CHAPTER XVII 

 ACIDITY 



No other quality of protoplasm and of the fluids that bathe it 

 has received more attention of late than has acidity. Perhaps 

 undue emphasis has been laid upon acidity; nevertheless, it is 

 still recognized as one of the most important conditions upon 

 which the prolonged life and continued well-being of organisms 

 rest. Body fluids are maintained fairly constantly at a definite 

 acid or alkaline value; pronounced changes usually result in an 

 unhealthy condition. The acidity of the surrounding solution 

 (of the soil water, the pond water, or the ocean) is quite as impor- 

 tant in the case of plants and aquatic animals as is that of the 

 body fluids. Many species of plants are so sensitive to the acid 

 condition of the soil that if the degree of acidity is not suitable, 

 the plant will not survive. Other species, however, show great 

 tolerance. The nonliving world is also very sensitive to its 

 acid condition. The rate and type of chemical reactions are in 

 large measure dependent upon acidity. 



Terms. — An acid is a compound that yields hydrogen ions 

 when in solution, or, an acid is a compound the hydrogen of which 

 is replaceable. Alkalinity is the opposite of acidity. An 

 alkali is a substance containing the hydroxyl radical and is 

 known as a base. The amount of replaceable hydrogen in an 

 acid can be ascertained by the amount of alkali necessary to 

 neutralize the acid. 



Degree of acidity may be determined and expressed in a 

 number of ways. The taste of a substance indicates whether 

 it is very sour, mildly sour, or not sour at all. Such a method 

 serves well for a crude determination of the acidity of food, but 

 it is obviously not precise, though many a chemist has used his 

 tongue as an acid indicator. Some dyes assume different colors 

 when in solutions of different degrees of acidity; the color is 

 usually definite for a definite degree of acidity. Acidity may, 

 therefore, be determined by color. This is done by comparison 



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