1.
Melting point
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The melting 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 change from liquid to solid. Because of the ability of some substances to supercool, the point is not considered as a characteristic property of a substance. For most substances, melting and freezing points are approximately equal, for example, the melting point and freezing point of mercury is 234.32 kelvins. However, certain substances possess differing solid-liquid transition temperatures, for example, agar melts at 85 °C and solidifies from 31 °C to 40 °C, such direction dependence is known as hysteresis. The melting point of ice at 1 atmosphere of pressure is close to 0 °C. In the presence of nucleating substances the freezing point of water is the same as the melting point, the chemical element with the highest melting point is tungsten, at 3687 K, this property makes tungsten excellent for use as filaments in light bulbs. Many laboratory techniques exist for the determination of melting points, a 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, differential scanning calorimetry gives information on melting point together with its enthalpy of fusion. A basic melting point apparatus for the analysis of crystalline solids consists of an oil bath with a transparent window, 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 with the aid of the melting of the individual crystals at a certain temperature can be observed. In large/small devices, the sample is placed in a heating block, the measurement can also be made continuously with an operating process. For instance, oil refineries measure the point of diesel fuel online, meaning that the sample is taken from the process. This allows for more frequent measurements as the sample does not have to be manually collected, for refractory materials the extremely high melting point 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
2.
Chromium
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Chromium is a chemical element with symbol Cr and atomic number 24. It is the first element in Group 6 and it is a steely-grey, lustrous, hard and brittle metal which takes a high polish, resists tarnishing, and has a high melting point. The name of the element is derived from the Greek word χρῶμα, chrōma, meaning color, Chromium metal is of high value for its high corrosion resistance and hardness. A major development was the discovery that steel could be highly resistant to corrosion and discoloration by adding metallic chromium to form stainless steel. Stainless steel and chrome plating together comprise 85% of the commercial use, trivalent chromium ion is an essential nutrient in trace amounts in humans for insulin, sugar and lipid metabolism, although the issue is debated. While chromium metal and Cr ions are not considered toxic, hexavalent chromium is toxic and carcinogenic, abandoned chromium production sites often require environmental cleanup. Chromium is remarkable for its properties, it is the only elemental solid which shows antiferromagnetic ordering at room temperature. Above 38 °C, it changes to paramagnetic, Chromium metal left standing in air is passivated by oxidation, forming a thin, protective, surface layer. This layer is a structure only a few molecules thick. It is very dense, and prevents the diffusion of oxygen into the underlying metal and this is different from the oxide that forms on iron and carbon steel, through which elemental oxygen continues to migrate, reaching the underlying material to cause incessant rusting. Passivation can be enhanced by short contact with oxidizing acids like nitric acid, passivated chromium is stable against acids. Passivation can be removed with a reducing agent that destroys the protective oxide layer on the metal. Chromium metal treated in this way readily dissolves in weak acids, Chromium, unlike such metals as iron and nickel, does not suffer from hydrogen embrittlement. However, it suffer from nitrogen embrittlement, reacting with nitrogen from air. Chromium is the 22nd most abundant element in Earths crust with a concentration of 100 ppm. Chromium compounds are found in the environment from the erosion of chromium-containing rocks, Chromium is mined as chromite ore. About two-fifths of the ores and concentrates in the world are produced in South Africa, while Kazakhstan, India, Russia. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan, although rare, deposits of native chromium exist
3.
Oxygen
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Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the group on the periodic table and is a highly reactive nonmetal. By mass, oxygen is the third-most abundant element in the universe, after hydrogen, at standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. This is an important part of the atmosphere and diatomic oxygen gas constitutes 20. 8% of the Earths atmosphere, additionally, as oxides the element makes up almost half of the Earths crust. Most of the mass of living organisms is oxygen as a component of water, conversely, oxygen is continuously replenished by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Oxygen is too reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone, strongly absorbs ultraviolet UVB radiation, but ozone is a pollutant near the surface where it is a by-product of smog. At low earth orbit altitudes, sufficient atomic oxygen is present to cause corrosion of spacecraft, the name oxygen was coined in 1777 by Antoine Lavoisier, whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosion. One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle, Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries later Leonardo da Vinci built on Philos work by observing that a portion of air is consumed during combustion and respiration, Oxygen was discovered by the Polish alchemist Sendivogius, who considered it the philosophers stone. In the late 17th century, Robert Boyle proved that air is necessary for combustion, English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. From this he surmised that nitroaereus is consumed in both respiration and combustion, Mayow observed that antimony increased in weight when heated, and inferred that the nitroaereus must have combined with it. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract De respiratione. Robert Hooke, Ole Borch, Mikhail Lomonosov, and Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element. This may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, which was then the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, one part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. The fact that a substance like wood gains overall weight in burning was hidden by the buoyancy of the combustion products
4.
Close-packing of equal spheres
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In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement. Carl Friedrich Gauss proved that the highest average density – that is, the same packing density can also be achieved by alternate stackings of the same close-packed planes of spheres, including structures that are aperiodic in the stacking direction. The Kepler conjecture states that this is the highest density that can be achieved by any arrangement of spheres and this conjecture was proven by T. C. Highest density is only in case of 1,2,3,8 and 24 dimensions. Many crystal structures are based on a close-packing of a kind of atom. The cubic and hexagonal arrangements are very close to one another in energy, there are two simple regular lattices that achieve this highest average density. They are called face-centered cubic and hexagonal close-packed, based on their symmetry, both are based upon sheets of spheres arranged at the vertices of a triangular tiling, they differ in how the sheets are stacked upon one another. The fcc lattice is known to mathematicians as that generated by the A3 root system. Cannonballs were usually piled in a rectangular or triangular wooden frame, both arrangements produce a face-centered cubic lattice – with different orientation to the ground. Hexagonal close-packing would result in a pyramid with a hexagonal base. In both the fcc and hcp arrangements each sphere has twelve neighbors, for every sphere there is one gap surrounded by six spheres and two smaller gaps surrounded by four spheres. The distances to the centers of these gaps from the centers of the spheres is √ 3⁄2 for the tetrahedral, and √2 for the octahedral. Relative to a layer with positioning A, two more positionings B and C are possible. Every sequence of A, B, and C without immediate repetition of the one is possible. The most regular ones are fcc = ABCABCA hcp = ABABABA, in close-packing, the center-to-center spacing of spheres in the xy plane is a simple honeycomb-like tessellation with a pitch of one sphere diameter. The coordination number of hcp and fcc is 12 and their atomic packing factors are equal to the mentioned above,0.74. The distance between the centers along the shortest path namely that straight line will therefore be r1 + r2 where r1 is the radius of the first sphere, in close packing all of the spheres share a common radius, r. Therefore two centers would simply have a distance 2r, hexagonal close packing of spheres, the coordinate points of the lattice will be the spheres centers
5.
Mohs scale of mineral hardness
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The Mohs scale of mineral hardness is a qualitative ordinal scale characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. Created in 1812 by German geologist and mineralogist Friedrich Mohs, it is one of several definitions of hardness in materials science, while greatly facilitating the identification of minerals in the field, the Mohs scale does not show how well hard materials perform in an industrial setting. Despite its lack of precision, the Mohs scale is highly relevant for field geologists, the Mohs scale hardness of minerals can be commonly found in reference sheets. Reference materials may be expected to have a uniform Mohs hardness, the Mohs scale of mineral hardness is based on the ability of one natural sample of mineral to scratch another mineral visibly. The samples of matter used by Mohs are all different minerals, Minerals are pure substances found in nature. Rocks are made up of one or more minerals, as the hardest known naturally occurring substance when the scale was designed, diamonds are at the top of the scale. The hardness of a material is measured against the scale by finding the hardest material that the material can scratch. For example, if material is scratched by apatite but not by fluorite. Scratching a material for the purposes of the Mohs scale means creating non-elastic dislocations visible to the naked eye, frequently, materials that are lower on the Mohs scale can create microscopic, non-elastic dislocations on materials that have a higher Mohs number. The Mohs scale is an ordinal scale. For example, corundum is twice as hard as topaz, the table below shows the comparison with the absolute hardness measured by a sclerometer, with pictorial examples. On the Mohs scale, a streak plate has a hardness of 7.0, using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale. The table below incorporates additional substances that may fall between levels, Comparison between Hardness and Hardness, Mohs hardness of elements is taken from G. V, samsonov in Handbook of the physicochemical properties of the elements, IFI-Plenum, New York, USA,1968. The Hardness of Minerals and Rocks
6.
Chlorite group
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The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the four elements in the silicate lattice, Mg, Fe, Ni. Clinochlore, O108 Chamosite, O108 Nimite, O108 Pennantite, 64O108 In addition, zinc, lithium, the great range in composition results in considerable variation in physical, optical, and X-ray properties. Similarly, the range of chemical composition allows chlorite group minerals to exist over a range of temperature and pressure conditions. For this reason chlorite minerals are ubiquitous minerals within low and medium temperature metamorphic rocks, some rocks, hydrothermal rocks. The name chlorite is from the Greek chloros, meaning green, the typical general formula is, 34O102·36. This formula emphasizes the structure of the group, chlorites have a 2,1 sandwich structure, this is often referred to as a talc layer. Unlike other 2,1 clay minerals, a chlorites interlayer space is composed of 6 and this 6 unit is more commonly referred to as the brucite-like layer, due to its closer resemblance to the mineral brucite. Therefore, chlorites structure appears as follows, -t-o-t-brucite-t-o-t-brucite, thats why they are also called 2,1,1 minerals. An older classification divided the chlorites into two subgroups, the orthochlorites and leptochlorites, the terms are seldom used and the ortho prefix is somewhat misleading as the chlorite crystal system is monoclinic and not orthorhombic. Chlorite is commonly found in rocks as an alteration product of mafic minerals such as pyroxene, amphibole. Chlorite is a common mineral associated with ore deposits and commonly occurs with epidote, sericite, adularia. Chlorite is also a metamorphic mineral, usually indicative of low-grade metamorphism. It is the species of the zeolite facies and of lower greenschist facies. It occurs in the quartz, albite, sericite, chlorite, within ultramafic rocks, metamorphism can also produce predominantly clinochlore chlorite in association with talc. Chlorite occurs naturally in a variety of locations and forms, for example, chlorite is found naturally in certain parts of Wales in mineral schists. Chlorite is found in large boulders scattered on the surface on Ring Mountain in Marin County. Clinoclore, pennantite, and chamosite are the most common varieties, several other sub-varieties have been described
7.
Chromium trioxide
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Chromium trioxide is an inorganic compound with the formula CrO3. It is the anhydride of chromic acid, and is sometimes marketed under the same name. This compound is a solid under anhydrous conditions, bright orange when wet. Millions of kilograms are produced annually, mainly for electroplating, Chromium trioxide is a powerful oxidiser and a suspected carcinogen. The solid consists of chains of tetrahedrally coordinated chromium atoms that share vertices, each chromium center, therefore, shares two oxygen centers with neighbors. Two oxygen atoms are not shared, giving an overall stoichiometry of 1,3, the structure of monomeric CrO3 has been calculated using density functional theory, and is predicted to be pyramidal rather than planar. In these oxidations, the Cr converts primary alcohols to the carboxylic acids. It is typically employed with additives that affect the plating process, the trioxide reacts with cadmium, zinc, and other metals to generate passivating chromate films that resist corrosion. It is also used in the production of synthetic rubies, Chromic acid solution is also used in applying types of anodic coating to aluminium, which are primarily used in aerospace applications. A Chromic Acid/ Phosphoric Acid solution is also the preferred stripping agent of anodic coatings of all types, Chromium trioxide is highly toxic, corrosive, and carcinogenic. It is the example of hexavalent chromium, an environmental hazard. The related chromium derivatives are not particularly dangerous, thus, reductants are used to destroy chromium samples, Chromium trioxide, being a powerful oxidizer, will ignite organic materials such as alcohols on contact. ATSDR Case Studies in Environmental Medicine, Chromium Toxicity U. S. Department of Health and Human Services Chromium Trioxide at The Periodic Table of Videos
8.
Jmol
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Jmol is computer software for molecular modelling chemical structures in 3-dimensions. Jmol returns a 3D representation of a molecule that may be used as a teaching tool and it is written in the programming language Java, so it can run on the operating systems Windows, macOS, Linux, and Unix, if Java is installed. It is free and open-source software released under a GNU Lesser General Public License version 2.0, a standalone application and a software development kit exist that can be integrated into other Java applications, such as Bioclipse and Taverna. A popular feature is an applet that can be integrated into web pages to display molecules in a variety of ways, for example, molecules can be displayed as ball-and-stick models, space-filling models, ribbon diagrams, etc. Jmol supports a range of chemical file formats, including Protein Data Bank, Crystallographic Information File, MDL Molfile. There is also a JavaScript-only version, JSmol, that can be used on computers with no Java, the Jmol applet, among other abilities, offers an alternative to the Chime plug-in, which is no longer under active development. While Jmol has many features that Chime lacks, it does not claim to reproduce all Chime functions, most notably, Chime requires plug-in installation and Internet Explorer 6.0 or Firefox 2.0 on Microsoft Windows, or Netscape Communicator 4.8 on Mac OS9. Jmol requires Java installation and operates on a variety of platforms. For example, Jmol is fully functional in Mozilla Firefox, Internet Explorer, Opera, Google Chrome, fast and Scriptable Molecular Graphics in Web Browsers without Java3D
9.
Ammonium dichromate
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Ammonium dichromate is the inorganic compound with the formula 2Cr2O7. In this compound, as in all chromates and dichromates, chromium is in a +6 oxidation state and it is a salt consisting of ammonium ions and dichromate ions. Ammonium dichromate is sometimes known as Vesuvian Fire, because of its use in demonstrations of tabletop volcanoes, however, this demonstration has become unpopular in schools due to the compounds carcinogenic nature. It has also used in pyrotechnics and in the early days of photography. At room temperature and pressure, the compound exists as orange, acidic crystals soluble in water and it is formed by the action of chromic acid on ammonium hydroxide with subsequent crystallisation. The 2Cr2O7 crystal contains a type of ammonium ion, at sites of symmetry C1. Each NH4+ centre is surrounded irregularly by eight oxygen atoms at N—O distances ranging from ca.2.83 to ca.3.17 Å, typical of hydrogen bonds. It has been used in pyrotechnics and in the days of photography as well as in lithography, as a source of pure nitrogen in the laboratory. It is also used as a mordant for dyeing pigments, in the manufacturing of alizarin, chrome alum, leather tanning and oil purification. Photosensitive films containing PVA, ammonium dichromate, and a phosphor are spin-coated as aqueous slurries in the production of the raster of television screens. The ammonium dichromate acts as the photoactive site and its decomposition reaction proceeds to completion once initiated, producing voluminous dark green powdered chromium oxide. Not all of the ammonium dichromate decomposes in this reaction, when the green powder is brought into water a yellow/orange solution is obtained from left over ammonium dichromate. The CrO3 has been identified as a possible molten intermediate participating in 2Cr 2O7 decomposition, ammonium dichromate is a strong oxidising agent and reacts, often violently, with any reducing agent. The stronger the reducing agent, the more violent the reaction and it has also been used to promote the oxidation of alcohols and thiols. Ammonium dichromate, in the presence of Mg2 and wet SiO2 can act as an efficient reagent for the oxidative coupling of thiols under solvent free conditions. The reactions produces reasonably good yields under relatively mild conditions, the compound is also used in the oxidation of aliphatic alcohols to their corresponding aldehydes and ketones in ZrCl4/wet SiO2 in solvent free conditions, again with relatively high yields. Ammonium dichromate, like all chromium compounds, is highly toxic and it is also a strong irritant. In sealed containers, ammonium dichromate is likely to explode if heated
10.
Corundum
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Corundum is a crystalline form of aluminium oxide typically containing traces of iron, titanium, vanadium and chromium. It is a transparent material, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red, all other colors are called sapphire, e. g. green sapphire for a green specimen. The name corundum is derived from the Tamil word Kurundam which originates from the Sanskrit word Kuruvinda meaning ruby, because of corundums hardness, it can scratch almost every other mineral. It is commonly used as an abrasive on everything from sandpaper to large machines used in machining metals, plastics, some emery is a mix of corundum and other substances, and the mix is less abrasive, with an average Mohs hardness of 8.0. In addition to its hardness, corundum is unusual for its density of 4.02 g/cm3, corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low silica igneous syenite and nepheline syenite intrusives, other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in pegmatites. It commonly occurs as a mineral in stream and beach sands because of its hardness. The largest documented crystal of corundum measured about 65×40×40 cm. The record has since surpassed by certain synthetic boules. Corundum for abrasives is mined in Zimbabwe, Russia, Sri Lanka, historically it was mined from deposits associated with dunites in North Carolina, US and from a nepheline syenite in Craigmont, Ontario. Emery-grade corundum is found on the Greek island of Naxos and near Peekskill, New York, abrasive corundum is synthetically manufactured from bauxite. Four corundum axes dating back to 2500 BCE from the Liangzhou culture have been discovered in China, in 1837, Marc Antoine Gaudin made the first synthetic rubies by fusing alumina at a high temperature with a small amount of chromium as a pigment. In 1847, Ebelmen made white synthetic sapphires by fusing alumina in boric acid, in 1877 Frenic and Freil made crystal corundum from which small stones could be cut. Frimy and Auguste Verneuil manufactured artificial ruby by fusing BaF2, in 1903, Verneuil announced he could produce synthetic rubies on a commercial scale using this flame fusion process. The Verneuil process allows the production of flawless single-crystal sapphires, rubies and it is also possible to grow gem-quality synthetic corundum by flux-growth and hydrothermal synthesis. Corundum crystallizes with trigonal symmetry in the space group R3c and has the lattice parameters a =4.75 Å and c =12.982 Å at standard conditions, the unit cell contains six formula units. In the lattice of corundum, the atoms form a slightly distorted hexagonal close packing
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