THE CHEMISTRY OF LIFE 45 



emulsifiers. Soap (sodium stearate) will have the sodium end of its 

 molecules soluble in water and the stearate end soluble in oil. Soap, 

 therefore, can remove oil from your hands, whereas plain water cannot. 

 The bile produced by the liver acts as an emulsifier of the fats which 

 we eat. When there is a deficiency of bile, there cannot be a proper 

 digestion of fats. 



Colloidal Dispersions. A colloidal dispersion is composed of 

 particles that are larger than the tiny units of matter in a solution, but 

 smaller than the relatively large particles of matter in a suspension or 

 an emulsion. The size of colloidal particles is usually given as lying 

 between about .001 and .1 micron in their greatest dimension. (A 

 micron is a thousandth of a millimeter.) This is the size that is just 

 below the limits of the ordinary optical microscope, and thus colloidal 

 dispersions appear homogeneous under such microscopes. Molecules 

 as a whole, on the other hand, lie below the smaller limits of size of 

 colloids. Colloids will not settle out or rise to the top upon standing 

 — the particles are so small that they are kept evenly dispersed by the 

 motion of the water molecules around them. In this respect they are 

 like solutions, but, unlike solutions, colloids do not cause any great 

 change in the chemical nature of the water in which they are dispersed. 



The white of an egg is a good example of a colloidal dispersion. 

 The albumin is present in the nature of finely divided particles dispersed 

 in water. This type of colloidal dispersion is sometimes called a col- 

 loidal suspension, but should not be confused with the true suspension 

 which we have previously discussed. Homogenized milk is another 

 example of a colloidal dispersion. In fresh cow's milk the oil or fat is 

 dispersed through the milk in rather large droplets and forms an ordi- 

 nary emulsion, for the cream will rise to the top of the milk upon 

 standing. When homogenized, however, the fat droplets are broken 

 up into much smaller sizes which fall within the range of colloids. 

 Thus, there will be no cream at the top of homogenized milk after it 

 stands. Such a colloidal dispersion is sometimes called a colloidal 

 emulsion, but again we should keep this distinct from the true emulsion 

 previously described. Except for a few cases, solid colloidal particles 

 do not form crystals when the water is evaporated. 



Sols and Gels. Before leaving our discussion of colloids we should 

 mention their unique ability to change from a liquid to a solid or semi- 

 solid state and back again. If we mix gelatin with hot water, the 

 particles of gelatin form a colloidal dispersion and a liquid results. 

 This is called the sol state. The water forms a continuous phase and 

 the particles of gelatin are in a discontinuous phase. As this mixture 

 cools, however, the gelatin particles come together to form the con- 



