Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 17 



the sodium ions (Na+) are attracted toward the negative pole where they 

 acquire electrons and their positive charge is neutralized. The chlorine ions 

 (CI") are attracted toward.the positive pole, give up an electron and become 

 neutral atoms. The moving ions conduct an electrical current and thus estab- 

 lish a complete circuit. Any substance which thus ionizes in water is called an 

 electrolyte because of its ability to conduct electricity. 



Fig. 2.7. Diagram of the ionization or dissolv- 

 ing of salt in water. When sodium chloride (salt) 

 is put into water the atoms Na (sodium) and CI 

 (chlorine) separate and become electrically 

 charged wandering atoms or ions, Na+, Cl~. 

 The movements of the sodium ions (-(-) and 

 chlorine ions ( — ) conduct an electrical current 

 in water. In general, water promotes the forma- 

 tion of ions and ions promote chemical reactions. 



The properties of electrolytes depend upon the kind of ions which they 

 produce in a solution. On the basis of the simpler theory of electrolytes there 

 are three classes: acids, alkalis, and salts. The degree of acidity or alkalinity 

 of a compound depends upon the degree to which it ionizes in water, that is, 

 the degree to which the molecules yield positive hydrogen ions (H+) or nega- 

 tive hydroxyl ions (OH~) in the solution. Acids are electrolytes that as a 

 group form positively charged hydrogen ions, giving the acid its sour taste. 

 Hydrochloric acid ionizes in water: 



HC1^H+ + C1-. 

 The alkalis or bases form negatively charged combinations of oxygen and 

 hydrogen, the hydroxyl ions, OH~, The alkali, sodium hydroxide, ionizes 

 thus: 



NaOH^Na+ -f OR-. 

 Some compounds of protoplasm yield both H+ and OH~ in solution. The 

 third class of electrolytes is the salts whose ionization produces neither H+ 

 nor OH~, Sodium chloride is an example: 



NaCFNa+ + C\-. 



Many of the important characteristics of cells, such as the permeability of 

 their membranes, their irritability or response, are associated with the existence 



