368 Common Science 



dry acid (cream of tartar or phosphates in the better 

 baking powders, alum in the cheap ones). These dry 

 acids cannot act on the soda until they go into solution. 

 As long as the baking powder remains dry in the can, 

 there is no effervescence. But when the baking powder 

 is stirred into the moist biscuit dough or cake batter, 

 the baking powder dissolves ; so the acid in it can act 

 on the baking soda and set free the carbon dioxid. 



In most cases it is the freeing of carbon dioxid that 

 constitutes effervescence, but the freeing of any gas 

 from liquid is effervescence. When you made hydrogen 

 by pouring hydrochloric acid (HC1) on zinc shavings, 

 the acid effervesced, the hydrogen gas was set free 

 and it bubbled up. 



Stirring or shaking helps effervescence, just as it does 

 crystallization. As the little bubbles form, the stirring 

 or shaking brings them together- and lets them join to 

 form big bubbles that pass quickly up through the 

 liquid. That is why soda pop will foam so much if 

 you shake it before you pour it, or if you stir it in your 

 glass. 



Application 87. Explain why we do not neutralize the 

 acid in sour milk gingerbread with weak caustic soda instead 

 of with baking soda ; why soda water which is drawn with 

 considerable force from the fine opening at a soda fountain 

 makes so much more foam than does the same charged water 

 if it is drawn from a large opening, from which it flows gently ; 

 why there is always baking soda and dry acid in baking 

 powder. 



Application 88. A woman wanted to make gingerbread. 

 She had no baking powder and no sour milk, but she had 

 sweet milk and all the other articles necessary for making 



