Chap. 2 



LIFE IS A CONCERN OF MATTER AND ENERGY 



21 



Mixtures of Substances 



Mixtures of substances may be of different kinds and states, those of solids, 

 liquids, gases, or a solid and a gas (Fig. 2.9). 



Solutions. These are homogeneous mixtures. We usually think of solutions 

 as aqueous since natural water is a solution containing dissolved air. Bubbles 

 of air leave water when it is heated, appearing just before it boils. When it is 

 freezing bubbles of air appear and are caged in the ice. Glass is also a ho- 

 mogeneous mixture, in spite of its hardness, a true solution. 



Suspensions. The particles of at least one of the substances in a suspension 

 are larger than molecules and remain undissolved. One or several kinds of 

 substances, or different states of one or more of them may be suspended in 

 another substance. Suspensions include various types of colloids all of which 

 consist of one or more substances dispersed in another. There is no escape 

 from colloids. We consume them as food, breathe them as fog and smoke, 

 and are composed of them. 



Colloids. These are gelatinous substances that include two or more com- 

 ponents: (1) a solid in a solid — the ruby glass of cathedral windows usually 

 containing metallic gold; (2) a solid in a liquid — sodium chloride (salt) in 

 water; (3) solid particles in a gas — blue cigarette smoke; (4) a liquid in a 

 solid — natural pearl, which is water in calcium carbonate (a secretion of 



Ooo 1 



.X=k- 



>.^k> 



Sol 



Gel 



Movement of ameba accompanied by 

 changes sol to gel and reverse 



Fig. 2.11. Diagrams of the colloidal states, sol and gel. In the sol state the par- 

 ticles and droplets (white) move about freely in fluid. In the gel state the surfaces 

 of the droplets are in contact and the substance is jellylike. The protoplasm of an 

 active ameba constantly changes from sol to gel and reverse. 



