Chemical substances simple

A single chemical substance, simple or compound, exists at once in two different solid forms, such as the yellow iodide of mercury with the red iodide. 

Relatively simple notions of attractive forces between opposite charges are suffi cient to account for many of the properties of chemical substances You will find it help ful to keep the polarity of carbon-oxygen and carbon-halogen bonds m mind as we develop the properties of alcohols and alkyl halides m later sections... 

The most suitable method of fast and simple control of the presence of dangerous substances is analytical detection by means of simplified methods - the so-called express-tests which allow quickly and reliably revealing and estimating the content of chemical substances in various objects. Express-tests are based on sensitive reactions which fix analytical effect visually or by means of portable instalments. Among types of indicator reactions were studied reactions of complex formation, oxidation-reduction, diazotization, azocoupling and oxidative condensation of organic substances, which are accompanied with the formation of colored products or with their discoloration. 

Chemicals are classed as either elements or compounds. The former are substances which cannot be split into simpler chemicals, e.g. copper. There are 90 naturally-occuiTing elements and 17 artificially produced. In nature the atoms of some elements can exist on their own, e.g. gold, whilst in others they link with other atoms of the same element to form molecules, e.g. two hydrogen atoms combine to form a molecule of hydrogen. Atoms of different elements can combine in simple numerical proportions 1 1, 1 2, 1 3, etc. to produce compounds, e.g. copper and oxygen combine to produce copper oxide hydrogen and oxygen combine to produce water. Compounds are therefore chemical substances which may be broken down to produce more than one element. Molecules are the smallest unit of a compound. 

The phase rule is a mathematical expression that describes the behavior of chemical systems in equilibrium. A chemical system is any combination of chemical substances. The substances exist as gas, liquid, or solid phases. The phase rule applies only to systems, called heterogeneous systems, in which two or more distinct phases are in equilibrium. A system cannot contain more than one gas phase, but can contain any number of liquid and solid phases. An alloy of copper and nickel, for example, contains two solid phases. The rule makes possible the simple correlation of very large quantities of physical data and limited prediction of the behavior of chemical systems. It is used particularly in alloy preparation, in chemical engineering, and in geology. 

The drying a chemical substance is not a simple process. Drying a mass of finely divided solid particles carrying 30 to 40% water, for example the rate of evaporation is constant and high as long as the surfaces exposed are wet. After the surface is dry, the water in the interstices must make its way to the surface, a process of diffusion that is slower than evaporation from a wet surface the rate will then drop. This second part of the process must be modified according to the case with which the material crumbles as it dries, exposing new surfaces. 

The ionic model describes a number of metal halides, oxides, and sulfides, but it does not describe most other chemical substances adequately. Whereas substances such as CaO, NaCl, and M 2 behave like simple cations and anions held together by electrical attraction, substances such as CO, CI2, and HE do not. In a crystal of Mgp2, electrons have been transferred from magnesium atoms to fluorine atoms, but the stability of HE molecules arises from the sharing of electrons between hydrogen atoms and fluorine atoms. We describe electron sharing, which is central to molecular stability, in Chapters 9 and 10. 

Sometimes it so happens that crystals of a new salt are formed when solutions of two simple salts are mixed and the mixed solution is evaporated. The salt thus obtained is a distinct chemical substance in the solid state as well as in solution. In aqueous solution, it does not dissociate into all the simple ions of the salts it is obtained from, but yields complex ions along with the simple ions. Such a salt is known as a complex salt. A characteristic feature of complex salts is that in these the constituents retain their separate entities both in the solid state and in solution. Potassium ferrocynide, K4Fe(CN)6, is a complex salt and is obtained on mixing the solution of a ferrous salt with an excess of potassium cyanide solution. From its composition [Fe(CN)2,4 KCN], it appears to be a mixture of ferrous cyanide and potassium cyanide in the ratio of 1 4, and is thus taken to be an ordinary double salt. This representation of the compound is, however, not satisfactory since it responds neither to tests for Fe2+ ions nor to those for CN ions but does respond to tests for K+ ions and tetravalent Fe(CN)Jj ions. The ionization reaction of the complex salt cited in the present example can be represented as ... 

An indirect method to determine reaeration in gravity sewers was developed by Parkhurst and Pomeroy (1972). They made the measurements in gravity sewers where the biofilm was removed mechanically followed by a shock load with caustic soda. During the measurement period, the biological activity was suppressed in the water phase by a chemical substance. Measurement of upstream and downstream DO concentrations in the sewer determined the reaeration by using a simple DO mass balance. 

A chemical substance or simple mixture of substances obtained from natural sources by distillation or extraction such as citral from lemongrass oil or eugenol from clove bud. 

Chemists working on the same problems could only know they were studying the same substances if they spoke the same language. The chemical language, like chemical instruments, defined the discipline.46 The instruments and the nomenclature were illustrated in elaborate diagrams and "tableaus." Lavoisier s "Tableau des substances simples" is one of the most famous. (See fig. 1.) Here he organizes thirty-three simple substances into four categories ... 

In snmmary, many of the specific chemicals in petroleum are hazardous because of their chemical reactivity, fire hazard, toxicity, and other properties. In fact, a simple definition of a hazardons chemical (or hazardous waste) is that it is a chemical substance (or chemical waste) that has been inadvertently released, discarded, abandoned, neglected, or designated as a waste material and has the potential to be detrimental to the environment. Alternatively, a hazardons chemical may be a chemical that may interact with other (chemical) snbstances to give a prodnct that is hazardous to the environment. Whatever the case, methods of analysis mnst be available to determine the nnrture of the released chemical (waste) and from the data predict the potential hazard to the environment. 

Chemistry affects every aspect of our daily lives. Even something as simple as frying sausages involves chemical processes And while it is well known that, say, car batteries contain acid, how often do we think of all the acids around us in the kitchen Yet a few simple tests will prove their presence, Obviously, far more complicated chemical processes are involved in the industrial manufacture of synthetic materials. But however they occur, naturally or otherwise, all chemical substances are made up of the basic elements, whose atomic structure is the key to their behavio r. 

Stoichiometry. Such a complicated word for such a simple idea. The Greek roots of the word mean measuring elements, which doesn t sound nearly as intimidating. Moreover, the ancient Greeks couldn t tell an ionic bond from an Ionic column, so just how technical and scary could stoichiometry really be Simply stated, stoichiometry is the quantitative relationship between components of chemical substances. In compound formulas and reaction equations, you express stoichiometry by using subscripted numbers and coefficients. 

The table of values contains a list of all the chemical substances (except carbon compounds containing more than 2 carbon atoms), for which there are thermochemical data, together with values for the heats of formation (or heat of transition, fusion, or vaporization) of each substance. It was deemed expedient not to include in the present table values for the heats of formation of carbon compounds containing more than two carbon atoms. It should be possible to obtain from this table, by simple addition and subtraction, the heat of any process involving any of the substances given, with an accuracy as great as is actually known. There are listed in the table 5840 values of heats of formation, and, in addition, 350 values of heats of transition, fusion, vaporization, or reaction, for substances for which no values are given for the heat of formation. 

The simplest example of a flame-supporting medium is a pure chemical compound which decomposes exothermically. The widespread interest in such flames is due to their possibilities as monopropellants. Many studies are motivated by purely fundamental considerations, since a decomposition flame can be a kinetically simple flame. The most widely used and studied combustion reactions are those between hydrocarbons or hydrocarbon derivatives and air or oxygen. However, many other chemical substances may be substituted for the common fuels and/or oxidizers. Flames of uncommon fuels and oxidizers are most important because of their possibility of surpassing ordinary hydrocarbon oxidation as a source of energy. Some unusual flames are discussed in reference (PI). 

This simple model provides reliable estimates for bioconcentration factors in fish of non-metabolizable hydrophobic chemical substances with log Kow between 1 and 5. It also reliably estimates bioaccumulation factors of the same substances under field conditions,... 

Absorption spectrometry is a traditional method used for the measurement of various chemical substances and makes it possible to carry out visual colorimetry allowing easy measurements. Conventional absorption spectrophotometry is the measurement of numerical values such as that of absorbance to carry out qualitative and quantitative analysis. In such cases, if the spectra obtained are complicated, the determination often becomes difficult. However, even if the spectral changes are quite complicated, our eyes recognize them simply as color changes. Determination utilizing the colors themselves is a perceptual method instead of simple absorption spectrophotometry. 

The selective absorption of ultraviolet, visible and infrared radiation by molecules is explained in a descriptive manner that stresses how the noncontinuous energy requirements of chemical substances can only be satisfied by photons that have energy values equivalent to that of the differences in energy levels of the molecule in question. The meaning and quantitative significance of Beer s Law is briefly discussed. The components of a simple spectrophotometer are illustrated, accompanied by a demonstration of the operation of a spectrophotometer in the laboratory. Actual applications of the techniques of spectrophotometry are described during the presentation of relevent topics, for example, in drug identification. 

The third fundamental component in the QSAR model is the mathematical algorithms. Many methods have been used, and in the last years, there has been an increase of the methods, and hence, quite probably this trend will continue, introducing many other methods [4—6]. Classical QSAR methods, used decades ago, were simple linear relationships. Corwin Hansch has been a pioneer of these methods [2]. An example can be the linear relationship between the fish toxicity and the partition coefficient between octanol and water, called Kow [3]. Kow, and its logarithm, called log P, is still the most popular chemical descriptor used in QSAR models for fish toxicity, and it is the base of software programs used by the US Environmental Protection Agency for fish toxicity [11]. The theoretical assumptions for the use of log P are that (1) octanol mimics the lipophylic component of the fish cell, and (2) the toxic effect is due to the adsorption of the chemical substance into the cell. 

For theoretical chemistry to succeed it must develop the power to elucidate the behaviour of chemical substances to the satisfaction of experimental chemists, known to operate at many different levels. Understanding is not promoted by the generation of numbers, however accurate or numerous, without a simple picture that tells the story. It is inevitable that the chain of reasoning must reduce the problem of understanding the behaviour of substances, to the understanding of molecules, atoms, electrons, and eventually the aether. Again, this ladder of understanding should not be obscured by complicated mathematical relationships that cannot be projected into a simple picture. Small wonder that the planetary model of the atom, inspired by Kepler, and discredited almost a hundred years ago, is still the preferred icon to represent nuclear installations and activity in the commercial world. Theoretical chemistry should also communicate with the predominantly nonscientist population of the world, but in order to tell a story it is first of all necessary to know the story. 

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