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.

S = 32.

Fe = 56, 0=16, FeO means 56 +16= 72 parts by weight of a compound called ferrous oxide; this formula also tells that one c. w. of iron combines with one c. w. of oxygen to form ferrous oxide. Fe2O, means (56 × 2) + (16 × 3) = 160 parts by weight of a compound called ferric oxide; also that 2 c. ws. of iron combine with 3 c. ws. of oxygen to form ferric oxide.

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FeSO, means 56 +32 + (16 × 4) = 152 parts by weight of a compound called ferrous sulphate; also that one c. w. of iron, one c. w. of sulphur, and four c. ws. of oxygen, combine to form ferrous sulphate.

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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 1 : 8 to form water.

These chemical reactions may be shortly expressed thus ;(1) S+ Fe FeS. (2) 2H+0=H2O.

==

Fe = 56, S = 32, O= 16. ratio 2 16 = 1:8.

The ratio 32: 56 = 1 : 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 = is equal to the total mass of matter on the other side.

Let us consider one or two rather more complex reactions. (1) Na + H2O + Aq = NaOHAq + H.

(2)

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(3) Zn + H2SO,Aq = ZnSO2Aq + 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.

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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 a definite mass, and also one combining weight, of that element.

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;

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These formulae suggest a question, the answer to which is of the utmost importance, but a question to which a satisfactory answer cannot yet be given.

as the

Why should we choose to represent hydrogen peroxide as composed of 2 combining weights of hydrogen with 2 c. ws. of oxygen? Water is represented as produced by the union of 1 c. w. of oxygen with 2 c. ws. of hydrogen; why should not the composition of peroxide of hydrogen be represented by the formula HO? The ratio H: O is the same ratio H2: 0,. Again, why should the formula of ferric chloride be Fe,Cl, rather than FeCl, Chloride of antimony, SbCl, is represented as formed by the union of 1 c. w. of antimony with 3 c. ws. of chlorine; why should we choose a formula for ferric chloride which represents the composition of that mass of this compound which is formed by the union of 2 c. ws. of iron with 6 c. ws. of chlorine?

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Similar questions are suggested by the other formulae. In some cases the formula appears to be the simplest that could be given to the compound, e.g. H2O, HS, HCl, SbCl,; in other cases a needless and foolish complication seems to be introduced. Why not HCO, in place of HCO; HC in place of HC; SCI in place of S,Cl,; HO in place of H2O,; FeCl, in place of Fe,Cl? Or if the more complex formulae are to be used, why should such formulae not be always used? Why not HO, or HO, in place of HO; HS, or HS, or H12S in place of HS; Sb,Cl, in place of SbCl,; &c.?

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There must be some reason for these apparent inconsistencies. There are several reasons; but we are not yet in a position fully to understand and appreciate these reasons. We may however gain some notion of the kind of reasoning employed in determining which of several possible formulae best represents the composition and reactions of a compound. The gist of the matter, as we shall hereafter find, is in the conception expressed by the words composition and reactions. So long as we look only at the composition of compounds we cannot find answers to our questions. If we disregard the composition and look only at the reactions of compounds we cannot find answers to our questions.

The symbol of an element represents a certain mass of 86 that element usually called its combining weight. Elements combine in the ratios of their combining weights, or in ratios bearing a simple relation to these.

The formula of a compound represents the composition of a certain mass of that compound; this mass we propose to call the reacting weight of the compound. Compounds interact in the ratios of their reacting weights, or in ratios bearing a simple relation to these.

The reacting weight of water is 18 (H2 = 2 + 0 = 16).

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The combining weight of sodium is 23 (this, as a matter of fact, is the mass of sodium which combines with 8 parts by weight of oxygen). Let us examine the interaction of water and sodium.

When sodium is thrown into water a reaction immediately occurs; the sodium rapidly disappears and hydrogen gas is produced. When the reaction is finished, let the solution be evaporated; water passes away as steam, and a white solid (caustic soda) remains. The composition of this solid is represented by the formula NaOH (Na = 23, O= 16), that is to say, this compound is produced by the combination of one combining weight of sodium, one c. w. of hydrogen, and one c. w. of oxygen. We know that water is a compound of hydrogen and oxygen, and that sodium is an element. Hence the oxygen and hydrogen which form part of the caustic soda Imust have come from the water. But besides caustic soda, hydrogen was produced; this must also have come from the water. Hence when sodium and water interact, a portion of the hydrogen which was combined with oxygen is evolved as hydrogen gas, and another portion enters into combination with the sodium and the oxygen to produce caustic soda. When this experiment is made quantitative, it is found that 23 parts by weight of sodium interact with 18 parts by weight of water, and there are produced 40 parts by weight of caustic soda and 1 part by weight of hydrogen. The 40 parts of caustic soda are composed of 23 parts of sodium, 16 parts of oxygen, and 1 part of hydrogen.

The conclusion from these experiments is, that, as regards the interaction of water with sodium, 18 is the reacting weight of water, and that the decomposition of one reacting weight of water results in the production of 2 combining weights of hydrogen and 1 c. w. of oxygen. But if H = 1 and 0 = 16, the formula H2O (18) summarises the results of this experiment.

M. E. C.

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