英文科學(xué)讀本 第六冊·Lesson 52 Acids

Lesson 52 Acids

I have here a bottle containing a liquid which has long been familiar to us by name, for we have made frequent use of it in our lessons, said Mr. Wilson at the next meeting of the class. "We know it as spirit of salt, or muriatic acid."

I think we are now in a position to make it teach us something new, he continued, and he poured some of the liquid into a test-tube, and added a little blue litmus solution.

The color, you see, he went on, "instantly changes to red. Now, Fred, dip your finger into the liquid, and put it to your tongue."

Fred did so, and found that it had a sharp, strong, sour taste. "We have met with these same characteristics before," continued Mr. Wilson, "in the dissolved oxides which we examined. The solutions of the oxides of sulphur and phosphorus had the same sharp, sour taste, and they both turned the blue litmus into a bright red. The carbon dioxide was sour too, although only very slightly; but it also changed the blue color of the litmus.

There are many substances, besides those we have examined, which have the same characteristics. The chemist has one name for them all. He calls them acids. In our everyday language the word acid calls up in our mind some sharp, sour taste, such as that of vinegar or unripe fruit. It comes from a Latin word acidus, which means sour. Many substances possess this peculiar sharp taste, and when the name acid was first used by the chemist, it was limited to such bodies. They all have the same effect, too, in turning the blue color of litmus into red. But it is now known that, although most acids are sour to the taste, there are some which are not sour. To the chemist, therefore, this sourness is a mere accident in spite of the name. Acids are now distinguished entirely by their action on vegetable coloring matter. Any substance which can change the natural blue color of litmus into a red is said to be an acid, whether it be sour to the taste or not.

We are now in a position to give the proper names to those substances which we have till now called dissolved oxides. Carbon dioxide when dissolved in water is carbonic acid. Sulphur dioxide similarly becomes sulphurous acid when it is dissolved in water. The oxide of phosphorus, which we produced and examined, became phosphoric acid when it was dissolved in water. These acids and many more like them were produced by the combustion of some substance in oxygen. There are other acids which are not formed in this way, and these are not oxides; they contain no oxygen it all. The spirit of salt or muriatic acid which we examined just now is one of them. It is also sometimes called hydrochloric acid, but this is not really a good name, for spirit of salt is really a solution of hydrochloric acid, and not the acid itself.

Hydrochloric acid, by its very name, of course, suggests that it is composed of two bodies—hydrogen and chlorine—with which our lessons have made us familiar. We will prepare some now, and see what we can learn about it. I have here some common salt, which has been fused in an iron ladle over a very fierce red fire, then allowed to cool into a solid mass, and afterwards broken up into pieces with a hammer. We must first weigh a certain quantity of this fused salt, and put it into a flask. Then we must weigh an equal quantity of oil of vitriol, and pour over the salt.

I ought to tell you that oil of vitriol has another name—sulphuric acid, but this is not the same as the sulphurous acid I mentioned a little while ago. It is one of the most powerful of acids, and like those we have examined, it has a sharp taste and turns blue litmus red. You see by this time there is a sort of action going on between the salt and the acid. If I now apply the flame of the Bunsen burner gently to the bottom of the flask, a gas will be rapidly given off. This gas is the hydrochloric acid we want.

Hydrochloric acid is heavier than air; we therefore collect it by displacement. All that is necessary is to guide the delivery tube into the jars as they stand, with their mouths open, on the table. The heavier gas will sink to the bottom of the jar and drive the air out at the top. As they fill we will cover them with the glass discs, and set them aside till we require them. Now to learn something about this new gas. We will take one of the jars, with the glass cover still closing the mouth, and invert it in this bowl of water. Notice what happens when I slide the disc away under the water. The water rushes rapidly up into the jar, dissolving the gas which it contains. There is a very strong affinity between this gas—hydrochloric acid—and water. Water will greedily dissolve 480 times its own bulk of this gas. You remember I told you that muriatic acid, or spirit of salt, is simply hydrochloric acid in solution.

I am now going to perform the same experiment again, but I will color the water with blue litmus this time. When the jar is inverted, and the disc removed, the water rushes up as before, but as it enters the jar it changes from blue to red. Now you can see why we call the gas an acid. You saw, moreover, that I prepared the gas from common salt. Hence you will not be surprised now at the name spirit of salt.