Common Science Part 48

539. The creases made in ironing the clothes will reappear even if you flatten the creases out with your hand.

540. After they have been worn, washed, and ironed a number of times, clothes are thinner than they were when they were new.

SECTION 57. _Effervescence._

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 gla.s.s of ordinary lemonade (half a lemon, 1-1/2 teaspoonfuls of sugar; fill the gla.s.s with water). Pour half of this lemonade into another cup or gla.s.s.

Into the remaining half gla.s.s 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 s.h.i.+pped 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 m.u.f.fins 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.

[Ill.u.s.tration: FIG. 185. Making a gla.s.s 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 was.h.i.+ng 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 const.i.tutes 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 pa.s.s 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 gla.s.s.

_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 gla.s.s 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

a.n.a.lYSIS

SECTION 58. _a.n.a.lysis._

How can people tell what things are made of?

If it were not for chemical a.n.a.lysis, most of the big factories would have to shut down, much of our agricultural experimentation would stop, the Pure Food Law would be impossible to enforce, mining would be paralyzed, and the science of chemistry would almost vanish.

a.n.a.lysis is finding out what things are made of. In order to make steel from ore, the ore has to be a.n.a.lyzed; and factories could not run very well without steel. In order to test soil, to test cow's milk, or to find the food value of different kinds of feed, a.n.a.lysis is essential. As to the Pure Food Law, how could the government find out that a firm was using artificial coloring matter or preservatives if there were no way of a.n.a.lyzing the food? In mining, the ore must be a.s.sayed; that is, it must be a.n.a.lyzed to show what part of it is gold, for instance, and what part consists of other minerals. Also, the a.n.a.lysis must show what these substances are, so that they can be treated properly. And the science of chemistry is largely the science of a.n.a.lyzing--finding out what things are made of and how they will act on each other.

The subject of chemical a.n.a.lysis is extremely important. But in this course it is impossible and unnecessary for you to learn to a.n.a.lyze everything; the main thing is for you to know what a.n.a.lysis is and to have a general notion of how a chemist a.n.a.lyzes things.

[Ill.u.s.tration: FIG. 186. The platinum loop used in making the borax bead test.]

When you tested a number of substances with litmus paper to find out which of them were acids, you were really doing some work in chemical a.n.a.lysis. Chemists actually use litmus paper in this way to find out whether a substance is an acid or a base.

THE BORAX BEAD TEST. This is another chemical test, by which certain substances can be recognized:

EXPERIMENT 113. Make a loop of wire about a quarter of an inch across, using light-weight platinum wire (about No. 30). Seal the straight end of the wire into the end of a piece of gla.s.s tubing by melting the end of the tube around the wire.

Hold the loop of wire in the flame of a Bunsen burner for a few seconds, then dip the looped end in borax powder. Be careful not to get borax on the upper part of the wire or on the handle. Some of the borax will stick to the hot loop. Hold this in the flame until it melts into a gla.s.sy bead in the loop. You may have to dip it into the borax once or twice more to get a good-sized bead.

When the bead is all gla.s.sy, and while it is melted, touch it lightly to _one small grain_ of one of the chemicals on the "jewel-making plate." This jewel-making plate is a plate with six small heaps of chemicals on it. They are: manganese dioxid, copper sulfate, cobalt chlorid, nickel salts, chrome alum, and silver nitrate. Put the bead back into the flame and let it melt until the color of the chemical has run all through it. Then while it is still melted, shake the bead out of the loop on to a clean plate. If it is dark colored and cloudy, try again, getting a still smaller grain of the chemical. You should get a bead that is transparent, but clearly colored, like an emerald, topaz, or sapphire.

Repeat with each of the six chemicals, so that you have a set of six different-colored beads.

[Ill.u.s.tration: FIG. 187. Making the test.]