GENERAL NATURE OF COLLOIDAL SYSTEMS 33 



owe many of their most distinctive properties to this fact. Few streams or 

 bodies of water are entirely free from matter in the colloidal condition. 

 Clouds, fogs, mists, and smoke also represent matter in the colloidal state. 



General Nature of Colloidal Systems. — If a little sucrose or sodium 

 chloride be shaken up in water the solid will soon disappear, and the result- 

 ing system will be a solution. Although there is abundant evidence which 

 indicates that the solute particles are dispersed throughout the solvent, mole- 

 cules and ions are so small that it is impossible to detect their presence by 

 direct observation, even with the most powerful optical systems available. 



If, instead of sucrose or sodium chloride, we stir some fine river bottom 

 silt into water, a different sort of a system will result. The silt particles do 

 not dissolve but simply become dispersed throughout the liquid. Such a 

 system is called a suspension. The particles in it are of such size that they 

 can readily be detected under a microscope. Suspensions are not stable, how- 

 ever, as the particles gradually settle out, and the system becomes separated 

 into its two original components within a relatively short period of time. 



Similar systems can be prepared by vigorously shaking together two im- 

 miscible liquids such as oil and water. Such sj'stems are called emulsions. 

 Emulsions are not stable unless there is also present in the system a third 

 component called an emu/sifier. 



Still another type of system consisting of particles dispersed through water 

 can be prepared. If a trace of sulfur be dissolved in a small volume of 

 alcohol which is then poured into a somewhat larger volume of water, a cloudy 

 opalescent liquid will result which is composed of sulfur particles dispersed 

 through water. This type of system is intermediate in its properties between 

 solutions and suspensions. The dispersed particles are not molecules, but, as 

 in suspensions, aggregates of molecules. Unlike suspensions, however, such 

 systems are relatively stable, as the particles will remain dispersed throughout 

 the liquid indefinitely. The dispersed particles of such systeins cannot be 

 seen under a microscope, showing that they are smaller than the particles in 

 suspensions or emulsions. They can, however, be detected with the aid of an 

 ultramicroscope (Chap. V). The sulfur-in-water system which has just been 

 described is a simple example of a colloidal system. 



Colloidal systems, as the preceding discussion has indicated, are two- 

 phased systems like solutions. Unlike solutions, however, the particles of the 

 dispersed phase are not in the molecular or ionic condition, but — with certain 

 exceptions to be noted shortly — are molecular aggregates. One colloidal 

 particle is often composed of hundreds or even thousands of molecules lumped 

 together. The molecular aggregates must not be so large, however, that the 

 particles readily settle out of the system, as stability is one of the essential 



