THE CHEMISTRY OF LIFE 41^ 



merit which reacts violently when it contacts water, and chlorine is a 

 greenish gas. Neither of these is formed when salt is dissolved in 

 water. Instead, the molecules break down into sodium and chlorine 

 ions. An ion differs from an atom in that it is unbalanced in its elec- 

 trons and protons — it has either one extra positive or an extra negative 

 charge. Table salt breaks down into a positively charged sodium ion 

 (Na + ) and a negatively charged chlorine ion (Cl~). Whenever water 

 is evaporated from a salt solution these ions easily reassemble because 

 of their opposite charges and the salt molecule is reformed. Thus, we 

 can see that, when ionization occurred, one of the electrons from the 

 sodium atom was transferred to the chlorine atom. This leaves the 

 sodium with an extra positive charge, since the number of protons 

 within the nucleus remains the same. The chlorine, on the other hand, 

 becomes negative in its charge because of the addition of the electron 

 to a system which was previously neutral. 



Acids and Bases 



Acids are substances which have a sour taste. We are all familiar 

 with common acids found in the kitchen — vinegar which contains acetic 

 acid, and sour milk which contains lactic acid. Other substances are 

 known as alkalies or bases. These commonly have a bitter taste and 

 feel greasy. Common baking soda dissolved in water forms an alkali 

 solution. The stomach produces a common acid, hydrochloric acid, in 

 very low concentration which aids in the digestion of food, but some- 

 times too much of this acid is produced with unpleasant symptoms. 

 Many people take a little baking soda or other alkali in water to neu- 

 tralize the excess acid. 



Now, what is it that makes an acid an acid and a base a base, and 

 why does one tend to neutralize the other? It is all dependent upon 

 the ionization of molecules. Plain water undergoes a very slight 

 amount of ionization — a few of its molecules will separate into hydrogen 

 ions (H + ) and hydroxyl ions (OH - ). There will always be an equal 

 number of each of these. Thus, we say that water is neutral — neither 

 acid nor basic. When there is an excess of hydrogen ions in a solution, 

 however, that solution will be acid. An excess of hydroxyl ions, on the 

 other hand, renders it basic. 



Suppose we dissolve some sodium hydroxide (NaOH) in water. It 

 breaks down into ions (Na + and OH"). This breaks the even balance 

 of hydrogen and hydroxyl ions in the water, and the excess of hydroxyl 

 ions makes the solution basic. Hydrogen chloride (HC1), on the other 

 hand, breaks down into ions which results in an excess of hydrogen 



