42 THE CHEMISTRY OF LIFE 



ions and an acid is formed (hydrochloric acid). If we mix these two 

 solutions together in the proper quantities, the excess hydrogen ions of 

 the acid will unite with the excess hydroxyl ions of the base to form 

 water and the solution will be neutral. It will contain free sodium and 

 chlorine ions, and, if the water is evaporated, common table salt will 

 result. Such a mixing is known as a chemical reaction and may be 

 expressed as an equation : 



HC1 + NaOH -> NaCl + H 2 



We shall have occasion to study a number of chemical reactions in 

 living matter which can be expressed in terms of equations. 



Some acids are much weaker than others of the same concentration, 

 because there is a lower degree of ionization. For instance, common 

 vinegar contains about 3 per cent acetic acid, but it does not burn the 

 mouth when used in foods. Neither does it react violently with alumi- 

 num cooking vessels when used in cooking. A 3 per cent solution of 

 hydrochloric or sulfuric acid, on the other hand, would severely injure 

 the mouth and literally eat up any aluminum containers into which it 

 was placed. Now what is the difference? Hydrochloric acid, as we 

 have already indicated, is almost completely ionized in water. On the 

 other hand, the molecules of acetic acid are less than 1 per cent ionized. 

 Thus, there are many more free hydrogen ions in a 3 per cent hydro- 

 chloric acid than in a 3 per cent acetic acid, and it is the free hydrogen 

 ion that gives acid its reaction properties. 



The degree of acidity or alkalinity is expressed in terms of pH 

 value. Without going into details of the meaning of this, we can say 

 that O represents maximum acidity and 14 represents maximum alka- 

 linity. The figure 7 lies halfway between these two extremes and rep- 

 resents neutrality. Human blood has a pH value of about 7.3, which 

 means that it is very slightly basic. Each point on the scale represents 

 a tenfold change in ion concentration. For instance, a solution with a 

 pH of 5 has ten times as many hydrogen ions as a solution with a pH 

 of 6. Likewise, a solution with a pH of 9 has ten times as many 

 hydroxyl ions as a solution with a pH of 8. 



All living matter must exist within a rather narrow range on the 

 pH scale. Human blood is normally about 7.3, but it can vary slightly 

 above or below this point. If it ever falls as low as 7.0 or goes as high 

 as 7.8, death results. Hence, we have a pH tolerance of less than 1 

 point on the scale in our blood. Plants grow best when the soil re- 

 action is at a certain point on the pH scale and grow less vigorously 

 when the pH goes above or below this point. If it gets too far above 

 or below, the plant will not grow at all. Each plant is specific in its 



