The density, or more the volumetric mass density, of a substance is its mass per unit volume. The symbol most used for density is ρ, although the Latin letter D can be used. Mathematically, density is defined as mass divided by volume: ρ = m V where ρ is the density, m is the mass, V is the volume. In some cases, density is loosely defined as its weight per unit volume, although this is scientifically inaccurate – this quantity is more called specific weight. For a pure substance the density has the same numerical value as its mass concentration. Different materials have different densities, density may be relevant to buoyancy and packaging. Osmium and iridium are the densest known elements at standard conditions for temperature and pressure but certain chemical compounds may be denser. To simplify comparisons of density across different systems of units, it is sometimes replaced by the dimensionless quantity "relative density" or "specific gravity", i.e. the ratio of the density of the material to that of a standard material water.
Thus a relative density less than one means. The density of a material varies with pressure; this variation is small for solids and liquids but much greater for gases. Increasing the pressure on an object decreases the volume of the object and thus increases its density. Increasing the temperature of a substance decreases its density by increasing its volume. In most materials, heating the bottom of a fluid results in convection of the heat from the bottom to the top, due to the decrease in the density of the heated fluid; this causes it to rise relative to more dense unheated material. The reciprocal of the density of a substance is called its specific volume, a term sometimes used in thermodynamics. Density is an intensive property in that increasing the amount of a substance does not increase its density. In a well-known but apocryphal tale, Archimedes was given the task of determining whether King Hiero's goldsmith was embezzling gold during the manufacture of a golden wreath dedicated to the gods and replacing it with another, cheaper alloy.
Archimedes knew that the irregularly shaped wreath could be crushed into a cube whose volume could be calculated and compared with the mass. Baffled, Archimedes is said to have taken an immersion bath and observed from the rise of the water upon entering that he could calculate the volume of the gold wreath through the displacement of the water. Upon this discovery, he leapt from his bath and ran naked through the streets shouting, "Eureka! Eureka!". As a result, the term "eureka" entered common parlance and is used today to indicate a moment of enlightenment; the story first appeared in written form in Vitruvius' books of architecture, two centuries after it took place. Some scholars have doubted the accuracy of this tale, saying among other things that the method would have required precise measurements that would have been difficult to make at the time. From the equation for density, mass density has units of mass divided by volume; as there are many units of mass and volume covering many different magnitudes there are a large number of units for mass density in use.
The SI unit of kilogram per cubic metre and the cgs unit of gram per cubic centimetre are the most used units for density. One g/cm3 is equal to one thousand kg/m3. One cubic centimetre is equal to one millilitre. In industry, other larger or smaller units of mass and or volume are more practical and US customary units may be used. See below for a list of some of the most common units of density. A number of techniques as well as standards exist for the measurement of density of materials; such techniques include the use of a hydrometer, Hydrostatic balance, immersed body method, air comparison pycnometer, oscillating densitometer, as well as pour and tap. However, each individual method or technique measures different types of density, therefore it is necessary to have an understanding of the type of density being measured as well as the type of material in question; the density at all points of a homogeneous object equals its total mass divided by its total volume. The mass is measured with a scale or balance.
To determine the density of a liquid or a gas, a hydrometer, a dasymeter or a Coriolis flow meter may be used, respectively. Hydrostatic weighing uses the displacement of water due to a submerged object to determine the density of the object. If the body is not homogeneous its density varies between different regions of the object. In that case the density around any given location is determined by calculating the density of a small volume around that location. In the limit of an infinitesimal volume the density of an inhomogeneous object at a point becomes: ρ = d m / d V, where d V is an elementary volume at position r; the mass of the body t
Glycolaldehyde is the organic compound with the formula HOCH2-CHO. It is the smallest possible molecule that contains both a hydroxyl group, it is a reactive molecule that occurs both in the biosphere and in the interstellar medium. It is supplied as a white solid. Although it conforms to the general formula for carbohydrates, Cnn, it is not considered to be a saccharide. Glycolaldehyde exists; as a solid and molten liquid, it exists as a dimer. In aqueous solution, it exists as a mixture of at least four species, which interconvert, it is the only possible diose, a 2-carbon monosaccharide, although a diose is not a saccharide. While not a true sugar, it is the simplest sugar-related molecule, it is reported to taste sweet. Glycolaldehyde is the second most abundant compound formed. Glycolaldehyde can be synthesized by the oxidation of ethylene glycol using hydrogen peroxide in the presence of Iron sulphate, it can form by action of ketolase on fructose 1,6-bisphosphate in an alternate glycolysis pathway.
This compound is transferred by thiamine pyrophosphate during the pentose phosphate shunt. In purine catabolism, xanthine is first converted to urate; this is converted to 5-hydroxyisourate, which decarboxylates to allantoic acid. After hydrolyzing one urea, this leaves glycolureate. After hydrolyzing the second urea, glycolaldehyde is left. Two glycolaldehydes condense to form erythrose 4-phosphate, which goes to the pentose phosphate shunt again. Glycolaldehyde is an intermediate in the formose reaction. In the formose reaction, two formaldehyde molecules condense to make glycolaldehyde. Glycolaldehyde is converted to glyceraldehyde; the presence of this glycolaldehyde in this reaction demonstrates how it might play an important role in the formation of the chemical building blocks of life. Nucleotides, for example, rely on the formose reaction to attain its sugar unit. Nucleotides are essential for life, because they compose the genetic information and coding for life, it is invoked in theories of abiogenesis.
In the laboratory, it can be converted to amino acids and short dipeptides may have facilitated the formation of complex sugars. For example, L-valyl-L-valine was used as a catalyst to form tetroses from glycolaldehyde. Theoretical calculations have additionally shown the feasibility of dipeptide-catalyzed synthesis of pentoses; this formation showed stereospecific, catalytic synthesis of D-ribose, the only occurring enantiomer of ribose. Since the detection of this organic compound, many theories have been developed related various chemical routes to explain its formation in stellar systems, it was found that UV-irradiation of methanol ices containing CO yielded organic compounds such as glycolaldehyde and methyl formate, the more abundant isomer of glycolaldehyde. The abundances of the products disagree with the observed values found in IRAS 16293-2422, but this can be accounted for by temperature changes. Ethylene Glycol and glycolaldehyde require temperatures above 30 K; the general consensus among the astrochemistry research community is in favor of the grain surface reaction hypothesis.
However, some scientists believe the reaction occurs within colder parts of the core. The dense core will not allow for irradiation; this change will alter the reaction forming glycolaldehyde. The different conditions studied indicate how problematic it could be to study chemical systems that are light-years away; the conditions for the formation of glycolaldehyde are still unclear. At this time, the most consistent formation reactions seems to be on the surface of ice in cosmic dust. Glycolaldehyde has been identified in gas and dust near the center of the Milky Way galaxy, in a star-forming region 26000 light-years from Earth, around a protostellar binary star, IRAS 16293-2422, 400 light years from Earth. Observation of in-falling glycolaldehyde spectra 60 AU from IRAS 16293-2422 suggests that complex organic molecules may form in stellar systems prior to the formation of planets arriving on young planets early in their formation; the interior region of a dust cloud is known to be cold. With temperatures as cold as 4 Kelvin the gases within the cloud will freeze and fasten themselves to the dust, which provides the reaction conditions conducive for the formation of complex molecules such as glycolaldehyde.
When a star has formed from the dust cloud, the temperature within the core will increase. This will cause the molecules on the dust to be released; the molecule will emit radio waves that can be analyzed. The Atacama Large Millimeter/submilliter Array first detected glycolaldehyde. ALMA consists of 66 antennas. On October 23, 2015, researchers at the Paris Observatory announced the discovery of glycolaldehyde and ethyl alcohol on Comet Lovejoy, the first such identification of these substances in a comet. "Cold Sugar in Space Provides Clue to the Molecular Origin of Life". National Radio Astronomy Observatory. September 20, 2004. Retrieved December 20, 2006
The Jmol applet, among other abilities, offers an alternative to the Chime plug-in, no longer under active development. While Jmol has many features that Chime lacks, it does not claim to reproduce all Chime functions, most notably, the Sculpt mode. Chime requires plug-in installation and Internet Explorer 6.0 or Firefox 2.0 on Microsoft Windows, or Netscape Communicator 4.8 on Mac OS 9. Jmol operates on a wide variety of platforms. For example, Jmol is functional in Mozilla Firefox, Internet Explorer, Google Chrome, Safari. Chemistry Development Kit Comparison of software for molecular mechanics modeling Jmol extension for MediaWiki List of molecular graphics systems Molecular graphics Molecule editor Proteopedia PyMOL SAMSON Official website Wiki with listings of websites and moodles Willighagen, Egon. "Fast and Scriptable Molecular Graphics in Web Browsers without Java3D". Doi:10.1038/npre.2007.50.1
A furanose is a collective term for carbohydrates that have a chemical structure that includes a five-membered ring system consisting of four carbon atoms and one oxygen atom. The name derives from its similarity to the oxygen heterocycle furan, but the furanose ring does not have double bonds; the furanose ring is a cyclic hemiketal of a ketohexose. A furanose ring structure consists of four carbon and one oxygen atom with the anomeric carbon to the right of the oxygen; the highest numbered chiral carbon determines whether or not the structure has a d-configuration or L-configuration. In an l-configuration furanose, the substituent on the highest numbered chiral carbon is pointed downwards out of the plane, in a D-configuration furanose, the highest numbered chiral carbon is facing upwards; the furanose ring will have either alpha or beta configuration, depending on which direction the anomeric hydroxy group is pointing. In a d-configuration furanose, alpha configuration has the hydroxy pointing down, beta has the hydroxy pointing up.
It is the opposite in an l-configuration furanose. The anomeric carbon undergoes mutarotation in solution, the result is an equilibrium mixture of alpha-beta configurations. Pyranose
The melting point of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium; the melting point of a substance depends on pressure and is specified at a standard pressure such as 1 atmosphere or 100 kPa. 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 always "the principle of observing the disappearance rather than the formation of ice", that is, the melting point. For most substances and freezing points are 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. 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 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 before freezing. The chemical element with the highest melting point is tungsten, at 3,414 °C; the often-cited carbon does not melt at ambient pressure but sublimes at about 3,726.85 °C. Tantalum hafnium carbide is a refractory compound with a high melting point of 4215 K. At the other end of the scale, helium does not freeze at all at normal pressure at temperatures arbitrarily close to absolute zero. 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 and a simple magnifier. The several grains of a solid are placed in a thin glass tube and immersed in the oil bath; the oil bath is heated and with the aid of the magnifier melting of the individual crystals at a certain temperature can be observed. In large/small devices, the sample is placed in a heating block, optical detection is automated; the measurement can 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 remote laboratory. For refractory materials the 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, 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. 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 this black-body; the temperature of the black-body is adjusted until a match exists between its intensity and that of the pyrometer filament. The true higher temperature of the black-body is determined from Planck's Law; the absorbing medium is 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 hi
Rhamnus is a genus of about 110 accepted species of shrubs or small trees known as buckthorns, in the family Rhamnaceae. Its species range from 1 to 10 meters tall and are native in east Asia and North America, but found throughout the temperate and subtropical Northern Hemisphere, more locally in the subtropical Southern Hemisphere in parts of Africa and South America. Both deciduous and evergreen species occur; the leaves are simple, 3 to 15 centimeters long, arranged alternately, in opposite pairs, or paired. One distinctive character of many buckthorns is the way the veination curves upward towards the tip of the leaf; the plant bears fruits which are red berry-like drupes. The name is due to the woody spine on the end of each twig in many species. In the United States, buckthorn is considered an invasive species by many local jurisdictions and state governments, including Minnesota and Wisconsin. Rhamnus species are shrubs or small to medium-sized trees, with deciduous or evergreen foliage.
Branches are end in a woody spine. The leaf blades are pinnately veined. Leaf margins are serrate or entire. Most species have yellowish green, bisexual or unisexual polygamous flowers. Calyx tube campanulate to cup-shaped, with 4 or 5 ovate-triangular sepals, which are adaxially ± distinctly keeled. Petals 4 or 5 but a few species may lack petals; the petals are shorter than the sepals. Flowers have 5 stamens which are surrounded by and equal in length the petals or are shorter; the anthers are dorsifixed. The superior ovary is free, with 2-4 chambers. Fruits are a 2-4 stoned, berrylike drupe, obovoid-globose or globose shaped. Seeds are obovoid or oblong-obovoid shaped, unfurrowed or abaxially or laterally margined with a long, furrow; the seeds have fleshy endosperm. Rhamnus has a nearly cosmopolitan distribution, with about 150 species which are native from temperate to tropical regions, the majority of species are from east Asia and North America, with a few species in Europe and Africa. North American species include alder-leaf buckthorn occurring across the continent, Carolina buckthorn in the east, cascara buckthorn in the west, the evergreen California buckthorn or coffeeberry and hollyleaf buckthorn in the west.
In South America, Rhamnus diffusus is a small shrub native to the Valdivian temperate rain forests of Chile. Buckthorns may be confused with dogwoods; the two plants are easy to distinguish by pulling a leaf apart. The genus has been divided into two subgenera, which are treated as separate genera: Subgenus Rhamnus: flowers with four petals, buds with bud scales, leaves opposite or alternate, branches with spines. Species include: Rhamnus alaternus – Italian buckthorn Rhamnus alnifolia – alderleaf buckthorn, alder-leaved buckthorn Rhamnus arguta – sharp-tooth buckthorn Rhamnus bourgaeana – endemic to Mallorca Rhamnus cathartica – common buckthorn, purging buckthorn Rhamnus crocea – redberry buckthorn, hollyleaf buckthorn Rhamnus davurica – Dahurian buckthorn Rhamnus diffusa Rhamnus globosa – Lokao buckthorn Rhamnus ilicifolia – hollyleaf redberry Rhamnus japonica – Japanese buckthorn Rhamnus lanceolata – lanceleaf buckthorn Rhamnus libanotica – Lebanese buckthorn Rhamnus ludovici-salvatoris – endemic to Mallorca Rhamnus lycioides Rhamnus palaestina – Rhamnus pallasii – Fisch.
& C. A. Meyer, Rhamnus persica – Persian Buckthorn, Boiss.& Hohen, Rhamnus petiolaris – Rhamnus pirifolia – island redberry buckthorn Rhamnus prinoides – shiny-leaf buckthorn Rhamnus pumila - dwarf buckthorn Rhamnus saxatilis – rock Buckthorn, Avignon buckthorn, Avignon berry Rhamnus serrata – sawleaf buckthorn Rhamnus smithii – Smith's buckthorn Rhamnus staddo – Staddo Rhamnus taquetii – Jejudo buckthorn Rhamnus utilis – Chinese buckthorn Subgenus Frangula: flowers with five petals, buds without bud scales, leaves always alternate, branches without spines. Species include: Rhamnus betulaefolia – birchleaf buckthorn Rhamnus californica – California buckthorn, coffeeberry Rhamnus caroliniana – Carolina buckthorn, Indian cherry Rhamnus frangula – alder buckthorn, glossy buckthorn, breaking buckthorn, black dogwood Rhamnus glandulosa – sanguino Rhamnus hintonii – M. C. Johnst. & L. A. Johnst. Rhamnus purshiana – cascara buckthorn Rhamnus rubra – red buckthorn Some species are invasive outside their natural ranges.
R. cathartica was introduced into the United States as a garden shrub and has become an invasive species in many areas there. It is a primary host of the soybean aphid, a pest for soybean farmers across the US; the aphids use the buckthorn as a host for the winter and spread to nearby soybean fields in the spring. Italian buckthorn, an evergreen species from the Mediterranean region, has become a serious weed in some parts of New Zealand on Hauraki Gulf islands. Buckthorns are used as food plants by the larvae of many Lepidoptera sp
Uncaria] of flowering plants in the family Rubiaceae. It has about 40 species, their distribution is pantropical, with most species native to tropical Asia, three from Africa and the Mediterranean and two from the neotropics. They are known colloquially as cat's claw or uña de gato; the latter two names are shared with several other plants. The type species for the genus is Uncaria guianensis. Indonesian Gambier is a large tropical vine with leaves typical of the genus, being opposite and about 10 cm long; the South American U. tomentosa is called Uña de Gato. Uncaria sinensis is common in China. Uncaria was named in 1789 by Johann von Schreber in his Genera Plantarum edition 8; the genus name is derived from the Latin word uncus, meaning "a hook". It refers to the hooks, formed from reduced branches, that Uncaria vines use to cling to other vegetation. Uncaria is a member of the tribe Naucleeae. Woody lianas. Stipules bifid. Inflorescences are compact heads at the ends of horizontal reduced branches.
Corolla lobes without appendages. Seeds with a long wing at each end, the lower wing bifid; the following species list may contain synonyms. Diplomat Edmund Roberts noted that, upon his visit to China in the 1830s, Chinese were using U. gambir for tanning, noted that the U. gambir made "leather porous and rotten." He noted that Chinese would chew it with areca nut. The plant extract contains some 150 identified phytochemicals, including catechins and chalcone-flavan-3-ol dimers, called gambiriins. Cat's claw and the Chinese Uncaria species are used in traditional medicine, although there is no high-quality clinical evidence they have any medicinal properties. Although cat's claw appears to be safe for human use below 350 milligrams per day over 6 weeks, its adverse effects may include nausea, upset stomach, an increased risk of bleeding if used with an anticoagulant drug. Germplasm Resources Information Network: Uncaria Search Uncaria, Royal Botanic Gardens, KewScience