What makes baking powder bubble?
What makes the foam on soda water?
Did you ever make soda lemonade? It is easy to make and is rather
good. Try making it at home. Here are the directions:
EXPERIMENT 112. Make a glass of ordinary lemonade (half
a lemon, 1-1/2 teaspoonfuls of sugar; fill the glass with
water). Pour half of this lemonade into another cup or glass.
Into the remaining half glass stir half a teaspoonful of soda.
Drink it while it fizzes. Does it taste sour?
When anything fizzes or bubbles up like this, we say that it
_effervesces_. Effervescence is the bubbling up of a gas from a
liquid. The gas that bubbled up from your lemonade was carbon dioxid
(CO_2), and this is the gas that usually bubbles up out of things when
they effervesce.
When you make bread, the yeast turns the sugar into carbon dioxid
(CO_2) and alcohol. The carbon dioxid tries to bubble up out of the
dough, and the bubbles make little holes all through the dough.
This makes the bread light. When bread rises, it really is slowly
effervescing.
HOW SODA WATER IS MADE. Certain firms make pure carbon dioxid
(commercially known as _carbonic acid_ _gas_) and compress it in
iron tanks. These iron tanks of carbon dioxid (CO_2) are shipped to
soda-water fountains and soda-bottling works. Here the compressed
carbon dioxid is dissolved in water under pressure,--this is called
"charging" the water. When the charged water comes out of the faucet
in the soda fountains, or out of the spout of a seltzer siphon, or out
of a bottle of soda pop, the carbon dioxid that was dissolved in
the water under pressure bubbles up and escapes,--the soda water
effervesces.
Sometimes there is compressed carbon dioxid down in the ground. This
dissolves in the underground water, and when the water bubbles up from
the ground and the pressure is released, the carbon dioxid foams
out of the water; it effervesces like the charged water at a soda
fountain.
But the most useful and best-known effervescence is the kind you got
when you stirred the baking soda in the lemonade. Baking soda is made
of the same elements as caustic soda (NaOH), with carbon dioxid (CO_2)
combined with them. The formula for baking soda could be written
NaOHCO_2, but usually chemists put all of the O's together at the end
and write it NaHCO_3. Whenever baking soda is mixed with any kind
of acid, the caustic soda part (NaOH) is used up in neutralizing
the acid. This leaves the carbon dioxid (CO_2) part free, so that it
bubbles off and we have effervescence. Baking soda mixed with an acid
always effervesces. That is why sour milk muffins and pancakes are
light as well as not sour. The effervescing carbon dioxid makes
bubbles all through the batter, while the caustic soda (NaOH) in the
baking soda neutralizes the acid of the sour milk.
[Illustration: FIG. 185. Making a glass of soda lemonade.]
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.
CHAPTER TWELVE
ANALYSIS