英文科學(xué)讀本 第六冊·Lesson 10 Chemical Combination

Lesson 10 Chemical Combination

We have already had occasion to mention certain metallic elements, began Mr. Wilson. "I have one in this bottle, of a very different nature from the metals we are accustomed to meet with every day. We call it potassium. It can exist in its pure state only while it is bottled up like this in paraffin-oil.

You have seen before now another element— phosphorus—which, in like manner, must be bottled up in water. But water is not able to keep this element— potassium—out of mischief; we have to put it in paraffin-oil.

Mr. Wilson then proceeded to take the potassium out of the bottle, and cut off a small pellet about the size of a pea. This he threw into a large basin of water standing on the table. The metal was so light that it floated on the surface, but the most wonderful thing of all was that the moment it touched the water it seemed to burst into flame. The boys saw the beautiful purple flame playing all round it, as it floated about on the surface of the water.

The piece of metal gradually became smaller and smaller, and at last disappeared altogether.

Now let us find out, said Mr. Wilson, "what all this means. You saw that we had to keep the potassium in paraffin. The fact is, this metal has such a great liking for oxygen that if it were left exposed to the air, it would go on abstracting oxygen from it. The two elements could not keep apart; they would rush together to form a new compound substance. It is for this very reason that we cannot keep it in water even, for it would rob the water of its oxygen for the same purpose. Hence we bottle it up in paraffin.

Now watch, while I cut another small pellet. Notice the bright, bluish, metallic lustre of the newly-cut edge. But even as you look at it, you can see the bright lustre gradually disappear. The fact is, the metal, as soon as it was exposed to the air, began to rob the air of its oxygen, and to combine with that oxygen, to form a new substance all over its surface. It is this new substance which dulls the lustre of the newly-cut surface. Let us try now and learn the meaning of that flame on the water. The moment the potassium touches the water, it begins to rob it of some of its oxygen. Now, as water is composed of only the two gases, oxygen and hydrogen, what must happen when the metal takes away some of the oxygen? A certain amount of hydrogen must be set free. The potassium and oxygen and some of this hydrogen combine, and form a new compound substance—caustic potash—which dissolves in the water. In other words, the oxygen, set free from the water, burns up or oxidizes the potassium, the result of the oxidation being the new substance—caustic potash.

We have already learned that whenever chemical combination takes place heat is given out. Now then let us see. Hydrogen, a very inflammable gas, is set free all round the floating piece of metal. The metal combining with oxygen gives off great heat, and the burning flame is really the hydrogen, set free from the water, and blazing round the pellet of potassium. All this takes place because of the great attraction that potassium and oxygen have for each other. We call this attraction chemical affinity, or chemical attraction. It is another of the great forces in nature. It was this chemical attraction, or chemical affinity, that enabled us to obtain carbonic acid gas from chalk by means of hydrochloric acid. The calcium of the chalk has a stronger affinity for the chlorine of the hydrochloric acid than for the carbonic acid. It consequently breaks its connection with the carbonic acid, and enters into new relations with the chlorine, forming a new substance, chloride of calcium, and setting free the carbonic acid gas.

In the same way, it is the strong affinity of phosphorus, sulphur, carbon, and hydrogen for oxygen, that causes them to burn so fiercely in that gas. We may sum up the results of our investigations as follows:

1. Certain elements have an attraction or affinity for others. 2. This attraction does not exist between all bodies, and it differs in degree. It is stronger between some than it is between other bodies.

3. It is so strong in some that they at once unite if they are brought into contact. 4. In others heat must be applied to start the chemical action. The water poured on the quick-lime gives a good illustration of the powerful affinity existing between these two substances. They instantly enter into combination, the lime drinking in the water with greedy avidity not only without the application of heat, but itself actually evolving intense heat during the combination.

Now let us have another experiment. I will put some flowers of sulphur in the bottom of this glass flask, and on the sulphur a small coil of fine copper wire. Watch what takes place when I heat the flask over the Bunsen burner. First the yellow powder melts and becomes liquid sulphur; then it begins to change colour, till at last it is quite black. By this time the liquid begins to boil, and the copper also becoming heated, begins to glow with an intensely bright light and great heat, and eventually melts and seems to disappear.

If the flask is now set aside to cool, and afterwards broken open, it will be seen to contain neither sulphur nor copper, but a black solid substance quite unlike either. The copper and the sulphur have combined to form this new substance, which contains in itself every particle of the two original elements. While they were combining in this way they gave out intense heat; it was this heat that melted and burned the metallic copper.

The great thing to remember in this chemical combination of two or more elements is that, although each element is present in the compound, and can be recovered from it, it loses entirely its distinctive properties while it is in combination.