a 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