44 



THE CHEMISTRY OF LIFE 



come evenly distributed in the water. The water will appear milky. 

 Each of these tiny droplets of oil will consist of many molecules, 

 because olive oil will not go into solution in water. Such a mixture 

 of a liquid in a liquid is known as an emulsion. If we allow the mix- 

 ture to stand for a time, the oil droplets will rise to the surface and 

 soon the oil will be separated from the water. 



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Photo by Winchester 



Fig. 3.3. The four types of dispersions which may occur in water according to the 

 size of particles of the dispersed substance. The first photograph shows the substances 

 before mixing with water, the second photograph shows them immediately after mixing, 

 and the third photograph shows them several hours later. (Heat was applied to 

 achieve the dispersion of the starch.) The salt forms a true solution, the clay forms 

 a suspension and settles out, the starch forms a colloidal dispersion, and the olive 

 oil forms an emulsion and rises to the top of the water. 



It is possible to prevent the separation of the two parts of an emul- 

 sion by the addition of an emulsifier. An emulsifier is a substance in 

 which the ends of the molecules are different — one is soluble in water 

 and the other in oil. Thus, when an emulsifier is added to a mixture of 

 oil and water it tends to bind the two together and prevent separation. 

 Ordinary soap or any of the many types of detergents are examples of 



