The melting point of a solid is the temperature range at which it changes state from solid to liquid. A state of matter (or physical state, or form of matter) has physical properties which are qualitatively different from other states of matter Liquid 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 Although the phrase would suggest a specific temperature and is commonly and incorrectly used as such in most textbooks and literature, most crystalline compounds actually melt over a range of a few degrees or less. 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.

Fundamentals

Melting points (in blue) and boiling points (in pink) of the first eight carboxylic acids (°C)

For most substances, melting and freezing points are essentially equal. Carboxylic acids are Organic acids characterized by the presence of a Carboxyl group, which has the formula -C(=OOH usually written -COOH or -CO2H Melting is a process that results in the phase change of a substance from a Solid to a Liquid. For freezing as a method of food preservation see Frozen food. For example, the melting point and freezing point of the element mercury is 234. A chemical element is a type of Atom that is distinguished by its Atomic number; that is by the number of Protons in its nucleus. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum 32 kelvin (−38. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic 83 °C or −37. The Celsius Temperature scale was previously known as the centigrade scale. 89 °F). Fahrenheit is a temperature scale named after Daniel Gabriel Fahrenheit (1686–1736 a German Physicist who proposed it in 1724 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. Agar or agar agar is a Gelatinous substance derived from Seaweed. 6 °F to 104 °F); this process is known as hysteresis. A system with hysteresis can be summarised as a system that may be in any number of states independent of the inputs to the system

Certain materials, such as glass, may harden without crystallizing; these are called amorphous solids. Glass in the common sense refers to a Hard, Brittle, transparent Solid, such as that used for Windows many An amorphous solid is a Solid in which there is no Long-range order of the positions of the Atoms (Solids in which there is long-range atomic order are 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 viscoelastic properties over a range of temperatures. Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Such materials are characterized by a glass transition temperature which may be roughly defined as the "knee" point of the material's density vs. The glass transition temperature, T g is the temperature at which an Amorphous solid, such as Glass or a Polymer, becomes brittle temperature graph.

The melting point of ice at 1 atmosphere of pressure is very close ^[1] to 0 °C (32 °F, 273. Ice is a Solid phase, usually crystalline, of a Non-metalic substance that is liquid or gas at Room temperature, such as Ammonia 15 K), this is also known as the ice point. In the presence of nucleating substances the freezing point of water is the same as the melting point, but in the absence of nucleators water can supercool to −42 °C (−43. Nucleation is the onset of a Phase transition in a small region 6 °F, 231 K) before freezing.

Unlike the boiling point, the melting point is relatively insensitive to pressure because the solid/liquid transition represents only a small change in volume. The boiling point of a liquid is the temperature at which the Vapor pressure of the liquid equals the environmental pressure surrounding the liquid Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface ^[2][3] Melting points are often used to characterize organic compounds and to ascertain the purity. 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 point.

The chemical element with the highest melting point is tungsten, at 3695 K (3422 °C, 6192 °F) making it excellent for use as filaments in light bulbs. A chemical element is a type of Atom that is distinguished by its Atomic number; that is by the number of Protons in its nucleus. Tungsten (ˈtʌŋstən also known as wolfram (/ˈwʊlfrəm/ is a Chemical element that has the symbol W and Atomic number 74 The often-cited carbon does not melt at ambient pressure but sublimates at about 4000 K; a liquid phase only exists above pressures of 10 MPa and estimated 4300–4700 K. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Sublimation of an element or compound is a transition from the Solid to Gas phase with no intermediate liquid stage Tantalum hafnium carbide (Ta₄HfC₅) is a refractory compound with a very high melting point of 4488 K (4215 °C, 7619 °F). Tantalum hafnium carbide (Ta4HfC5 is a refractory compound with an extremely high melting point of 4488 K (4215 °C 7619 °F A refractory is a material that retains its strength at high Temperatures ASTM C71 defines refractories as "non-metallic materials having those chemical and physical ^[4] At the other end of the scale, helium does not freeze at all at normal pressure, even at temperatures infinitesimally close to absolute zero; pressures over 20 times normal atmospheric pressure are necessary. Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical Absolute zero is the point at which molecules do not move (relative to the rest of the body more than they are required to by a quantum mechanical effect called Zero-point

Melting point measurements

Many laboratory techniques exist for the determination of melting points. Laboratory techniques are the sum of procedures used on Natural sciences such as Chemistry, Biology, Physics in order to conduct an experiment A Kofler bench 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 calorimetry gives information on melting point together with its enthalpy of fusion. Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of Heat required to increase the Temperature The standard Enthalpy of fusion (symbol \Delta{}H_{fus} also known as the heat of fusion or specific melting heat, is the amount of

A basic melting point apparatus for the analysis of crystalline solids consists of a oil bath with a transparent window (most basic design: a Thiele tube) and a simple magnifier. An oil bath is a Laboratory heating device which uses Boiling Oil as the Temperature regulator The Thiele tube, named after the German chemist Johannes Thiele, is a Laboratory glassware designed to contain and Heat an Oil bath. 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 contemporary devices, the sample is placed in a heating block, and optical detection is automated.

Thermodynamics

Pressure dependence of water melting point (MPa/K)

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. The standard Enthalpy of fusion (symbol \Delta{}H_{fus} also known as the heat of fusion or specific melting heat, is the amount of

From a thermodynamics point of view, at the melting point the change in Gibbs free energy (ΔG) of the material is zero, because the enthalpy (H) and the entropy (S) of the material are increasing (ΔH,ΔS > 0). In Thermodynamics, the Gibbs free energy ( IUPAC recommended name Gibbs energy or Gibbs function) is a Thermodynamic potential which In Thermodynamics and molecular chemistry, the enthalpy (denoted as H, h, or rarely as χ) is a quotient or description of In Thermodynamics (a branch of Physics) entropy, symbolized by S, is a measure of the unavailability of a system ’s Energy 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 "T","ΔS", and "ΔH" in the above are respectively the temperature at the melting point, change of entropy of melting, and the change of enthalpy of melting. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature In Thermodynamics (a branch of Physics) entropy, symbolized by S, is a measure of the unavailability of a system ’s Energy In Thermodynamics and molecular chemistry, the enthalpy (denoted as H, h, or rarely as χ) is a quotient or description of

Carnelley’s Rule

In organic chemistry Carnelley’s Rule established in 1882 by Thomas Carnelley, states that high molecular symmetry is associated with high melting point ^[5]. Organic chemistry is a discipline within Chemistry which involves the scientific study of the structure properties composition reactions, and preparation Molecular symmetry in Chemistry describes the Symmetry present in Molecules and the classification of molecules according to their symmetry 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. Structural isomerism, or constitutional isomerism, is a form of isomerism in which Molecules with the same Molecular formula have Atoms A chemical formula is a way of expressing information about the Atoms that constitute a particular Chemical compound, and how the relationship between those atoms changes Isopentane, C5[[hydrogen H12]] also called methylbutane or 2-methylbutane, is a branched-chain Alkane with five Pentane is any or one of the Organic compounds with the formula C5H12 8 °C (143 K) and neopentane −18 °C (255 K). Neopentane, also called dimethylpropane or 22-dimethylpropane, is a double-branched-chain Alkane with five Carbon atoms Likewise in xylenes and also dichlorobenzenes the melting point increases in the order meta, ortho and then para. o-Xylene|m-Xylene|p-XyleneThe term xylene or xylol refers to a mixture of three Benzene derivatives which is used as a Solvent in the Printing Arene substitution patterns are part of Organic chemistry IUPAC nomenclature and pinpoint the position of Substituents other than Hydrogen in Pyridine has a lower symmetry than benzene hence its lower melting point but the melting point again increases with diazine and triazines. Pyridine is a Chemical compound with the formula C5[[Hydrogen H5]] N. Benzene, or benzol, is an organic Chemical compound and a known Carcinogen with the molecular formula C 6 H 6 Diazine refers to a group of Organic compounds having the Molecular formula C4H4N2 A triazine is one of three Organic chemicals Isomeric with each other whose Empirical formula is 333 Many cage-like compounds like adamantane and cubane with high symmetry have very high melting points. Adamantane (tricyclodecane is a colourless crystalline compound with a Camphor -like odour Cubane (C8H8 is a synthetic Hydrocarbon Molecule that consists of eight Carbon Atoms arranged at the corners of a

A high melting point results from a high heat of fusion or a low entropy of fusion or a combination of both. The standard Enthalpy of fusion (symbol \Delta{}H_{fus} also known as the heat of fusion or specific melting heat, is the amount of The entropy of fusion is the increase in Entropy when Melting a substance 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

• Boiling point
• Freezing-point depression
• List of elements by melting point
• Melting
• Melting Points for various elements
• Phases of matter
• Triple point
• Vicat softening point - the determination of the softening point for materials that have no definite melting point. The boiling point of a liquid is the temperature at which the Vapor pressure of the liquid equals the environmental pressure surrounding the liquid Freezing-point depression describes the phenomenon that the freezing point of a Liquid (a Solvent) is depressed when another compound is added meaning This is a list of the Chemical elements sorted by Melting point measured at normal pressure Melting is a process that results in the phase change of a substance from a Solid to a Liquid. This page lists Melting points for various Chemical elements at standard Atmospheric pressure in Kelvin, Celsius, and Fahrenheit. In the Physical sciences a phase is a Set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties In Thermodynamics, the triple point of a substance is the Temperature and Pressure at which three phases (for example Gas, Liquid Vicat softening point or Vicat hardness is the determination of the Softening point for materials such as Polyethylene, which have no definite Melting

References

External links

• Melting and boiling point tables vol. 1 by Thomas Carnelley (Harrison, London, 1885-1887)
• Melting and boiling point tables vol. 2 by Thomas Carnelley (Harrison, London, 1885-1887)