英文科學(xué)讀本 第六冊·Lesson 02 Elements and Compounds

Lesson 02 Elements and Compounds

This morning we are going to commence a series of simple lessons in chemistry, said Mr. Wilson, and the boys were at once full of curiosity as to what they were to expect next.

Let us begin with some of the red powder in this bottle. We call it red oxide of mercury. I think you know something about it already. I will put a little of it into this test-tube, and heat it over the flame of the Bunsen burner.

You showed us this experiment once before, sir, said Fred, "and I remember that by heating the solid powder over the flame you can make it give off an entirely different substance—a gas—and that gas is oxygen."

Quite right, Fred, and oxygen is now passing off from the red powder while I heat it. Our present business, however, is not to collect the gas for further experiment, as we did in our other lesson, but merely to show that it is coming off. How can I do this?

If you plunge a red-hot splinter of wood into the mouth of the tube, sir, we shall soon see whether the tube contains oxygen.

Very well, my lad, you come and do it, said Mr. Wilson. Fred did so, and immediately the red-hot splinter burst into a brilliant glow, thereby proving the presence of oxygen.

We know, then, continued Mr. Wilson, "that this solid, dry red powder contains the gas—oxygen. But what are these little silvery-white, shining globules all round the sides of the tube? They look like little balls of silver. They are not at all like the red powder we put into the tube. If we continued to heat the powder till we could get no more oxygen from it, and then stood the tube aside for a while to cool, we should be able to scrape off and pour out the shining little balls and leave the vessel empty. Moreover, as we poured out these tiny silvery balls, they would run together and form a little pool of liquid metal—the metal mercury. The red powder, therefore, which is a mass of minute solid particles, contains a liquid metal—mercury, and a gas—oxygen. We call it oxide of mercury.

But not only so. It has been found to be impossible, either by still further heating the powder or by any other treatment, to make it yield anything but these two substances—mercury and oxygen. We say, therefore, that oxide of mercury is a compound substance; it is made up of two other substances—mercury and oxygen. The red powder may be actually made by heating the metal mercury in a closed vessel. The metal, as it is heated, robs the air in contact with it of its oxygen, the two substances unite, and form the entirely new body—oxide of mercury. The process is long and tedious, and takes two or three days to complete. Hence we shall not attempt to do it now. I merely want you to know that it can be done. Let us take another experiment. I have here two lumps of chalk. I will place one piece in the middle of the fire, where it is bright and red; and while it is heating we will deal with the other piece on the table.

I want you to carry your minds back to an experiment I once showed you with some chalk by pouring over it dilute hydrochloric acid. I put the pieces of chalk into a bottle, covered them with water, and added some hydrochloric acid, till little bubbles began to form all round the chalk.

Oh, I remember, sir, said Will. "You made the chalk give off carbonic acid gas; that was what the bubbles were. You collected the gas in a bottle that stood on the table."

Perfectly true, my lad, said Mr. Wilson, "and I will do the same thing now, in a somewhat different way. As, however, I do not wish to collect the carbonic acid gas this time, but merely to show that it is actually passing off, it will be sufficient to put the chalk into a basin and pour the liquid over it. You can see the bubbles rising up through the liquid. Those bubbles are bubbles of carbonic acid gas, and that gas came from the solid chalk.

Your earlier lessons have made you familiar with the fact that this carbonic acid gas is composed of two substances—the solid substance, carbon, and the gas, oxygen. We can make it for ourselves by burning carbon in oxygen. Therefore carbonic acid gas is itself a compound substance. Let us next turn our attention to the lump of chalk which I placed in the fire. I will take it out now with the tongs, and stand it on this iron tray.

As soon as it is cool I will pour some of the dilute hydrochloric acid on it as we did on the other. It does not give off bubbles of gas as the other did, you see. Why not? The heat of the fire has driven off all the carbonic acid it contained, and the substance we have here now is not chalk. It is lime. In its present state we call it quicklime. So then chalk is a compound substance, made of lime and carbonic acid, and you have already learned that carbonic acid is itself a compound of carbon and oxygen. Chalk, therefore, consists of lime, carbon, and oxygen.

Now, instead of dilute acid, I will pour water on this hard, solid quicklime. Watch what happens. The water instantly disappears—it seems to be sucked up by the quicklime; tremendous heat is generated, and the solid substance falls away in powder. This powder we call slaked lime.

Let us see what has happened. The solid substance, quicklime, has become united with water. We say that the two have combined to form a new compound substance—slaked lime.