(iii) Compound decomposed by heat into water and another compound, but not formed by bringing together water and that other compound; CuO, H2O. We also notice that the compounds under (i) are decomposed by heat at different temperatures, varying from a little above 0o in the case of C1.5H,O, to 400° or 500° in the case of CaO.H,O. Inasmuch as each substance we have been considering is either formed by the combination of water with another substance, or is resolved into water and another substance, we are justified in calling them all compounds of water with other elements or compounds. But in many of its interactions with elements and com- 105 pounds water is decomposed, and new bodies are formed which cannot be regarded as compounds of water. We have learned that when sodium and water interact the products are hydrogen and sodium hydroxide; the equation Na + H2O + Aq= NaOHAq + H expresses the composition of the system before and after this interaction. 2 A similar change occurs when potassium is thrown into water; K+ H2O + Aq = KOHAq + H. In each of these reactions much heat is produced; in the case of potassium the reaction proceeds very rapidly, and the temperature of the hydrogen produced is raised so much that this gas takes fire. Fig. 15. M. E. C. 6 When water-gas is passed over hot magnesium, or hot finely divided iron, in an apparatus as represented by fig. 15, hydrogen is obtained, and oxide of magnesium or iron is formed and remains in the tube in which the magnesium or iron was heated. Quantitative experiments have proved that for every 18 parts by weight of water decomposed 2 parts by weight of hydrogen are obtained, and an oxide of magnesium or iron is formed by the union of the 16 parts by weight of oxygen, formerly combined with the 2 parts of hydrogen, with 24 parts of magnesium or 42 parts of iron. The combining weights of magnesium and iron are 24 and 56, respectively; the reacting weights of the oxides formed in the process just described are 40 and 232, respectively, and the compositions of these oxides are represented by the formulae MgO and FeO. Knowing that the combining weight of oxygen is 16, and the reacting weight of water is 18 (H2O), we can summarise the changes of composition which occur in the reactions between steam and heated magnesium or iron in these equations (Mg=24, Fe = 56, O= 16);— When the gaseous element chlorine is passed into boiling water and the mixture of steam and chlorine thus obtained is passed through a porcelain tube, loosely packed with pieces of porcelain, and heated to bright redness, oxygen and hydrogen chloride are produced. The apparatus represented in fig. 16 may be used. The exit end of the porcelain tube is connected with a vessel containing caustic potash solution (A). The gases coming from the tube bubble through this solution. Hydrogen chloride is absorbed by caustic potash, but oxygen is not. The gas which is not absorbed by the caustic potash is collected and proved to be oxygen. Quantitative experiments shew that the compositions of the interacting substances and of the products of the interaction are expressed by the equation (Cl = 35·5, O = 16);— H2O+2C1 = 2HCl + 0. 18 +71 = 73 + 16. For every 18 parts by weight of water decomposed, 71 parts of chlorine are used, and 73 parts of hydrogen chloride and 16 parts of oxygen are produced. As all the substances taking part in this reaction are gases under the conditions of the experiment, and as we know (i) that the symbol of an elementary gas (with a few exceptions) represents the mass of Fig. 16. it which occupies 1 volume (i.e. the volume occupied by unit weight of hydrogen), and (ii) that the formula of a compound 106 107 gas represents the mass of it which occupies 2 volumes (v. anțe, par. 88), the foregoing equation tells that 2 volumes of watergas react with 2 volumes of chlorine gas to produce 4 volumes of hydrogen chloride gas and 1 vol. of oxygen gas. H2O+2C1 = 2HCl + O. The volumes on each side of the sign = are not the same; the masses on each side of the sign = are, and in chemical equations always are, the same. Our study of the properties of water has served to illustrate the nature of chemical change; to emphasise the distinctions between mixtures, elements, and compounds; to shew the importance of the laws of chemical combination; to familiarise us with the use of chemical formulae and equations; and to illustrate the meanings of the terms analysis and synthesis. This study has kept before us the notion of each element and compound interacting chemically with other elements and compounds in certain definite masses which are all simple multiples of one and the same mass. It seems as if a quantity of water, for instance, were composed of a vast number of little particles of water the masses of all of which are the same, and as if chemical interaction occurred between 1, 2, 3,......n of these little particles and a definite number of little particles of the element or compound with which the water interacts. The chemical conception of every element or compound having its own reacting weight leads to some such physical conception as this of small definite particles. Finally, the slight examination we have given to the chemical properties of water has shewn very clearly how closely interwoven chemical changes are with physical changes, and how impossible it is to arrive at any trustworthy conclusions regarding either otherwise than by quantitative experiments and accurate reasoning. Air. When magnesium is burnt in air magnesia is produced; but magnesia is a compound of magnesium and oxygen, therefore, the chemical change which occurs during the burning of magnesium in air consists in the combination of magnesium and oxygen. Therefore, in all probability, oxygen is a constituent of air. When mercury is heated in a measured quantity of air, mercury oxide is produced, and some of the air disappears; when the oxide is collected and strongly heated, oxygen and mercury are formed, and the quantity of oxygen is equal to the quantity of air which disappeared. Therefore the heated mercury combined with a part of the air in which it was heated, and this part was oxygen (v. ante, par. 18). From these experimental results we may conclude that if magnesium or mercury is burnt in air, the air which remains after burning will almost certainly differ in properties and composition from the air which was present before burning began. And if this is so we may further conclude that when any element which is known to combine with oxygen is burnt in an enclosed volume of air, the whole or a part of the oxygen in the air will combine with the element, and the air which remains will most probably differ from the original air. Phosphorus is an element which is easily burnt, and which 108 very readily combines with oxygen. Let an apparatus be arranged as shewn in fig. 17. A is a glass jar; the space from the cork to within about 3 or 4 inches of the open end is divided into 5 equal parts. The jar is placed, open end downwards, in such a quantity of water that the level of the water stands at the point where the graduation of the jar begins. B is an iron cup supported on an iron pillar with a broad foot. Let a piece of dry phosphorus be placed on B; let the jar be put over the phosphorus and iron stand; let the end of the brass chain be highly heated, and then let the chain be brought quickly into the jar, as shewn in the figure, so that the heated part of the chain touches the phosphorus. The phosphorus begins to burn, white clouds of phosphorus oxide fill the jar, and the water slowly rises in the jar. When the burning is finished and the clouds have disappeared the phosphorus oxide produced dissolves in the water-let water be poured into the outer vessel until the level of the water inside and outside A is the same. It is seen that the air has disappeared. Withdraw the cork, and plunge a lighted taper into A; the flame is instantly extinguished. Therefore the air in A after the burning of phosphorus is not the same as the air before the phosphorus was burnt. Fig. 17. B of By a little careful manipulation, portions of the air which remain after the phosphorus is burnt may be transferred from A to glass tubes or bottles, and the properties of this air may |