Physical change definitional

In order to discuss energy and its interconversions, thermochemists have agreed on a number of terms and definitions. You will learn about these terms and definitions over the next few pages. Then you will examine the energy changes that accompany chemical reactions, physical changes, and nuclear reactions. 

Sublimation. Accdu to definition ziven in Perry (Ref 13, p660), it covers the physical changes encountered by a substance in passing from a solid phase to a gas and back to a solid phase. It is characterized by the absence of the liquid phase and is used for purification of volatile substances like iodine, camphor, etc. Detailed description of the process is given in Refs 12, 13, 15 18). 

Electrochemistry, according to the definition given in Perry .s, is the science which treats of the chemical changes produced by an electric current and of the production of electricity from the energy of chemical reactions. Theoretically, the two branches are of equal importance. Industrially, however, the chemical and physical changes produced by the use of an electric current are by far the most important... 

Although the model may be worked on in isolation and, as a system of mathematical equations, has a life of its own, it is at all stages part of an ongoing analysis of some other entity. This is sometimes represented as a commutative diagram in which the state of the nonmathematical (let us call it physical, for definiteness) system, S, is mirrored in the model, 2, by the modeling relationship h. Changes in the physical, say, Si - S2 = gS, are mirrored in the model as — 22 - The model is satisfactory if the di-... 

A pure substance has a definite composition, which stays the same in response to physical changes. A lump of copper is a pure substance. Water (with nothing dissolved in it) is also a pure substance. Diamond, carbon dioxide, gold, oxygen, and aluminum are pure substances, too. 

For the purposes of this discussion, fiber variants will be divided into two types chemical and physical. Chemical variants will be those involving a small but significant change in composition, whereas physical changes will be those involving a change in either the dimensions of the fiber or its stress/strain or stability features. The definitions of the two variants also could be based on modification of either esthetics or functionality. 

Lavoisier discovered the law of conservation of matter, which states that matter cannot be created or destroyed during chemical reactions or physical changes. This generalization increased chemists efforts to measure the masses of elements in compounds and resulted in two more laws. The law of definite proportions states that the percentage of each element in any sample of a pure compound is always the same. According to the law of multiple proportions, if the mass of one of the elements in two or more compounds of the same elements is held constant, the masses of each other element form a small, whole-number ratio. (Section 3.1)... 

As to whether the addition of sulphur is a chemical or physical change we shall say little. Evidence appears on both sides and all we need say here is that, in whichever manner the sulphur really acts, it affects the caoutcHouc in a very definite way, is absorbed by it, remains there in some kind of union and is unable to be removed by sulphur solvents. The amount of sulphur thus definitely held by the caoutchouc is about 3 per cent, in the case of soft rubber, while in hard rubber it may be as much as 32 per cent. In both cases more than this amount of sulpher is usually present but the excess is as free sulphur which may be removed by solvents. 

Snow is a porous medium formed of air, ice crystals and small amounts of chemical impurities. Because ice has a high vapor pressure (165 Pa at -15°C, 610 Pa at 0°C), the vertical temperature gradient that is almost always present within the snowpack generates sublimation and condensation of water vapor that change the size and shape of snow crystals. This results in changes in physical variables such as density, albedo, heat conductivity, permeability and hardness. These physical changes have formed the basis for the definition of snow metamorphism. ... 

The dissolution of an ionic salt is an example of physical change, since the ions are present in both solid salt and in solution. By definition, covalent substances contain no ions, but some covalent substances may produce ions as a result of their chemical reaction with water. The presence of such ions may be confirmed by testing the solution to see if it conducts electricity, but it is the new reactions of the solution - the reactions of the ions produced - that conclusively prove that chemical change has taken place. We now look at three important examples the reactions with water of hydrogen chloride, pure sulfuric acid and ammonia. 

Generally speaking, physical changes are more readily reversible than chemical changes. However, again this is not a very definitive criterion. The idea that chemical reactions tend to either go or not... 

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