Spreading of Substances

Until now, when we have considered the chemical potential we have primarily concentrated upon chemical reactions and phase transitions. However, there is another characteristic of substances that is almost as important. This is the tendency to spread out or disperse in space whether it is empty or filled with matter. This phenomenon can easily be illustrated by everyday processes. Mostly, substances migrate extremely slowly and in infinitesimally small amounts so that this migration remains imperceptible. However, there are many examples of spreading that 

Experiment 12.1 Redistribution of water between zwieback and bread A piece of zwieback stored for 2 or 3 days in a bread box or plastic bag together with fresh bread absorbs moisture and becomes soft and bendable, while a slice of bread in a bag full of zwieback becomes hard and brittle from losing moisture to the dry zwieback. 

Experiment 12.2 Spreading ofKMn04 in agar gel (view from above on a thin layer of gel in a Petri dish) A few small KMn04 crystals are cautiously distributed on the agar gel. Immediately, a kind of red violet halo is formed around each crystal. Because of its color, the spreading of the halo away from the source can be observed easily. 

It is easy to foUow the spreading of strong smelling or colored substances. The scent of a bouquet of lilacs, a peeled orange, or the pungent odor of potent cheese quickly fills a whole room. The spreading of colored low-molecular chemicals such as potassium permanganate in a liquid, or better, in a gel (to hinder convection) happens so fast that it can readily be observed (Experiment 12.2). 

Gases spread out in the atmosphere even faster. This can be observed very easily in the case of reddish-brown bromine vapor (Experiment 12.3). 

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The sharpening and spreading of substance zones are determined by sorption conditions the formation of a sharp boundary at X, A > 4 and boundary spreading at X, A < 4. 

Depending on their physical, chemical and biochemical properties and the specific conditions under which they are released, environmental chemicals may be more or less widely dispersed or accumulate in a particular sector of the environment. Both the global spread of substances and regional pollution have to be taken into account. 

These examples may suffice to show that spreading of substances is a very general characteristic of matter. 

In the following, we will apply this statement to the spreading of substances and other phenomena related to it. Chapter 13 will deal with the special features that result when the mle mentioned above no longer applies which is the case at higher concentrations. 

Diffusion In Chap. 12, we were introduced to the driving force behind the spreading of substances that we call diffusion. This is the diffusion force Fb, which is dependent upon the gradient of the chemical potential ... 

A discussion of the chemical drive of solvation and hydration processes, respectively, leads to the introduction of the basic concept of electrolytic dissociation, the disintegration of a substance in solution into mobile ions. Subsequently, we learn about the migration of these ions along an electric potential gradient as a special case of spreading of substances in space. The ionic mobilities provide a link to conductance and the related quantities conductivity as well as molar and ionic conductivity. For determining the conductivity of ions experimentally, the introduction of the term transport number which indicates the different contribution of ions to the electric current in electrolytes is very useful. In the last section, the technique for measuring conductivities is presented as well as its application in analytical chemistry where conductometric titration is a routine method. 

In the previous chapter we dealt with rates of spreading of substances and their causes. Migration of ions is influenced by additional forces caused by the electric charge of the particles. The force F acting upon a charge (2 in an electric field E is given by... 

Design and operate processes and activities to minimize emission, release and spread of substances hazardous to health. 

Choose the most effective and reliable control options which minimise the escape and spread of substances hazardous to... 

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