Combining weights of nitrogen, phosphorus, and iron. This list might be largely extended; in very few cases should we find but one value for the combining weight (as defined) of an element. To adopt several combining weights for each element would introduce endless confusion into our system of representing the composition of compounds. It is absolutely necessary to adopt one value and one value only, not merely for convenience but also for cogent reasons which will be given later. Sometimes the highest value found by the method already stated is adopted, e.g. for nitrogen (N=14; comp. above results with the table in par. 76); sometimes a simple multiple of this highest value is adopted, e.g. for iron (Fe = 56: s. table in par. 76). If we define combining weight as has been already done, then the definition generally leads to several values for the combining weight of each element. If we call the numbers in the table in par. 76 combining weights, then we cannot accurately define the term combining weight. The best compromise, at any rate for us at present, is to 79 say, that the actually used combining weight of an element is a number which expresses either the largest mass of the element which combines with 1 part by weight of hydrogen, or 8 parts of oxygen, or 16 of sulphur, or 35.5 of chlorine, or it expresses a simple multiple of this mass. The following table presents; in column I., the largest mass of each element which is known to combine with either 1 part by weight of hydrogen, or 8 of oxygen, or 16 of sulphur, or 35.5 of chlorine; and in column II., the actually used values for what are generally called the combining weights of the elements. 80 81 As we advance in our study of chemical events we shall learn that there is no purely chemical, and general, method, by using which a decision may be arrived at regarding the best value to be given to the combining weight of an element. Each case must be discussed by itself; the result is at best a compromise. But we shall also find that the application of certain physical conceptions to chemical phenomena leads to a generally applicable method, based on one definite principle, whereby values may be obtained for what we at present call the combining weights of the elements. The symbol of an element, then, expresses a definite mass of that element. The formula of a compound expresses the masses of the elements, stated as a certain number of combining weights of each element, which combine to form a specified mass of the compound. A number placed beneath (or sometimes above) the symbol of an element in the formula of a compound tells that the symbol is to be multiplied by this number. A number placed at the beginning of the formula of a compound multiplies the whole of the formula, or if a full stop occurs in the formula the number multiplies all as far as that stop; sometimes the formula is put in brackets and the multiplier is placed outside the bracket. The following formulae will illustrate these points. Fe=56, 0=16, S = 32. FeO means 56 +16= 72 parts by weight of a compound called 4 Fe..3SO, or Fe,(SO), means (56 × 2) + 3 (32 + 64) = 400 parts by weight of a compound called ferric sulphate; also that two c. ws. of iron, three c. ws. of sulphur, and twelve c. ws. of oxygen, combine to form ferric sulphate. 3Fe 3SO or 3Fe (SO), means 3{(56 × 2) + 3(32 + 64)} = 1200 parts by weight of ferric sulphate. Chemical changes are also expressible in formulae, so far 82 at least as the composition of the elements or compounds before and after such changes is concerned. Thus, we have learned that (1) Sulphur and iron combine when heated in the ratio 1: 1.75, to form iron sulphide; (2) Hydrogen and oxygen combine in the ratio 18 to form water. These chemical reactions may be shortly expressed thus ;(1) S+ Fe FeS. (2) 2H+0=H2O. = Fe56, S32, O= 16. ratio 2 16 = 1:8. : The ratio 32: 561: 1.75; the The sign + signifies reacts chemically with; the sign signifies with production of. The total mass of matter on one side of the sign to the total mass of matter on the other side. = = is equal Let us consider one or two rather more complex reactions. (1) Na + H2O + Aq = NaOHAq + H. (2) 2 3Fe + 4H20 = Fe ̧O̟ ̧ + 8H. (3) Zn + H2SO,Aq = ZnSO,Aq + 2H. Na = 23, O=16, Fe = 56, Zn = 65, S = 32. (1) When sodium and water interact, 23 parts by weight of sodium and 18 parts by weight of water disappear, and there are produced 40 parts of sodium hydroxide, which remains dissolved in the water that has not been changed, and 1 part by weight of hydrogen. (2) When iron and water interact, 168 parts of iron and 72 of water are changed to 232 parts of iron oxide and 8 parts of hydrogen. (3) When zinc and a solution in water of sulphuric acid interact, 65 parts by weight of zinc and 98 of the acid are changed into 161 parts of zinc sulphate, which remains in solution, and 2 parts of hydrogen. These chemical equations, as they are called, also represent the compositions of the compounds before and after the change, expressed as so many combining weights of each elementary constituent of each compound; when elements take part in the reactions, the equations also express the number of combining weights of this or that element which interacts with a certain mass of a compound, or with a certain number of combining weights of another element, and the number of combining weights of this or that element which is produced by the interaction. Thus (1) states, more shortly than can be done in words, the fact that one c. w. of sodium interacts with 18 parts of water to produce 40 parts of sodium hydroxide and 1 c. w. of hydrogen; and (3) states that one c. w. of zinc interacts with 98 parts by weight of sulphuric acid dissolved in water (which 98 parts are composed of 2 c. ws. of hydrogen, 1 c. w. of sulphur, and 4 c. ws. of oxygen) to produce 161 parts of zinc sulphate (composed of 1 c. w. of zinc, 1 c. w. of sulphur, and 4 c. ws. of oxygen) which remain in solution, and 2 c. ws. of hydrogen. The symbol Aq is used here, and generally in this book, to mean a large (indefinite) quantity of water; when placed after the formula of a compound or element it means that that body is in solution in a large quantity of water. 2 Chemical formulae express other facts regarding chemical 83 changes; these we shall learn as we advance. It is advisable to note here that these formulae and equations do not say anything regarding the conditions under which the chemical interactions occur. Thus Fe + S = FeS only tells us that a certain mass of iron combines with a certain mass of sulphur to produce the sum of these masses of iron sulphide. So Na + H2O + Aq = NaOHAq + H tells that certain masses of sodium and water interact to produce certain masses of sodium hydroxide [which is dissolved in the excess of water (s. ante par. 56)] and hydrogen, and that the sum of the masses of sodium and water is equal to the sum of the masses of sodium hydroxide and hydrogen. The equations in no way indicate the facts that iron and sulphur only combine when heated, but that sodium and water interact at ordinary temperatures. The equation Zn + H2SO,Aq= ZnSO,Aq + 2H expresses certain definite quantitative facts (s. ante); but it does not indicate or even suggest that the compositions of the products of the interaction of zinc and sulphuric acid vary with variations in the temperature at which the interaction occurs, and that the interaction proceeds according to the representation given by the equation only at the ordinary temperature. Chemical equations evidently give very incomplete representations of chemical changes. But nevertheless chemical formulae are of the greatest value, inasmuch as they enable us to exhibit, in a simple and intelligible way, the composition of compounds, and those changes of composition, the study of which forms one part of chemical science. We have learned that the symbol of an element represents 84 definite mass, and also one combining weight, of that element. a The formula of a compound also represents a definite mass of the compound, and tells the composition of that definite mass, both in parts by weight, and also in combining weights, of each of the elements by the combination of which the compound has been formed. The following are the formulae of some well-known compounds;— H2O. Water 2 2 Formic acid H.CO. |