Common Science Part 46

SECTION 54. _Acids._

Why are lemons sour?

How do acids act?

Some acids are very powerful. There is one, called _hydrofluoric acid_, that will eat through gla.s.s and has to be kept in wax bottles; and all acids tend to eat or corrode metals. You saw what hydrochloric acid did to the zinc shavings when you wanted to make a balloon; or, to be more accurate, you saw what the zinc shavings did to the acid, for the hydrogen gas that bubbled off was driven out of the acid by the zinc. Then the zinc combined with the rest of the acid to form what chemists call a _salt_.

If we were to let the soft metal, sodium, act on hydrochloric acid, we should get hydrogen also; but the salt that formed would be regular table salt (NaCl). You cannot do this experiment, however, as the sodium does its work so violently that it is dangerous.

EXPERIMENT 105. _To be done by the teacher before the cla.s.s.

If acid spatters on any one's skin or clothes, wash it of immediately with ammonia or a strong soda solution._

Drop a little candle grease on a piece of copper about 3/4 inch wide and 2 or 3 inches long. In the flame of a Bunsen burner, gently heat the end of the copper that has the candle grease (paraffin) on it, so that the paraffin will spread out all over the end. Let it harden. With a nail, draw a design in the paraffin on the copper, scratching through the thin coat of paraffin to the copper below. Pour a couple of drops of concentrated nitric acid on the paraffin-covered end of the piece of copper, and spread the acid with a match so that it can get down into the scratches. Let it stand by an open window for 5 or 10 minutes. Do not inhale the brown fumes that are given off. They are harmless in small amounts, but if breathed directly they are very irritating. Now wash off the acid by holding the copper under the hydrant, and sc.r.a.pe off the paraffin.

[Ill.u.s.tration: FIG. 182. Etching copper with acid.]

The nitric acid did to the copper in this experiment exactly what the hydrochloric acid did to the zinc shavings when you made the toy balloon. The copper drove the hydrogen out of the nitric acid and incidentally broke down some of the nitric acid to make the brown gas, and then the copper joined the rest of the nitric acid to make a salt called _copper nitrate_. This salt is green, and it dissolves in water. When you washed the copper, the green salt was washed away and a dent remained in the copper where the copper salt had been.

Here is a more practical experiment showing the action of acid on metal:

EXPERIMENT 106. Use two knives, one of bright steel and the other a silver-plated one. If the steel knife is not bright, scour it until it is. Drop a little lemon juice on each knife and let it stand for a few minutes, while the teacher does the next experiment. Then rinse both knives and examine them. What has the lemon juice done to the silver knife? to the steel one?

The lemon juice acts in this way because it is acid. Acids act on the taste nerves in the tongue and give the taste of sourness; everything sour is an acid. The black stuff formed on the steel knife by the lemon juice is an iron salt. The iron in the knife drove the hydrogen out of the lemon juice, but there was not enough for you to see it coming off; then the iron combined with the rest of the lemon juice to form an iron salt.

Whenever an acid acts on a metal, the metal drives off the hydrogen and forms a salt. The salt is not always good to eat; for instance, the salt that tin forms with acids is poisonous.

ACTION OF ACIDS ON OTHER SUBSTANCES. Acids do not act on metals only, however. Watch the next experiment to see what a strong acid will do to cloth.

EXPERIMENT 107. _To be done by the teacher._ Put a drop of concentrated nitric or sulfuric acid on a piece of colored wool cloth, or on a piece of colored silk. Let it stand for a few minutes, then rinse it thoroughly. Test the cloth where the acid has been to see whether or not it is as strong as the rest of the cloth. How has the acid affected the color?

[Ill.u.s.tration: FIG. 183. Strong acids will eat holes like this in cloth.]

ACTION OF ACIDS ON THE NERVES OF TASTE. Acids act on the taste nerves in the tongue and give the taste of sourness; everything sour is an acid. Lemon juice, sour milk, and sour fruits are all too weak acids to injure clothes or skin, but their sour taste is a result of the acid in them acting on the nerves of taste.

_APPLICATION 79._ A girl wanted to make lemonade. She did not know which of two knives to use, a steel-bladed one or a silver-plated one. Which should she have used?

_APPLICATION 80._ A woman was going to put up some tomatoes.

She needed something large to cook them in. She had a shiny new tin dish pan, an older enamelware dish pan, a galvanized iron water pail, and an old-fashioned copper kettle. Which would have been best for her to use?

Make a list of as many acids as you can think of.

INFERENCE EXERCISE

Explain the following:

511. Sugar dissolves readily in _hot_ coffee.

512. The sugar disappears, yet the coffee flavor remains and so does the sweetness of the sugar.

513. A tin spoon left overnight in apple sauce becomes black.

514. If one's clothes are on fire, rolling over on the ground is better than running.

515. Lemon juice bleaches straw hats.

516. Will-o'-the-wisps glow at night, deceiving travelers by their resemblance to moving lanterns.

517. Tomatoes should never be left in a tin can after it has been opened.

518. Boiled milk tastes different from ordinary milk.

519. Your hands become very cold after you have washed things in gasoline.

520. Wood decays more quickly when wet than when dry.

SECTION 55. _Bases._

Why does strong soap make your face sting?

How is soap made?

"Contains no free alkali," "Will not injure the most delicate of fabrics," "99-44/100% pure,"--such phrases as these are used in advertising soaps. What is meant by 99-44/100% pure? What is free alkali? Why should any soap injure fabrics? What makes a soap "strong"?

The answer to all these questions is that there are some substances called _bases_, which are the opposites of acids, and some of which are as powerful as acids. Lye, ammonia, caustic soda, and baking and washing soda are common bases. The strong bases, like lye and caustic soda, are also called _alkalies_. If you want to see what a strong base--an alkali--will do to "the most delicate of fabrics," and to fabrics that are not so delicate, for that matter, try the following experiment:

EXPERIMENT 108. _To be done by the teacher._ If you get any alkali on your skin or clothes, wash it off immediately with vinegar or lemon juice.

Put half a teaspoonful of lye and a quarter of a cup of water into a beaker, a small pan, or an evaporating dish. Bring it to a _gentle_ boil. Drop a small piece of woolen cloth and a small piece of silk cloth into it and let them boil gently for a couple of minutes. What happens to them? Try a piece of plain cotton cloth, and then a piece of cloth that is mixed wool and cotton or mixed silk and cotton. What happens to them? This is a very good test to determine whether any goods you buy are pure silk or wool, or whether there is a cotton thread mixed with them. Drop one end of a long hair into the hot lye solution. What happens to it? Drop a speck of meat or a piece of finger nail into it.

From this experiment you can readily see why lye will burn your skin and ruin your clothes. You can also see how it softens the food that sticks to the bottom of the cooking pan and makes the pan easy to clean. Lye is one of the strongest bases or alkalies in the world.

[Ill.u.s.tration: FIG. 184. The lye has changed the wool cloth to a jelly.]

HOW SOAP IS MADE. When lye and grease are boiled together, they form soap. You cannot very well make soap in the laboratory now, as the measurements must be exact and you need a good deal of strong lye to make it in a quant.i.ty large enough to use. But the fact that soap is made with oil, fat, or grease boiled with lye, or caustic soda, which is almost the same thing, shows why a soap must be 99-44/100% pure, or something like that, if it is not to injure "the most delicate fabric." If a little too much lye is used there will be free alkali in the soap, and it will make your hands harsh and sore and spoil the clothes you are washing. A "pure" soap is one with no free alkali in it. A "strong" soap is one that does have some free alkali in it; there is a little too much lye for the oil or fat, so some lye is left uncombined when the soap is made. This free alkali cleans things well, but it injures hands and clothes.

When the drainpipe of a kitchen sink is stopped up, you can often clear it by sprinkling lye down it, and then adding boiling water.

_If you ever do this, stand well back so that no lye will spatter into your face; it sputters when the boiling water strikes it._ The grease in the drainpipe combines with the lye when the hot water comes down; then the soap that is formed is carried down the pipe, partly dissolved by the hot water.

When you sponge a grease spot with ammonia, the same sort of chemical action takes place. The ammonia is a base; it combines with the grease to form soap, and this soap rinses out of the cloth.

THE LITMUS TEST. To tell what things are bases and what are acids, a piece of paper dyed with litmus is ordinarily used. Litmus is made from a plant (lichen). This paper is called _litmus paper_. Try the following experiment with litmus paper:

EXPERIMENT 109. Pour a few drops of ammonia, a base, into a cup. Into another cup pour a few drops of vinegar, an acid.

Dip your litmus paper first into one, then into the other, and then back into the first. What color does the vinegar turn it? the ammonia? Try lemon juice; diluted hydrochloric acid; a _very_ dilute lye solution.

This is called the _litmus test_. All ordinary acids, if not too strong, will turn litmus pink. All bases or alkalies will turn it blue. If it is already pink when you put it into an acid, it will stay pink, of course; if it is already blue when you put it into a base, it will stay blue. But if you put a piece of litmus paper into something that is neither an acid nor a base, like sugar or salt, it will still stay the same color. So, to test for a base, use a piece of litmus paper that is pink and see if it turns blue, or if you want to test for an acid, use blue litmus paper. Do this experiment: