Absorption physical changes

There is absorption of latent heat not only in physical changes of state (fusion, evaporation), but also in many chemical reactions which occur at a transition temperature. In all cases the transition temperature is more or less dependent upon the... 

This was got over by saying that in such cases there were physical changes in which solid salt became liquid, as well as chemical changes in which the salt combined with the water. The absorption of heat attending the first change exceeded the evolution in the second. To all such exceptions it was thought sufficient to answer that they were not of a purely chemical nature. ... 

Some drains act on the body by changing the cellular environment, either physically or chemically. Physical changes in the cellular environment include changes in osmotic pressures, lubrication, absorption, or the conditions on the surface of the cell membrane An example of a drag that changes osmotic pressure is mannitol, which produces a change in the osmotic pressure in brain cells, causing a reduction in cerebral edema A... 

The use of the term leuco dye is a common paradox. Leuco color formers are materials that undergo controlled chemical or physical changes resulting in a shift from a colorless state to an intense color. The preparation of leuco color formers takes advantage of the very nature of colored materials themselves. The existence of extended conjugated -system in dyes is responsible for the absorption in the visible region. The chemistry of such rc-system is noted for facile reactivity, particularly to reactions such as reduction, oxidation, and hydrolysis (not hydrolytic cleavage). When n-... 

In the preceding ten chapters of this book, we have described various important chemical and physical changes brought about by the absorption of ionizing radiation in gaseous and condensed media. Wherever possible, we have tried to elucidate the underlying mechanism with a discussion of the properties and reactivities of the intermediate species. However, the book would remain incomplete without discussion of some of the various uses that have been found for radiation-induced reactions in science and industry. 

All the chemical changes and many of the physical changes that we have studied so far involve alterations in the electronic structures of atoms. Electron-transfer reactions, emission and absorption spectra, and X rays result from the movement of electrons from one energy level to another. In all of these, the nuclei of the atoms remain unchanged, and different isotopes of the same element have the same chemical activity. Nuclear chemistry, or radioactivity, differs from other branches of chemistry in that the important changes occur in the nucleus. These nuclear changes also are represented by chemical equations. However, because the isotopes of the same element may, from a nuclear standpoint, be very different in reactivity, it is necessary that the equations show which isotopes are involved. 

Two factors determine the spontaneity of a chemical or physical change in a system a release or absorption of heat (AH) and an increase or decrease in molecular randomness (AS). To decide whether a process is spontaneous, both enthalpy and entropy changes must be taken into account ... 

Nitrate explosives. The term explosion is applied to the effect produced by a sudden change in the pressure of one or more gases. This may be the result of either chemical or physical changes—that is, the sudden liberation or absorption of gases in chemical reactions or the sudden formation of gases from either liquids or solids. Because certain nitrates decompose readily with liberation of gaseous products, these substances are useful in compounding a variety of commercial explosives. Ammonium nitrate is stable under ordinary atmospheric conditions and may be handled safely in small quantities, even at elevated temperatures. When the dry salt is heated, it decomposes with liberation of nitrous oxide and water,... 

Physical change in the polymer, including absorption of solvents... 

This idea has been further developed by Sutherland,2 who concludes that ordinary water vapour is monohydrol, H20 liquid water is an equilibrium mixture of tnhydrol, (H20)3, and dihydrol, (H20)2 whilst ice consists entirely of trihydrol. The high value found for the latent heat of fusion of ice thus receives explanation, for it is not due merely to the heat absorption consequent upon physical change of solid to liquid it is enhanced by the heat required to effect the simultaneous dissociation or depolymerisation of a large proportion of trihydrol molecules to the dihydrol form, as indicated by the thermal equation which Sutherland writes as ... 

Isoclinic point A wavelength, wavenumber, or frequency at which the first derivative of an absorption spectrum of a sample does not change upon a chemical reaction or physical change of the sample. 

Isooptoacoustic point A wavelength, wavenumber, or frequency at which the total energy emitted by a sample as heat does not change upon a chemical reaction or physical change of the sample. Its position depends on the experimental conditions. The spectral differences between the isosbestic points and the isooptoacoustic points are the result of the nonlinear relationship between the molar absorption coefficient and the photoacoustic signal. 

All chemical reactions are accompanied either by an absorption or evolution of energy, which usually manifests itself as heat. It is possible to determine this amount of heat—and hence the temperature and product composition—from very basic principles. Spectroscopic data and statistical calculations permit one to determine the internal energy of a substance. The internal energy of a given substance is found to be dependent upon its temperature, pressure, and state and is independent of the means by which the state is attained. Likewise the change in internal energy, A , of a system that results from any physical change or chemical... 

Another thermal theory suggests that fire retardants cause chemical and physical changes so that heat is absorbed by the chemical to prevent the wood surface from igniting. This thermal absorption theory is based on chemicals that contain much water of crystallization. 

Applications dependent upon changes in physical or chemical properties that accompany the more easily observed color change during the photochromic reaction. Examples of such properties are conductivity, refractive index, electrical moment, dielectric constant, chelate formation, ion dissociation, phase transitions, solubility, and viscosity. Certain physical changes that occur when the photochromic entity is chemically attached to the macromolecular backbone of polymers are of special interest (see Chapter 1). Some examples of potential applications utilizing the physical or chemical changes which accompany the observed shift in the absorption maxima are ... 

Chemical reactions and physical changes occur with either the simultaneous evolution of heat (exothermic processes) or the absorption of heat (endothermic processes). The amount of heat transferred in a process is usually expressed in joules or in calories. 

Energy changes accompany physical changes, too (Ohapter 13). For example, the melting of one mole of ice at 0°O at constant pressure must be accompanied by the absorption of 6.02 kJ of energy. 

DSC instruments are sensitive pieces of modem equipment, having the capability to measure heat flows of the order of microwatts. This feature makes the applicability of the technique almost unlimited every physical change or chemical reaction takes place with a change of enthalpy and consequently absorption or release of heat. 

A timp of treatment of 30 min was used. Preliminary experiments showed that no rapid changes took place after 30 min, although slow changes, thought to result from changes in properties of the fiber, occurred more or less continuously. The time chosen was selected as the best compromise between essentially complete absorption and minimum chemical or physical change. 

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