CHAPTER II 



PROPERTIES OF SOLUTIONS 



Water is the most abundant compound present in all physiologically active 

 plant cells. The water which occurs in a liquid state in plant cells invariably 

 contains other substances dissolved in it and usually also contains dispersed 

 particles which are not in true solution. When the particles dispersed through- 

 out the water are within a certain range of sizes the system of water plus 

 particles falls into the category of colloidal systems. The complicated dynam- 

 ics of living systems can be largely interpreted in terms of the physico- 

 chemical properties of solutions and colloidal systems, one component of which 

 is water, although it should not be inferred that non-aqueous solutions and 

 colloidal systems are entirely absent in living organisms. 



Similarly liquid water never occurs in the pure state in the natural environ- 

 ment of living organisms. The water of streams, lakes, and oceans invariably 

 contains various substances in solution and usually in the form of dispersed 

 particles as well. This is likewise true of the soil water. Even raindrops, the 

 products of natural distillation, contain gases which have dissolved in them 

 from the atmosphere. 



General Nature of Solutions. — Simple solutions are systems in which 

 one component (the solute) is dispersed throughout the other (the solvent) 

 in the form of molecules or molecules and ions. Theoretically the solvent 

 may be a gas, a solid, or a liquid, but solutions in which the solvent is a liquid 

 are by far the most important in living organisms. Except in extremely con- 

 centrated solutions the average distance between the solute particles is usually 

 very great relative to their size. Naturally occurring solutions, whether in 

 living organisms or their environment, usually contain a number of different 

 solutes and are often exceedingly complex. Water is the commonest and most 

 important of all solvents both in the inorganic world and in the realm of living 

 organisms. The further discussion will be principally in terms of aqueous 

 solutions. 



Solutions of a Gas in a Liquid. — The water present in living organisms 

 usually contains dissolved gases. Those most commonly present are carbon 

 dioxide, oxygen, and nitrogen. Practically all the water in the environment 



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