英文科學讀本 第六冊·Lesson 20 Water, a Compound of Oxygen and Hydrogen

Lesson 20 Water, a Compound of Oxygen and Hydrogen

Our lesson on combustion led us to form a clear conception of the whole process of burning, said Mr. Wilson. "We know that in the burning nothing is lost, nothing is destroyed; the substance, whatever it is, enters into chemical combination with oxygen to form a new compound body. That is all.

I intend to devote our lesson this morning to the consideration of one element, and the results of its combustion, or chemical union with oxygen. What is the name of that very inflammable gas which takes fire immediately a flame is brought near it?"

Hydrogen.

Right. This gas, then, and its action in combining with oxygen, shall form the subject of our lesson. We shall want some hydrogen for our experiments, and as you have seen me prepare the gas before, perhaps you will be able to tell me how to proceed to make some now."

You must put some zinc clippings into a flask, sir, with just enough water to cover them, said Fred, "and then carefully pour in, through the funnel, some sulphuric acid, till the liquid mixture begins to bubble all round the zinc. Those bubbles will be bubbles of hydrogen; the zinc separates the hydrogen from the dilute sulphuric acid."

Perfectly correct, my lad. Now do you remember that, when I last prepared this gas for you, I was very careful to wait for some few minutes before I attempted to collect it for actual use?

Yes, sir. You waited till you were quite sure there was nothing coming from the flask but pure hydrogen. The flask was at first filled with air, and, if any of that air were allowed to mingle with the hydrogen, there would be an explosion as soon as a light was brought near it.

You are right again, Fred; but why should there be an explosion? Let us try and find out. I will follow your instructions carefully and prepare some hydrogen now. But while we are waiting for the hydrogen, I will also prepare some oxygen, as I have the apparatus all ready.

Mr. Wilson waited till both gases were coming off pretty freely. He then filled an ordinary soda-water bottle at the pneumatic trough, and inverted it in the water in the usual way. When all was ready he held the delivery tube from the hydrogen flask under the mouth of the bottle till about two-thirds of the water it contained had been displaced! He then rapidly removed this tube, and replaced it with the one from the oxygen flask, so that the oxygen rose and filled the remainder of the bottle.

Now, said he, "we have in the bottle a mixture of the two gases, hydrogen and oxygen, and there is twice as much hydrogen as oxygen. Watch what takes place next."

He bound a thick towel well round the bottle, and holding it firmly in one hand, lifted it out of the water, while with the other hand he brought a long lighted taper near the mouth. In an instant there was a sudden, violent explosion.

This, Mr. Wilson explained, as soon as the class had got over the shock, "is due to the strong affinity which hydrogen has for oxygen. While the two were merely mixed together in the bottle, no explosion took place, although the only thing wanted was a flame. But what has become of the two gases now? The next experiment will tell us."

He proceeded to fit a small glass jet-tube to the cork in the hydrogen flask, in place of the long delivery-tube, and when that was done he poured a few more drops of sulphuric acid down the funnel-tube. The bubbling immediately began afresh, and hydrogen was again given off as before.

This time, he said, "we will light the gas as it comes off at the jet."

He did so, and it burned there with a dull flame. While it was burning he held a large test-tube over the flame, and in a short time it became covered on the inside with a dew-like deposit. These, he explained, were drops of water; the hydrogen had burned, and formed water.

Here we have, he said in conclusion, "another instance of combustion. Hydrogen and oxygen have united chemically to form a new compound substance—water, and while uniting have evolved light and heat.

The combustion goes on quietly here, because only a very small quantity of hydrogen can escape from the jet at one time. When the gases were mixed in larger quantities in the soda-water bottle, the combustion was very sudden and very violent, and we had a loud explosion. Then, as now, however, the two gases combined to form water. Water always consists of these two gases, hydrogen and oxygen, chemically united in the proportion of two to one.

Remember that all wood, coal, oil, candles, gas—whatever we burn for fuel or lighting purposes—contains hydrogen as well as carbon; therefore, whenever these things burn water-vapor must be formed, and poured out into the air.