Pure substances crystalline

This agrees quite well with the rate constants for intramolecular proton transfer in 2,4-bis(dimethyl-amino )-6-(2-hydroxy-5-methylphenyl)-5-triazine which had been measured by Shizuka et al. ( l6) using laser picosecond spectroscopy. The fluorescence decay constant t of (TIN) was found to be 60 20 ps. Because of the weak intensity all fluorescence lifetimes refer to the pure substance in crystalline form at room temperature. 

In contrast to sulfurous acid, H2S03, which is not known as pure substance, the respective selenium compound can be obtained in crystalline form. H2Se03 crystallizes with the non-centrosymmetric space group P2 2,2] and contains pyramidal H2Se03 molecules.54 Within the molecules the bond lengths Se-O and SeOH differ by about 10 pm and the molecules are connected to puckered layers by strong hydrogen bonds. 

The Third Law of Thermodynamics states that the entropy of a pure, perfect crystalline substance is zero at 0 K. 

Spectroscopists interested in elucidation of the molecular energy schemes studied the phosphorescence emission of over 200 compounds, of which 90 were tabulated by Lewis and Kasha in 1944. They classified phosphorescing substances in two classes, based on the mechanism of phosphorescence production. The first group comprises minerals or crystals named phosphors, where the individual molecule is not phosphorescent as such, but emits a shining associated with the presence of some impurity localized in the crystal. This type of phosphorescence cannot be attributed to a concrete substance. The second type of phosphorescence emission is attributed to a specific molecular species, being a pure substance in crystalline form, adsorbed on a suitable surface or dissolved in a specific rigid medium [22],... 

All the material world is formed of mixtures, aggregates or more complex combinations of pure substances. For example, it is well known that the bark of the Cinchona tree Cinchona calisaya) shows a remarkable antimalarial activity, which is due, not to the bark as such, but to some "pure substance" which forms an integral part of it. In 1820, the French pharmacists Pelletier and Caventou isolated the active principle of the Cinchona bark, which they called quinine, as a pure, crystalline substance, m.p. 177 °C (dec), -169°, and assigned an elemental... 

Nemst asserted his postulate although the available data were inconclusive. In fact, Richards extrapolated some of his data to give a graph such as that shown in Figure 11.2, which suggests that Equation (11.1) is valid but that Equation (11.3) is not. Numerous subsequent experiments have conhrmed Nemst s postulate if it is limited to perfect crystalline systems. Apparent exceptions have been accounted for satisfactorily. The term perfect implies a single, pure substance. Other restrictions are implied by this term, but they will be discussed later. 

Third Law of Thermodynamics. Also referred to as the Nernst heat theorem, this law states that it is impossible to reduce the temperature of any system, via a finite set of operations, to absolute zero. For any changes involving perfectly crystalline solids at absolute zero, the change in total entropy is zero (thus, A5qk = 0). A corollary to this statement is that every substance, at T > 0 K, must have a positive and finite entropy value. The entropy of that substance is zero only at absolute zero when that substance is in pure, perfect crystalline form. See Entropy... 

Polymerization in electrostatic systems like the ones mentioned above is stericaUy inhibited by alkyl substitution at the a-carbon which must assume a coordination number greater than 4. Coates and Glockhng have treated this inhibition of polymerization in terms of decreased electronegative character of the branched alkyl groups. Therefore, stimulated by the idea that f-afkylhthium compounds may exist as low polymers or even as monomeric molecules, Weiner and coworkers and Kottke and Stalke have isolated f-butyllithium as a pure substance for the first time and characterised it by spectroscopic methods and X-ray diffraction. The colourless crystalline solid was found to be tetrameric over a range of concentrations in both benzene and hexane ... 

The third law of thermodynamics says that the entropy of pure, perfect crystalline substance is zero at absolute zero. But, in actual practice, it has been found that certain chemical reactions between crystalline substance, do not have DS = 0 at 0°K, which indicates that exceptions to third law exist. Such exceptional reactions involve either ice, CO, N2O or H2. It means that in the crystalline state these substances do not have some definite value of entropy even at absolute zero. This entropy is known as Residual Entropy. At 0°K the residual entropies of some crystalline substances are... 

Sulfur dioxide dissolves in water, forming a weak acid solution of sulfiirous acid, H2S03 however, the pure substance H2S03 has never been isolated. At 101.3 kPa (1 atm) sulfur dioxide pressure, the solubility of sulfur dioxide is 18.5% at 0°C and 5.1% at 40°C. Lower solubilities are observed when other diluent gases, eg, air, are present. At 25°C, the hypothetical H2S03 has a first ionization constant of 1.72 x 10-2 and a second ionization constant of 1.1 x 10-9. Atlow temperatures, concentrated sulfurous acid affords a crystalline hydrate, S02 6H20 [24402-69-5]. 

The bluish-green crystalline mass is very unstable and decomposes fairly rapidly even at the ordinary temperature, giving sulphur dioxide and sulphur. The pure substance may be preserved for several hours in a vacuum. It is hygroscopic, and on absorbing moisture first becomes brown and then deposits sulphur, with concurrent formation of sulphur dioxide, sulphuric acid and possibly other sulphur acids. It is soluble in fuming sulphuric acid, an acid rich in trioxide giving a bluish solution, whilst with less rich acids the solution is brown ordinary pure sulphuric acid causes decomposition. The colour of the brown solution is regarded by some as due to colloidal sulphur formed by partial decomposition of the sesquioxide.6... 

An apparatus lor carrying out a dialysis usually consists of two chambers separated by a semipermeable membrane of parchment paper latex, animal lissue. or oilier colloid. In one chamber the solution is placed, and in Ihc other, the pure solvent. Crystalline substances diffuse from the solution through the membrane and into the solvent much more rapidly than amorphous substances, colloids or large molecules. 

Tihe term lyotropic mesomorphism is used to describe the formation of thermodynamically stable liquid crystalline systems through the penetration of a solvent between the molecules of a crystal lattice. In contrast to the thermotropic mesomorphism shown by many pure substances, lyotropic mesomorphism always requires the participation of a solvent. Lyotropically mesomorphous systems, however, are usually as sensitive to changes in temperature as thermotropic systems. So far, lyotropic mesomorphism has been observed almost exclusively in lipid systems containing water. Lipids that show lyotropic mesomorphism frequently... 

Dissolve 34 g (0.25 mol) of o-nitroaniline in a warm mixture of 63 ml of concentrated hydrochloric acid and 63 ml of water contained in a 600-ml beaker. Place the beaker in an ice-salt bath, and cool to 0-5 °C while stirring mechanically the o-nitroaniline hydrochloride will separate in a finely divided crystalline form. Add a cold solution of 18 g (0.26 mol) of sodium nitrite in 40 ml of water slowly and with stirring to an end-point with potassium iodide-starch paper do not allow the temperature to rise above 5-7°C. Introduce, while stirring vigorously, a solution of 40 g (0.36 mol) of sodium fluoroborate in 80 ml of water. Stir for a further 10 minutes, and filter the solid diazonium fluoroborate with suction on a Buchner funnel. Wash it immediately once with 25 ml of cold 5 per cent sodium fluoroborate solution, then twice with 15 ml portions of rectified (or industrial) spirit and several times with ether in each washing stir the fluoroborate well before applying suction. The o-nitrobenzenediazonium fluoroborate weighs about 50 g (86%) the pure substance melts with decomposition at 135 °C. 

Stilbenoid dendrimers are able to undergo aggregation. Depending upon the generation number, some of the pure substances form liquid-crystalline phases (Dha discotic hexagonal disordered phase Dra discotic rectangular disordered phase Dob discotic distorted phase). Differential scanning calorimetry (DSC) revealed phase transitions between 99°C and 0°C. 

In the first discussion of equilibrium (Ch. 5) we recognized that there may be states of a system that are actually metastable with respect to other states of the system but which appear to be stable and in equilibrium over a time period. Let us consider, then, a pure substance that can exist in two crystalline states, a and p, and let the a phase be metastable with respect to the p phase at normal temperatures and pressures. We assume that, on cooling the a. phase to the lowest experimental temperature, equilibrium can be maintained within the sample, so that on extrapolation the value of the entropy function becomes zero. If, now, it is possible to cool the p phase under the conditions of maintaining equilibrium with no conversion to the a phase, such that all molecules of the phase attain the same quantum state excluding the lattice vibrations, then the value of the entropy function of the p phase also becomes zero on the extrapolation. The molar absolute entropy of the a phase and of the p phase at the equilibrium transition temperature, Tlr, for the chosen... 

Some pure substances from a glass-like solid, on cooling, rather than a crystalline solid. In such solids the molecules have a certain randomness in their spatial distribution, more like a liquid than a crystal. This is an example in which a metastable state rather than an equilibrium state is obtained on cooling. If such a solid is obtained on cooling to the lowest experimental temperature, then the value of the entropy function at 0 K, obtained on extrapolation, will be greater than zero. 

Third law of thermodynamics The entropy of a pure substance in a perfect crystalline state is zero at absolute zero. 

The modem method for quantifying the goodness of fit between two adjacent grains in a pure poly crystalline substance (homophase interfaces) or in a multiphase solid (heterophase interfaces) counts the number of lattice points (not... 

When a pure substance is liquefied from the solid state or vaporized from the liquid at constant pressure, there is no change in temperature but there is a definite transfer of heat from the surroundings to the substance. These heat effects are commonly called the latent heat of fusion and the latent heat of vaporization. Similarly, there are heats of transition accompanying the change of a substance from one solid state to another for example, the heat absorbed when rhombic crystalline sulfur changes to the monoclinic structure at 95°C and 1 bar is 360 J for each gram-atom. 

Most pure substances have a definite melting temperature below which the change from a random liquid structure to a well ordered, periodic crystalline structure can occur this transformation is called crystallisation the reverse process is called melting. 

The pure substance is a white crystalline solid and the technical grade is a soft, waxy solid which contains about 72% of heptachlor and 28% of related compounds. Heptachlor may be found as an impurity in chlordane. M.p. 95° to 96° (pure substance), 46° to 74° (technical grade). 

A homogeneous crystalline material is not necessarily a pure substance. Thus natural crystals of sulfur are sometimes deep yellow or brown in color, instead of light yellow. They contain some selenium, distributed at random throughout the crystals in place of some of the sulfur, the crystals being fiomogeneous. and with faces as well formed as those for pure sulfur. These crystals are a crystalline solution (or solid solution). The gold-copper alloy used in jew elry is another example of a crystalline solution. It is a homogeneous material, but its composition is variable. 

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