英文科學讀本 第六冊·Lesson 57 Bases, Alkalies, Salts

Lesson 57 Bases, Alkalies, Salts

You, no doubt, remember a very interesting experiment, in which a piece of the metal, potassium, thrown into a basin of water, takes fire spontaneously, and burns with a beautiful purple flame, as it floats about on the surface, said Mr. Wilson.

Oh yes, sir, replied Will. "You showed us that experiment to prove the strong affinity of potassium for oxygen. The metal burned by robbing the water of its oxygen."

I remember too, sir, said Fred, "you explained that as the metal took away some of the oxygen from the water, a certain amount of hydrogen was set free. This hydrogen, being a very inflammable gas, burns with the potassium on the surface of the water. The potassium, oxygen, and hydrogen enter into combination, and form a new substance, caustic potash, which dissolves in the water as it forms."

Those are two excellent answers, my lads, and they prove that you follow each lesson carefully, step by step. Yes, the potash is in the water after the experiment, although we cannot see it, because it is in a state of solution. Let us have the experiment again now. This time, however, I want to prove to you that the potash is really in the water. I have therefore colored the water in the basin with red-cabbage infusion. As soon as the piece of metal touches the water it bursts into flame as before, but what else do we see? The reddened water has changed at the same time into a bluish green.

Potash is a substance which is used for many purposes, and it is usually sold in sticks. It has a very greedy affinity for water, so that it must be kept bottled up close. I have a stick of the substance here. Of course, if I put some of it into water it will dissolve immediately. We will make a solution, and see what it will tell us.

Let us first pour a little of the potash solution, slightly diluted, into a test-tube, containing some red-cabbage infusion. The color instantly changes to a bluish-green, just as our water changed in the basin. This proves conclusively that the combustion of the potassium on the water just now produced potash. We could not see it, but we saw its effect on the red coloring matter of the water. I have here some litmus, which has been reddened by an acid. Notice that when I pour a little of our potash solution on it the original blue color of the litmus is at once restored.

Lastly, Fred shall pour a few drops of the potash solution into the palm of his hand and taste it. It has an intensely acrid taste—a taste like that of common washing soda—and produces a strong caustic or burning sensation on the tongue. This is why it is sometimes called caustic potash. This substance—potash—is a sample of a great many bodies, which have the same peculiar acrid taste, and the same power of changing vegetable reds to blue or green.

Any substance which has this acrid taste, and is able to turn vegetable red to blue, the chemist calls a base. Many of the bases resemble caustic potash in being soluble in water. These have another name. They are called alkalies. That is to say, an alkali is a base of a special kind. It is soluble in water, and it gives to the water a soapy taste and feel.

Here is some olive-oil. I will pour a little of it into a test-tube, add to it some of the potash solution, and shake it up for a minute or two. If you now pour some of the mixture into the palm of your hand, you will at once notice its soft, smooth, soapy feel. In fact, we have made soap. I think now you are clear as to the nature of an acid, a base, and an alkali.

I have now another acid for you to examine. It is called nitric acid, and is one of the most powerful of all. I will dilute a little of it and put it into a test-tube. Diluted as it is, it has a very sour taste, as you may easily test for yourselves. You of course can tell me what will happen if I dip this blue litmus-paper into the dilute acid? The color will instantly change to red, as it does with all acids.

Now, let us turn again to our solution of potash. I dip the reddened litmus into it, and the blue color is immediately restored like magic. This, you know, is the usual characteristic of a base. I want you to watch carefully the next step. I will mix the acid and the base together, and then test the mixture, as before, with litmus. This time the liquid has no effect upon either to change its color. The blue litmus remains blue, and the red remains red. The liquid is neither an acid nor a base. It is neutral to both the blue and the red litmus.

Perhaps we can learn something more about this liquid. Let us see. If a little of it be poured on a watch-glass, and allowed to evaporate, we shall find, when all the water has disappeared, that the surface of the glass will be covered with crystals of a new body. This substance—the product of an acid and a base—is called a salt. The name of this particular salt is nitre or saltpeter.