英文科學(xué)讀本 第六冊(cè)·Lesson 28 Chlorine

Lesson 28 Chlorine

In the course of our lessons I have made you acquainted with a great many gases, began Mr. Wilson. "We have had them before us in bottles and other glass vessels; we have experimented with them; and yet we have never seen them. You have been obliged, in each case, to trust to my statement at first that the gas was there at all, although we have always proved its presence afterwards by its action. This morning I am going to actually show you a gas. I will put equal quantities of common salt and black oxide of manganese into this mortar, and you shall crush and mix the two well together for me, Fred, while I get everything else ready."

He first diluted a little sulphuric acid with an equal quantity of water, and stood the mixture aside for a time to cool, and then arranged a flask, the Bunsen burner, and a number of perfectly dry gas-jars on the table ready to hand. As soon as the dilute acid was cool enough, he put into the flask the powdered mixture which Fred had prepared, and poured the dilute acid on it, till it began to look like a thin paste at the bottom. He then passed the flame to and fro under the flask, so as to apply the heat gradually and gently. In a very short time a dense yellowish-green gas was seen to rise rapidly, and pass along the delivery-tube into the dry bottles, one after the other.

This gas is the element chlorine, said Mr. Wilson. "It is a heavy gas—2.5 times as heavy as air. Hence, as it collects at the bottom of the bottle, it forces the air out at the top. We are collecting the gas by displacement, just as we collect carbonic acid gas."

As soon as all the gas jars were filled, Mr. Wilson had them covered, and the whole of the apparatus removed from the room. He explained to the boys that this gas is a violent irritant, and would set them coughing if they breathed it.

Now, said he, when everything was ready again, "we have plenty of chlorine; let us see what we can learn about it."

He began by taking one of the jars and lowering into it the deflagrating spoon with a piece of phosphorus in it about the size of a pea. The phosphorus immediately took fire, and burned in the chlorine with a pale green flame, till it was all consumed. This bottle, closed as it was, he then set aside. He next showed the class a leaf of Dutch metal, and told them that it was really copper-leaf. This he lowered into the second jar of chlorine, and it was no sooner placed in the gas than it ignited and burned with a smoky flame.

Now, boys, said he, "before we go any further let us think over what has taken place. In each case we have seen a spontaneous chemical action going on. Both the phosphorus and the Dutch metal ignited of their own accord in the jars of chlorine gas.

In the first bottle the phosphorus united with the chlorine gas to form a new compound, giving off light and heat during the combination. In the second bottle the copper also united with chlorine to form a new compound, and in this case too heat and light were evolved during the process. Now we have already given a name to this kind of chemical action. We call it combustion. But we have hitherto always associated oxygen with the act of combustion. Here, however, we find a new gas—chlorine—as a supporter of combustion, and we see that the usual heat and light are evolved during the chemical combination. When chlorine combines with another element the new compound formed is called a chloride. Thus the compound formed by the combustion of phosphorus in chlorine is a new substance—chloride of phosphorus; the compound formed, in like manner, by the combustion of the copper is a substance known as chloride of copper.

Similarly, chlorine forms compounds or chlorides with nearly all the elements. I have a most important chloride to show you now. You remember, of course, the metal potassium. Here is another metal very similar to it, and like it, this one must be kept bottled up in naphtha or paraffin. We call it sodium."

He took a piece out of the bottle, and cut off a small pellet from it, calling attention, as he did so, to the bright silvery lustre of the newly-cut edge. He placed the pellet of sodium in the deflagrating spoon, and held it in the flame of the Bunsen burner till the metal was in a molten state. In this condition he lowered it into the other jar of chlorine, and in an instant the liquid metal took fire, and burned with intense heat and a very bright glowing light. As the light died away the jar was seen to be filled with dense white fumes.

Now it is clear from what we have already seen, began Mr. Wilson again, "that as the sodium has been burned in chlorine, there must be a new compound formed, and that compound must be chloride of sodium."

He pointed out the deposit on the bottom of the jar, left there by the condensation of the white fumes.

This substance then, said he, "must be the chloride of sodium. Let us see what it is like."

He poured a little water into the jar, and after washing it round, asked the boys to dip their fingers into it and taste it.

Why, it tastes just like salt, said Fred.

Yes, my lad, it does, said Mr. Wilson, "for it is really salt. The chemical name for the common salt which we use every day is chloride of sodium.

It is important to remember that neither the gas, chlorine, nor the metal, sodium, is ever found in the pure or free state in nature. The waters of the ocean and the vast beds of rock-salt in the crust of the earth contain abundant supplies of them in this compound form— chloride of sodium.