a transition from the solid to gas phase with no intermediate liquid stage. Sublimation is an endothermic phase transition that occurs at temperatures and pressures below the triple point ....
at about 4000 K; a liquid phase only exists above pressures of 10
MPaThe pascal is the SI derived unit of pressure, stress, Young's modulus and tensile strength. It is a measure of force per unit area, defined as one newton per square metre...
and estimated 4300–4700 K.
Tantalum hafnium carbideTantalum hafnium carbide is a refractory compound with the highest melting point of all known compounds--4488 K ....
(Ta
4HfC
5) is a
refractoryA refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that made them applicable for structures, or as components of systems, that are exposed to environments above...
compound with a very high melting point of 4488 K (4215 °C, 7619 °F). At the other end of the scale,
heliumHelium is the chemical element with atomic number 2, and is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table...
does not freeze at all at normal pressure, even at temperatures very close to
absolute zeroAbsolute zero is a temperature marked by a 0 entropy configuration. It is the coldest temperature theoretically possible and cannot be reached by artificial or natural means, because it is impossible to decouple a system fully from the rest of the universe...
; pressures over 20 times normal atmospheric pressure are necessary.
Melting point measurements
Many
laboratory techniquesLaboratory techniques are the sum of procedures used on natural sciences such as chemistry, biology, physics in order to conduct an experiment, all of them follow scientific method; while some of them involves the use of complex laboratory equipment from laboratory glassware to electrical devices...
exist for the determination of melting points.
A
Kofler benchA Kofler bench is a metal strip with a temperature gradient . Any substance can be placed on a section of the strip revealing its thermal behaviour at the temperature at that point....
is a metal strip with a temperature gradient (range 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 calorimetryDifferential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature. Both the sample and reference are maintained at nearly the same...
gives information on melting point together with its
enthalpy of fusionFor information about the plastic welding technique, see Heat fusionThe standard enthalpy of fusion , also known as the heat of fusion or specific melting heat, is the amount of thermal energy which must be absorbed or evolved for 1 mole of a substance to change states from a solid to a liquid or...
.
A basic melting point apparatus for the analysis of crystalline solids consists of a
oil bathAn oil bath is a laboratory heating device which uses boiling oil as the temperature regulator. Since different oils have different boiling points it is possible to obtain a temperature near to the desired temperature by selecting an oil with a boiling point as close as possible to the desired...
with a transparent window (most basic design: a
Thiele tubeThe Thiele tube, named after the German chemist Johannes Thiele, is a laboratory glassware designed to contain and heat an oil bath. Such a setup is commonly used in the determination of the melting point of a substance...
) 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.
Thermodynamics
Not only is heat required to raise the temperature of the solid to the melting point, but the melting itself requires heat called the heat of fusion.
From a thermodynamics point of view, at the melting point the change in
Gibbs free energyIn thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from an isothermal, isobaric thermodynamic system...
of the material is zero, because the
enthalpyIn thermodynamics and molecular chemistry, the enthalpy is a thermodynamic property of a thermodynamic system. It can be used to calculate the heat transfer during a quasistatic process taking place in a closed thermodynamic system under constant pressure...
and the
entropyEntropy is a concept of information maintaining great importance in physics, chemistry, and information theory...
of the material are increasing . Melting phenomenon happens when the 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 that:
The "","", and "" in the above are respectively the
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
at the melting point, change of
entropyEntropy is a concept of information maintaining great importance in physics, chemistry, and information theory...
of melting, and the change of
enthalpyIn thermodynamics and molecular chemistry, the enthalpy is a thermodynamic property of a thermodynamic system. It can be used to calculate the heat transfer during a quasistatic process taking place in a closed thermodynamic system under constant pressure...
of melting.
Carnelley’s Rule
In
organic chemistryOrganic chemistry is a discipline within chemistry which involves the scientific study of the structure, properties, composition, reactions, and preparation of chemical compounds that contain carbon...
Carnelley’s Rule established in 1882 by Thomas Carnelley, states that
high molecular symmetryMolecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...
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 C
5H
12 the melting point increases in the series
isopentaneIsopentane, C5H12, also called methylbutane or 2-methylbutane, is a branched-chain alkane with five carbon atoms. Isopentane is an extremely volatile and extremely flammable liquid at room temperature and pressure. The normal boiling point is just a few degrees above room...
−160 °C (113 K) n-pentane −129.8 °C (143 K) and
neopentaneNeopentane, also called dimethylpropane or 2,2-dimethylpropane, is a double-branched-chain alkane with five carbon atoms. Neopentane is an extremely flammable gas at room temperature and pressure which can condense into a highly volatile liquid on a cold day, in an ice bath, or when compressed to...
−18 °C (255 K). Likewise in
xyleneThe term xylene or xylol refers to a mixture of three aromatic hydrocarbon isomers which is used as a solvent in the printing, rubber, and leather industries. Xylene is a clear, colorless, sweet-smelling liquid that is very flammable. It is usually refined from crude oil in a process called...
s and also
dichlorobenzeneThere are three distinct chemical compounds which are dichlorobenzenes:*1,2-Dichlorobenzene or ortho-dichlorobenzene;*1,3-Dichlorobenzene or meta-dichlorobenzene;*1,4-Dichlorobenzene or para-dichlorobenzene....
s the melting point increases in the order
meta, ortho and then paraArene substitution patterns are part of organic chemistry IUPAC nomenclature and pinpoint the position of substituents other than hydrogen in relation to each other on an aromatic hydrocarbon.- Ortho, meta, and para substitution :...
.
PyridinePyridine is a simple aromatic heterocyclic organic compound with the chemical formula C
5H
5N used as a precursor to agrochemicals and pharmaceuticals, and is also an important solvent and reagent. It is structurally related to benzene, wherein one CH group in the aromatic...
has a lower symmetry than
benzeneBenzene, or benzol, is an organic chemical compound with the molecular formula C6H6. It is sometimes abbreviated Ph–H. Benzene is a colorless and highly flammable liquid with a sweet smell and a relatively high melting point...
hence its lower melting point but the melting point again increases with
diazineDiazine refers to a group of organic compounds having the molecular formula C
4H
4N
2. Each contains a benzene ring in which two of the C-H fragments have been replaced by isolobal nitrogen...
and
triazineA triazine is one of three organic chemicals, isomeric with each other, whose empirical formula is
333.- Structure :...
s. Many cage-like compounds like
adamantaneAdamantane is a colourless, crystalline compound with a camphor-like odour. With a formula C10H16, it is a cycloalkane and also the simplest diamondoid. Adamantane was discovered in petroleum in 1933. Its name derived from the Greek adamantinos , due to its diamond-like...
and
cubaneCubane is a synthetic hydrocarbon molecule that consists of eight carbon atoms arranged at the corners of a cube, with one hydrogen atom attached to each carbon atom. A solid crystalline substance, cubane is one of the Platonic hydrocarbons. It was first synthesized in 1964 by Philip Eaton, a...
with high symmetry have very 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.
See also
- Degree of frost
A degree of frost is a non-standard unit of measure for air temperature meaning degrees below melting point of water...
- Freezing-point depression
Freezing-point depression describes the phenomenon that the freezing point of a liquid is depressed when another compound is added, meaning that a solution has a lower freezing point than a pure solvent. This happens whenever a solute is added to a pure solvent, such as water...
- List of elements by melting point
- Phases of matter
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
- Triple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which three phases of that substance coexist in thermodynamic equilibrium...
- Vicat softening point
Vicat softening point or Vicat hardness is the determination of the softening point for materials that have no definite melting point, such as polycarbonate. It is taken as the temperature at which the specimen is penetrated to a depth of 1 mm by a flat-ended needle with a 1 square mm circular or...
- the determination of the softening point for materials that have no definite melting point.
- Liquidus temperature
The liquidus temperature, TL or Tliq, is mostly used for glasses and alloys. It specifies the maximum temperature at which crystals can co-exist with the melt in thermodynamic equilibrium. Above the liquidus temperature the material is homogeneous...
- Slip melting point
The Slip melting point or "slip point" is one conventional definition of the melting point of a waxy solid. It is determined by casting a 10 mm column of the solid in a glass tube with an internal diameter of about 1 mm and a length of about 80 mm, and thenimmersing it in a temperature-controlled...
- Solidus temperature
External links