Exp. 3. To a solution of silver nitrate add a dilute solution of hydrochloric acid; a white solid forms at once (silver chloride). C. Chemical change is frequently brought about by the interaction of two or more kinds of matter aided by heat. Compare Exp. 1, Chap. I. Exps. 2, 3, 5, and 6, Chap. II. and Exp. 3, Chap. III. Exp. 4. Place a little sulphur in the cup of a deflagrating spoon (Fig. 9), and bring it into a jar of oxygen; no change occurs. Now heat the sulphur in a Bunsen-lamp until it begins to burn and then plunge it into the jar of oxygen. The sulphur burns brilliantly; when the burning has ceased, withdraw the spoon, pour some distilled water into the jar and shake briskly. Taste a small drop of the solution; it is sour. Pour some blue litmus into the solution; the colour changes to red. The sulphur and oxygen have combined to form a new substance, sulphurous oxide, which has dissolved in the water. Fig. 9. Exp. 5. Pour a little dilute solution of nitric acid over a piece of copper in a test tube; no visible change occurs. Warm the contents of the tube; chemical change soon begins. How do you know that the change which occurs is chemical? Exp. 6. Put a small piece of charcoal with some potassium chlorate in a dry test tube; no change occurs. Heat the contents of the tube until the potassium chlorate melts and gives off gas; the carbon burns brilliantly forming carbon dioxide. Prove that carbon dioxide is produced by allowing some of the gas to fall into a tube containing a little lime water (solution of lime in water), and shaking up; a white pp., calcium carbonate, is produced (s. Fig. 10). Fig. 10. The production of this white pp. in lime water is a test whereby the presence of the gas carbon dioxide may be detected. Carbon dioxide is a compound of carbon and oxygen: what was the source of the oxygen in this experiment? In the preceding experiments it is to be noticed that when a chemical change occurred by the interaction of two or more substances, one at least of the substances was in the liquid or gaseous state. The change of a mixture of iron and sulphur to iron sulphide appears to be an exception; but when we remember that sulphur is easily melted, this change will be seen to be no exception to the statement. D. Chemical change is sometimes brought about by the electric current. Compare Exp. 3, Chap. III. Exp. 7. Pass an electric current through a solution of silver nitrate to which a little nitric acid has been added. Silver separates at the negative electrode. The arrangement shewn in Fig. 11 is a simple one for such B A Fig. 11. experiments as the electrolytic decomposition of silver nitrate solution. A and B are plates of platinum attached to platinum wires which pass through a cork in a small wide-mouthed bottle; the cork fits loosely so that any gas evolved during the electrolysis may escape. The platinum plates are called electrodes; that on which the silver (or copper, 8. Exp. 3, Chap. III.) separates is sometimes called the kathode and the other the anode. Exp. 8. Pass an electric current through water containing a little sulphuric acid. Arrange the apparatus so that each electrode (of platinum) passes a little way into a large tube full of acidulated water inverted over it (Fig. 12). Gas collects in each tube; after a time stop the current; cover the mouth of the tube which contains the larger volume of gas with the thumb, keeping the tube under the liquid all the time; invert the tube, withdraw the thumb, and at once bring a lighted taper to the mouth of the tube; the gas takes fire with a slight explosion, and burns with a nearly colourless flame. Now get ready a glowing chip of wood; cover the mouth of the other tube with the thumb, invert the tube, withdraw the thumb, and plunge the glowing chip into the gas; the wood bursts into flame, but the gas does not take fire. The first gas is hydrogen, the second is oxygen. You have learned in this experiment how to detect hydrogen and oxygen and to distinguish them from each other. CHAPTER VI. CHEMICAL PROPERTIES OF WATER. Exp. 1. Place a small piece of sodium in a little cage of wire gauze attached to a glass rod (Fig. 13). Fill a large test tube with water, and invert it in a small basin of water; hold the tube with one hand, and with the other bring the wire cage containing the sodium under the water so that the gas which at once begins to bubble through the water passes into the tube and collects there. When the tube is full of gas, cover the mouth with the thumb, invert the tube, and bring a lighted taper to the mouth; the gas takes fire and burns with a pale almost non-luminous flame. The gas is hydrogen. Fig. 13. Evaporate the water in the basin to dryness; the white solid which remains is a compound of sodium, hydrogen, and oxygen; it is called sodium hydroxide, or caustic soda. (The composition of this compound cannot be proved at present.) By the interaction of sodium and water, hydrogen and a compound of sodium with hydrogen and oxygen, have been formed. Sodium is an element; if this is taken as proved, it follows that the hydrogen evolved as gas in the foregoing experiment, and also the hydrogen and oxygen which combined with the sodium, must have formed part of the water at the beginning of the experiment. (Here we assume that the material of the vessels was not chemically changed during the process.) Water therefore is a compound of hydrogen and oxygen. In Exp. 8, Chap. V. water was decomposed into hydrogen and oxygen by the action of an electric current. In Exp. 3, Chap. II. water was produced by passing hydrogen over hot copper oxide. Water interacts with certain elements at high temperatures to produce oxygen and a compound of hydrogen with the interacting element*. The composition of water can be proved only by very accurate quantitative experiments. The composition of water is ; ; By weight. Hydrogen 1 part form 9 parts By volume; water gas. Oxygen 1 vol.form 2 vols. Hydrogen 2 vols. Jof water-gas. (8. "ELEMENTARY CHEMISTRY." Chap. VII. pars. 92 to 96.) Hydrogen and oxygen are not generally prepared by decomposing water. Exp. 2. Fit up an apparatus like that represented in Fig. 14. Place some granulated zinc in the bottle, cover it Fig. 14. with water, and pour a little dilute sulphuric acid into the bottle through the funnel-tube. The zinc and sulphuric acid react to produce zinc sulphate, which remains dissolved in the water, and hydrogen, which passes off as gas. While the air * The production of oxygen and hydrogen chloride by the interaction of chlorine and steam at a red heat should be demonstrated to the student. |