Physical changes of state

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... 

The substitution of more benign solvents in many formulations will have global ramifications. For example, water-reducible coatings are products where the solvent system used to disperse and suspend solids is primarily water. The remainder of the solvent system may contain liquids that are classified as flammable or combustible liquids. Inherent limitations in these systems relate to product viscosity, accuracy in predicting overall fire hazard, abihty to assess physical changes of state when a product is tested, and test method rehabihty and ease of use (Scheffey and Tabar, 1996). 

Physical changes of state are observable under suitable conditions as well-defined phenomena. However the very frequent occurrence of superheating and supercooling in liquids, supersaturation of vapors (e.g., in closed chambers), and the persistence of metastable solids (e.g., monoclinic sulfur at 0°C) show that these phase changes can be at times exceedingly... 

Hindering influences characteristic of chemical change From the molecular-mechanical point of view, chemical change, as we have remarked, has, in common with physical change of state, that movements in space are only necessary in the case of polymolecular reactions, as with evaporation and condensation that in changes taking place in solids, orientation of the molecules is necessary, as in solidification and that, in the latter case especially, capillary influences, e.g. presence of the substance to be formed, may make themselves felt. All these mechanical necessities correspond to influences that stand in the way... 

Thus, the rate expression is replaced by an algebraic constraint on the concentrations. This treatment is customary for physical changes of state such as vaporization and is often applied to fast chemical steps as well. Note that the true equilibrium condition of vanishing TZj is replaced here by the less stringent condition of Eq. (2.5-2a), which permits step j to proceed at a nonzero rate in response to concentration changes caused by other reactions. 

S7 The study of chemical reactions and the study of physical changes of state depend on the same theory, that of chemical mechanics.—These observations clearly show that chemical reactions and physical changes of state sometimes obey exactly fflmilar laws consequently every theory applicable to chemical reactions in general should include also physical changes of state. 

Thomsen took care to limit the application of this law to purely chemical reactions it is but too clear in fact that it may not be applied to physical changes of state at a temperature kept constant, one may observe the vaporization of a liquid, the fuedon of a solid, the solution of sea-salt in water, yet all these changes of state absorb heat. 

The chemical actions are not the only ones which make exception to the rules laid down by thermodynamics the physical changes of state and allotiopic modifications also furnish objections to this theory. 

The application of the second law of thermodynamics to chemistry, first occurring notably in the study of dissociation phenomena in solids, influenced Horstman in 1873 to point out that such changes are exactly similar to physical changes of state, and that the thermodynamic equation derived by Clapeyron and Clausius for changes of state are also applicable here, i.e., dp/dT QjT d - v), where p is the dissociation pressure, Q is the heat of dissociation, T is the absolute temperature, and C and v are the volumes of the system after and before dissociation. In the case in which a gas or vapor formed is supposed to behave as an ideal gas, and the volume of the solids is neglected in comparison with that of the gas, the equation becomes d log p/dT Q/RT, where R is the universal gas constant. ... 

Calorimeter n. An instrument for measuring the heat liberated (or absorbed) during chemical reactions or physical changes of state. 

The physical appearance of a substance can change in other ways too. Suppose that you dissolve some salt in water. The appearance of the salt changes, but you could re-form the salt crystals by evaporating the water. Thus in a physical change of state, no new substances are produced. Table 3.3 gives more examples of physical changes. 

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