CHAPTER III 



SOME PHYSICAL CHARACTERS OF ORGANIC 

 SUBSTANCES 



Crystalloids and Colloids. — ^A solid substance, when 

 placed in water, may behave in one of several ways. 

 First, it may react chemically with the water, as for 

 instance sodium, which breaks up the molecule of 

 water, combining with the oxygen and liberating 

 hydrogen. Secondly, it may dissolve in the water to 

 form a true solution, as for instance common salt 

 (sodium chloride), the salt seeming to disappear in 

 the solution (where it is called the solute, the dis- 

 solving substance being the solvent). Thirdly, it may 

 remain practically unaltered, as for instance quartz 

 sand (silica). If, however, such a substance as the 

 coarse grains of silica are ground in a very finely 

 grinding mill into finer and finer particles, a point of 

 fineness is ultimately reached at which the particles 

 stay suspended in the water, without settling, to 

 form what is known as a colloid » solution, or sol, which 

 differs from a true solution in several important 

 respects. This fourth kind of behaviour with water 

 is a very characteristic feature of many organic 

 compounds. 



Broadly speaking, the substances which form true 

 solutions with water, or some other liquid solvent, 

 have small or comparatively small molecules, such for 

 instance as simple inorganic salts, and many of the 



■ Greek /ctSAAa, glue, from the jelly-like nature of the most familiar 

 colloids {gels). See p. 53. 



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