Effervescence generally due to the freeing of carbon dioxid. Since baking soda is so much used in the home for neutralizing acids, people sometimes get the idea that whenever there is neutralization there is effervescence. Of course this is not true. Whenever you neutralize an acid with baking soda or washing soda, the carbon dioxid in the soda bubbles up and you have effervescence. But if you neutralize an acid with ammonia, lye, or plain caustic soda, there is not a bit of effervescence. Ammonia, lye, and plain caustic soda have no carbon dioxid in them to bubble out.
Baking powder is merely a mixture of baking soda and 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 (HCl) 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 gingerbread. She had also baking soda, caustic soda, lemons, oranges, vanilla, salad oil, vinegar, and lye. Was there any way in which she might have made the gingerbread light without spoiling it?
Inference Exercise
Explain the following:
541. Harness is oiled to keep it flexible.
542. When you pour nitric acid on copper filings, there is a bubbling up of gas.
543. The flask or dish in which the action takes place becomes very hot.
544. The copper disappears and a clear green solution is left.
545. In making cream of tomato soup, soda is added to the tomatoes before the milk is, so that the milk will not curdle How does the soda prevent curdling?
546. The soda makes the soup froth up.
547. A wagon squeaks when an axle needs greasing.
548. Seidlitz powders are mixed in only half a glass of water.
549. The work of developing photographs is all done with a ruby light for illumination.
550. Coal slides forward off the shovel into a furnace when you stop the shovel at the furnace door.