CHAPTER II. NON-METALLIC DIADS. (Oxygen, Sulphur, Selenium, Tellurium.) OXYGEN. Symbol, O 16. Formula, Og OXYGEN is the most abundant of the elements, for it is probable that not less than two-thirds of all that portion of our globe which is known to us consists of it. About one twobillionth part of this oxygen exists in the atmosphere in a free, or uncombined state; the remainder occurs in combination with various other elements. It constitutes eight-ninths of the weight of water, and about one-half of the weight of the chief rocks, and it is an important constituent of all animals and vegetables. In the pure state it is a gas of specific gravity 16, that is, about one-tenth heavier than common air. It is colourless, tasteless, and inodorous, and is very slightly soluble in water. Its compounds are called oxides, and oxides of all the elements, except fluorine, are known. The gas is magnetic; for a balloon filled with it is attracted by the poles of a powerful magnet. Oxygen cannot easily be prepared from air, and in the great majority of its most abundant compounds it is held in combination by a force too strong to be easily overcome; but a certain number of oxygen-compounds are known, which are so unstable, that either by heat, or by some other agency, the element may be displaced and collected in a pure state. PREPARATION. Oxygen was first prepared by Dr. Priestley, in 1774, from mercuric oxide, by the process already described (page 61). It is also obtained when water is decomposed by electricity (page 12). Experiment 1.-Into the same apparatus that was used for heating mercuric oxide, introduce 100 grains of potassium chlorate, and heat it carefully with the spirit-lamp. The salt will soon melt, and afterwards boil. As soon as the boiling commences the flame must be lowered, to prevent the mass from frothing over. When the liquid thickens, if some of the substance should be found adherent to the colder parts of the tube, approach it with the flame of the lamp until it is again melted down. The gas may be collected over the pneumatic trough, just like hydrogen; but when it ceases to come off, the delivery tube must immediately be removed from the water. Instead of using pure potassium chlorate, it is much better to mix the salt with about half its weight of carefullydried manganese peroxide. The oxygen is then given off at at a lower temperature and more easily, but the manganese peroxide remains unaltered after the experiment. The following equation shows the change which takes place in this important reaction : Potassium Potassium Chloride. 2KC10, = 2K C1+302. Potassium chlorate contains for every one hundred grains nearly forty grains of oxygen chemically combined; by the application of heat, these become free and escape. Mercuric oxide contains nearly eight per cent. of oxygen; therefore the former will yield five times more oxygen than the latter. If phials of twelve ounces' capacity are selected for receiving the gas, we shall be able to fill five of them, and shall have in each about eight grains, or nearly twenty cubic inches, of oxygen. Potassium chlorate may, under some circumstances, as when strongly rubbed, or treated with sulphuric acid, occasion very dangerous explosions; but no danger is to be apprehended from the application of it in the manner above directed. Experiment 2.-Add warm water to the salt remaining in the test-tube after the expulsion of the oxygen, and place the tube in a warm place until the salt is dissolved; evapo rate the solution gradually, when small cubic crystals of potassium chloride will be deposited. The potassium chlorate crystallises in thin tables or plates, the heated mass in cubes; this difference in the form of the crystals indicates that, by the heating of the former, an entirely new salt is formed. It is indeed one which no longer contains oxygen. Oxygen may also be prepared by the following processes, which, however, are less convenient on the small scale than the foregoing: 1. By heating manganese peroxide to redness in an iron tube closed at one end: 2. By passing the vapour of sulphuric acid through a redhot tube: 2 H2SO1 = 2 H2O + 2 SO2+ O2. The sulphurous anhydride may be absorbed by passing the gas through lime. This, on the large scale, is the cheapest process for preparing oxygen. PROPERTIES. Experiment 3.-Introduce a glowing shaving into a bottle of oxygen; it will kindle and burn for some time with great brilliancy and with a very dazzling flame, and then be extinguished. The same takes place when a piece of lighted tinder is fastened to a wire and suspended in the oxygen; the tinder burns with a lively flame, while, as is well known, it merely smoulders away in the open air. Oxygen, at a high temperature, combines eagerly with the component parts of wood and tinder, and heat and light are developed. When the combination is ended, and the oxygen is consumed, the combustion ceases. The product of the combustion, that is, the combination of the wood with the oxygen, is also aeriform; but burning substances are extinguished in the newly-formed gas. If the bottle be rapidly whirled round, the gas formed by the combustion will escape, and atmospheric air will supply its place. Air contains free oxygen; and a kindled shaving will burn in it for some time, but far slower and less briskly than in pure oxygen; because common air contains only one-fifth part of oxygen. Accordingly, combustion proceeds much more rapidly and violently in oxygen than in atmospheric air. Experiment 4.-Fasten a piece of charcoal to a wire, and kindle it in the flame of a lamp, and then introduce it into a bottle of oxygen; it will burn very vividly, and, if the charcoal had much bark on it, with beautiful sparks. If a piece of moistened blue litmus-paper be introduced into the bottle, after the combustion, it will be reddened; consequently an acid oxide has been formed from the charcoal and the oxygen; it is called carbonic anhydride. Pour into the bottle a little lime-water (which is prepared by shaking slaked lime with water for a short time and filtering). The clear lime-water becomes milky from the formation of the insoluble calcium carbonate. This is a good test for carbonic anhydride. Fig. 45. Fig. 46. Experiment 5.—If some pieces of sulphur are fastened to a longer wire, kindled and suspended in a second bottle, they will burn with a beautiful blue flame. The gas formed from this union of sulphur and oxygen has a very irritating odour; when dissolved in water, it likewise turns litmus-paper red, and consequently it is of an acid nature. It is called sulphurous anhydride. Experiment 6.-Take a small piece of phosphorus, which, on account of its inflammability, must be cut off under water from the stick, and place it, after it has been well dried between blottingpaper, in a scooped-out piece of chalk. Fasten the latter to a wire, and introduce it into a third bottle of oxygen. Affix the wire to a cross piece of wood, so that the chalk may hang a little below the centre of the bottle. If the phosphorus be now touched with a hot wire, it will kindle and burn with a dazzling brilliancy, filling the bottle with a thick white smoke. This smoke consists of a chemical compound of oxygen and phosphorus; it reddens the blue test-paper, and consequently is also an acid oxide; it is called phosphoric anhydride. If the bottle be allowed to stand for a time, the smoke will sink to the bottom and dissolve in the water which remains there, which thus acquires an acid taste. The two last experiments, and many others, can be more conveniently performed in deflagrating spoons, which are merely little ladles of iron or brass fastened at right angles on the end of iron wires, which pass through a cork and a piece of tin plate. The tin plate rests on the mouth of the bottle. Experiment 7.-Heat a small piece of sodium in a deflagrating spoon till it begins to burn, and then plunge it in a bottle of oxygen. It will burn with great brilliancy, and with a yellow flame. A white oxide, called sodium oxide, Na2O, is formed, which, when dissolved in water, turns red litmus-paper to blue. Oxides of this kind are called basic oxides (page 98). Potassium yields a similar compound. Experiment 8.-A piece of fine iron wire is so wound round a slate- or common lead-pencil, that, on the withdrawal of the latter, the wire may have a spiral form. Fasten the upper part of this wire, as in Experiment 6, to a cross-piece of wood, and place on the lower end of it a small portion of tinder. When this is kindled, introduce the wire into the oxygen; the burning tinder heats the iron to redness, which then burns brilliantly, throwing out sparks. The Fig. 47. iron, when red-hot, combines with the oxygen. The burnt or oxidized iron (iron scales) melts, and falls to the bottom in black globules, which are so hot that they are apt to melt into the glass, though it be partly filled with water. This black oxide of iron, Fe,О, is the same that is formed when steam is passed over red-hot iron in a tube. The inside of the bottle becomes covered during the experiment with a brickred oxide, FeО,, which is called ferric oxide. Common iron rust is ferric oxide. Ferric oxide is a basic oxide, although, being insoluble in water, it cannot be tested with litmuspaper. The black oxide is a neutral oxide. It is neither acid, nor basic. These experiments show that there are three kinds of oxides : 1. Acid oxides, or anhydrides, which by combination with water yield acids. |