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What Is Melting Points

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Most organic compounds have melting points below 200 ºC. Some decompose before melting, a few sublime, but a majority undergo repeated melting and crystallization without any change in molecular structure. When a pure crystalline compound is heated, or a liquid cooled, the change in sample temperature with time is roughly uniform. However, if the solid melts, or the liquid freezes, a discontinuity occurs and the temperature of the sample remains constant until the phase change is complete. This behavior is shown in the diagram on the right, with the green segment representing the solid phase, light blue the liquid, and red the temperature invariant liquid/solid equilibrium. For a given compound, this temperature represents its melting point (or freezing point), and is a reproducible constant as long as the external pressure does not change. The length of the horizontal portion depends on the size of the sample, since a quantity of heat proportional to the heat of fusion must be...
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What Is Atomic And Molecular Orbitals

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A more detailed model of covalent bonding requires a consideration of valence shell atomic orbitals. For second period elements such as carbon, nitrogen and oxygen, these orbitals have been designated 2 s , 2p x , 2p y   & 2p z . The spatial distribution of electrons occupying each of these orbitals is shown in the diagram below.   Very nice displays of orbitals may be found at the following sites: J. Gutow, Univ. Wisconsin Oshkosh R. Spinney, Ohio State  M. Winter, Sheffield University The valence shell electron configuration of carbon is 2 s 2 , 2p x 1 , 2p y 1  & 2p z 0 . If this were the configuration used in covalent bonding, carbon would only be able to form two bonds. In this case, the valence shell would have six electrons- two shy of an octet. However, the tetrahedral structures of methane and carbon tetrachloride demonstrate that carbon can form four equivalent bonds, leading to the desired octet. In order to explain this covalent bonding,...
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Is Coffee Good for You

Is coffee good for you is something which has definitely flashed in the minds of coffee aficionados. The aroma is tantalizing and heavenly . The taste is alluring and takes you on a different plane of gastronomic delight. All over the world it is amongst the most popular beverages. Any guesses? Well, actually it would not have been difficult, eh? Because it is our beloved beverage, the coffee! Have it hot or have it cold, it is awesome nevertheless. But it is not without its baggage of positives and negatives on account of its main substance, caffeine. Hence, the skepticism is coffee good for you revolves around this drink. Take a look as to what makes coffee good or bad for you. Is Coffee Good for Your Health? Too much of everything spoils the party. The same goes with coffee. However, for the coffee enthusiasts, there is a good news that the health benefits of coffee outweigh the side effects, but the amount consumed has to be in moderation. The first and foremost r...
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