Most acids contain oxygen as well as hydrogen, though none of those we have mentioned yet do so. For instance, there is an acid made of hydrogen, nitrogen, and oxygen, with the formula HNO, known as nitric acid. AN N ACID UPON WHICH THE WHOLE OF EUROPE AND AMERICA DEPEND FOR FOOD It and the salts it forms are very important in themselves, and also because of their services to the vegetable world, and so to us. The whole of Western civilisation at this moment depends on wheat grown with the aid of salts of nitric acid, called nitrates, which are added to the soil. Similarly hydrogen, sulphur, and oxygen form an acid which has the formula H2SO, and is known as sulphuric acid; and just as the salts formed from nitric acid are called nitrates, so the salts formed from sulphuric acid are called sulphates. They are also very important in many ways. These same elements form other acids in which the elements are combined in different proportions, such as nitrous acid, HNO2, and sulphurous acid, H2SO. But the acid about which we have heard most is carbonic acid, and especially we have heard about carbonic acid gas, CO2. You will say at once that there is something wrong here, for an acid is a compound of hydrogen, and there is no hydrogen in CO. That is true, but only because in CO2 we have all that is left of the acid when the atoms corresponding to water have been removed from it. If we add water to carbonic acid gas, we get real carbonic acid, thus: HgO + CO, = H,CO. HE DIFFERENT POWERS OF THE ATOMS TIN COMING TOGETHER This HCO, is a true acid, as we find directly we test it. As in the case of all acids, the hydrogen of it can be replaced by a metal, and so we get a salt, which is called a carbonate. For instance, there is calcium carbonate, CaCO3, one of the commonest salts in the world, which we know as chalk and marble. Then there is sodium carbonate, Na,CO, which is all-important in our blood. Here are the formulas of some of the principal acids we have mentioned: HC Hydrochloric HNO, Nitrous acid acid H.SO, Sulphuric acid HCN Hydrocyanic H2SO, Sulphurous acid or prussic acid HNO, Nitric acid H,CO, Carbonic acid We notice that the hydrogen of an acid is always written first in its formula, and we notice that none of the other elements found in these acids is a metal. We also see that in some of these acids there is only one hydrogen atom to each molecule, while in others there are two. Then we might have mentioned phosphoric acid, H,PO,, in which there are three hydrogen atoms to the molecule. This difference between acids illustrates a very important fact about all compounds, which is that the atoms of the various elements differ in their combining power. It is as if they had different numbers of hands with which to hold on to other atoms. The hydrogen atom always has only one hand, the chlorine atom has one hand, the carbon atom has four, the nitrogen atom has either three or five, the oxygen atom has two, and so on. Ho OW THE CHEMIST WRITES IN "PICTURES," OR GRAPHIC FORMULAS The formulas of these acids illustrate this very important fact. You will not imagine that I am speaking of real hands, but it is as if each atom had a certain number of hands or hooks or whatever it is that enables it to hold on to other atoms. Thus we can now learn to write our formulas in the form of little pictures, or diagrams. These are called "picture," or graphic, formulas. The graphic formula of water is : H-O-H and shows us that each of the two hands of the oxygen atom is holding on to the one hand of a hydrogen atom. Then we may write hydrochloric acid : H-Cİ each of the two atoms having just one hand. The next acid on our list is more of a puzzle, for we remember the number of hands that carbon has, and that the handed and sometimes five-handed, as nitrogen atom is sometimes threeit were. So how do we write the graphic formula of prussic acid? Either like this: C=N-H showing the nitrogen atom as having five hands, or like this: N=C-H showing the nitrogen atom with three hands. In each case the carbon atom has four and the hydrogen atom one. As to which of these formulas really represents the way in which the molecule of prussic acid is built up, it is for chemists to find out by THREE KINDS OF COMPOUNDS studying its behaviour when it is broken up or when salts are made of it. These acids we have named and looked at are all very simple compared with the extraordinary acids which are made in and by the bodies of living creatures. For instance, there is citric acid, which we find in oranges and limes and lemons; uric acid, which is made in our own bodies; malic acid (meaning apple acid), which is found in apples; and so on. The graphic formulas of these would almost fill half of this page, so large is the number of But always atoms in the molecule. we find that the acid has no metallic atoms, and that it has a certain number of atoms of hydrogen, which can be replaced by metallic atoms to form salts. THE COMPOUNDS CALLED OF METALS, ALKALIES, THE OPPOSITES OF ACIDS Now we must turn to another class of compounds which we always think of as the opposites of acids. These are compounds of metals. They may or may not contain hydrogen, but they are quite different in every respect from acids. These compounds we call bases, or alkalies; and when we study any liquid in chemistry we always want to know whether it is an acid liquid or an alkaline liquid, or a liquid that is neither There is a very easy acid nor alkaline. way of finding this out in most cases. There is a dye called litmus, which always turns red in the presence of an acid, and always turns blue in the presence of an alkali. We put some of this dye into a sort of blotting-paper, cut it up into strips, and use it for testing the of liquids, as we call it. "" reaction We usually have both sorts of paperblue and red. Then, if we want to find out the reaction of a liquid, we dip a piece of blue litmus in it, and find, perhaps, that it turns bright red. That proves that the liquid is acidperhaps hydrochloric acid or nitric acid. Now, if we take that piece of reddened paper, and dip it into a solution of such a thing as ammonia, or even hold it above a bottle containing a solution of ammonia (which is really a gas), we shall find that it quickly turns blue againproving that it has been exposed to an alkali. The good milk of a cow should give a faint acid reaction, while human. milk gives a faint alkaline reaction. Thus, in feeding a baby on cow's milk, we add something to make it alkaline, WAY IN WHICH THE ALKALIES ARE TMADE UP Now we must look at the composition is rather different from almost any we Ca (slaked lime). AMMONIA, AN IMPORTANT ALKALI THAT ABOUT These bases, or alkalies, are called fixed, just as we call certain oils fixed, because they do not give off gases, but stay where they are. There is, however, another very important base, or alkali, which is a gas and flies about, and so is called the volatile alkali, just as we call certain oils volatile. This alkali we already know, for it is ammonia. And now I hope you will say that here is an exception to what was said as to the way in which bases, or alkalies, are made. For we said that these are compounds of metals, and if you remember the formula of ammonia, you will see that it has no metal in it. Its formula is NH. If we add the formula of water to this, we get NH,OH, which represents ammonia in water; and this substance is a true alkali, and acts like one. There is some reason to suppose that the combination of atoms NH, acts like a metal, and has something like the properties of a metal. Anyhow, this compound gives an alkaline reaction to litmus paper, and it acts chemically in precisely the same way as the fixed alkalies, such as KOH, NaOH, Ca(OH)2, and many others which might be named. You will have noticed the way in which OH turns up again and again in the formulas of these alkalies; and, indeed, OH is such a common and important combination of atoms that it has been given the special name of hydroxyl. We find it in chemistry wherever we turn. And I want to show you that these various bases, or alkalies, really owe their OH to the fact that they are combined with water. Let us start with slaked lime, as that is simple. WHEN QUICK-LIME QUENCHES ITS THIRST There is a compound called quicklime, which really means live lime. It is called live lime because it acts so powerfully on things, as you soon find out if a speck of it gets into your eye. It is a white powder, and its formula is CaO. It is a very powerful alkali, formed by heating calcium carbonate, or chalk; thus WHAT HAPPENS CaCO = CaO + CO,. Now, when we add water to quicklime, we are said to slake it, and the product is called slaked lime; thusCaO +H2O Ca(OH)2. This slaked lime, then, is an oxide which has combined with water, and the same is true of the other alkalies we have named. For instance, there is an oxide KO, the oxide of potassium; and another, Na2O, the oxide of sodium. When water is added to these-and, indeed, there is no need to add water, for they take it from the air if they are exposed to itthe following reactions occur: K2O + H2O = 2KOH. Na2O + H2O = 2NaOH. You will notice that these correspond exactly to what happens when quicklime is "slaked"—that is, when it has its thirst satisfied-in the water; and when ammonia gas is added to water. So now we understand why these alkalies have the -OH, or hydroxyl, in them, and we can easily remember that the proper chemical name for them is hydroxides. a And now we have to study a third great group of compounds, called salts; and the reason why we have kept their study to the last is that salt is made when an acid meets an alkali, or base. In studying the acids we saw that, while every acid contains hydrogen, and no acid contains a metal, yet the hydrogen of an acidor part of it can always be replaced by a metal, forming a salt. We shall now see how this happens. WHEN AN Acid meets an ALKALI A SALT IS MADE Suppose we take some hydrochloric acid, HCl, and let it meet some caustic soda, NaOH. This is a very simple case. At once a powerful reaction happens, and a salt is formed. Here is the equation: HCl + NaOH -NaCl + H2O. This means that we take an acid and an alkali, and we get a salt-in this case, the common salt, or sodium chloride, that we know so well. The hydrogen and oxygen of the acid and the alkali have combined to form water; and we get a solution of salt in water. Now let us take another instance. Let us add slaked lime to sulphuric acid, and see what happens. We know that lime is a compound of calcium, and that salts formed from sulphuric acid are called sulphates, so we get calcium sulphate, thus: H2SO + Ca(OH)2 = CaSO, + 2H2O. SHFAMILIAR FORMS OF SALT, AND HOW THEY MADE If you test this rather difficult equation, you will find that it is right. CaSO, is calcium sulphate, and water is formed as in the last case. But this time the salt is almost insoluble in water, and so we find a white mass of stuff, which is the salt. This salt occurs in nature in great masses of rock called gypsum or alabaster, and is very much prized for its beauty. Another form of it is called plaster of Paris, and occurs as a powder. If water is added to the powder it soon "sets," and so we can use it for making casts, or images, of all sorts of things. In these instances we see the way in which the hydrogen of an acid can always be replaced by a metal. The method is to bring an alkali to act upon the acid. We see, too, that every salt consists THREE KINDS OF COMPOUNDS of two parts. It is a sort of double thing, having one part derived from an acid, and one part derived from an alkali, or base. These parts are called radicles, which means little roots. Thus the salt CaSO, consists of an alkaline radicle and an acid radicle, as we see when we look at it. How A STRONG ACID WILL TURN A WEAK ACID OUT OF A SALT And now we are to learn that different acids have varying degrees of power, and that a powerful acid will commonly turn out from a salt the acid radicle that goes to make it up, and will replace it by its own acid radicle. The strongest acids are those we began by mentioning-hydrochloric, sulphuric, and nitric acids; and among the weakest are prussic acid and carbonic acid. We call prussic acid weak because its salts can always be decomposed by other acids. Let us take an easy instance. act on sodium carbonate with hydro If we chloric acid, we find that the carbonate is decomposed, and the acid radicle of the stronger acid replaces the acid radicle of the weaker acid, thus: = 2HC1+ Na CO, 2NaCl + HCO3. We get sodium chloride again, and true carbonic acid. But instead of H.CO, we might have written H2O + CO2, for some of the carbonic acid gas of the acid is given off to the air. In so doing it forms bubbles, or it effervesces, as we say; and we commonly know that we have been acting on a carbonate when we add an acid to a salt and find that bubbles are produced. This will teach us that one of the easiest ways of making an acid is to take one of its salts and act upon it with a stronger acid, which turns it out and takes its place. For instance, if we want hydriodic acid, HI, we have only to take an iodide, such as KI, and act upon it with hydrochloric acid, thus : KI + HCl = KCl + HI, which tells us that the iodide is decomposed, potassium chloride and hydriodic acid being formed. 1801 potassium; but there is also the salt KHSO, in which only one of the hydrogen atoms has been replaced by potassium. We call these acid salts. Acids and alkalies vary in their strength. If we have a salt made of a weak acid and a strong alkali, the salt will be really more alkaline than acid; and though it should be neutral to litmus paper, we find that it turns red litmus blue, just as if it were an alkali. Thus, sodium chloride is neutral to litmus paper, for it is composed of a very strong alkali and a very strong acid, and these balance one another. But if instead of sodium chloride we take sodium carbonate, Na,CO, (which we usually call washing soda), we have a salt made of a strong alkali and a weak acid, and this salt turns red litmus blue. This case also illustrates for us what was said about cases where only half the hydrogen of an acid is replaced by a metal. HALWAYS GOING ON IN OUR BODIES SODA ILLUSTRATES A CHANGE Just as we have KHSO,, so we have NaHCO3, and this is usually called sodium bi-carbonate, or baking soda. It is called bi-carbonate because from one point of view it contains twice as much carbonic acid as the carbonate; but that is really only another way of saying that it contains half as much sodium. The way to make it is to add another dose of carbonic acid to the carbonate, thus : Na2CO+H.CO1 = 2NaHCO3. This reaction is one of the most important in the world, and is constantly happening in our blood as it runs in our tissues, and by this means the carbonic acid they produce is picked up and carried to the lungs, where the equation works in the opposite direction, CO, and H.O (that is, H.CO) being given off by the lungs, and Na CO, being re-formed in the blood to do its work again. When a reaction, like this one, may work in both directions, we write it in a special way, thus: Na CO+H2CO 2NaHCO. The arrows show us that the change may go in either direction, or is reversible. Of all the millions of compounds we have only glanced at a few, but we know what is meant by the words acid, alkali (or base), and salt, and what are the relations between these three kinds of compounds. now The next part of this is on page 1869. |