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 glass 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 class. 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 scrape off the paraffin.

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?

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.