Molecules of Solids and Liquids.-The formulæ applied to gases are generally extended for convenience sake, but by imperfect reasoning, to the same substances in the liquid and solid state. But it is right to point out that neither hypothesis nor experiment tell us anything of the number of atoms contained in the molecules of solids and liquids. We have good reason for believing that a molecule of iodine vapour contains only two atoms; but, for aught we know, a molecule of solid iodine may contain a thousand. The formula for common salt, which is non-volatile, is written NaCl, on account of its analogy, not with solid, but with gaseous HCl. The formula NaCl does indeed tell accurately, as far as our knowledge goes, the relative number of atoms of sodium and chlorine that are present in the compound, but it does not tell us the absolute number that are present in each molecule. The true formula for common salt, as compared with that of hydrochloric acid gas, HCl, would be Na, Cl, n standing for an unknown and probably very large number. In other words, the formula for common salt, or for any other solid or liquid, is probably only the formula for the nth part of one molecule. There is both convenience and propriety in the use of the ordinary formulæ for solids and liquids, inasmuch as they truly represent the relative quantities of matter which are concerned in chemical changes; but it should not be forgotten that these formulæ differ from those of gases in that the latter tell us, in addition, how much matter is present in a certain space, and also, if the molecular hypothesis be adopted, the absolute number of atoms in each molecule. Hypothesis to Account for Atomicity.-The different equivalent value of different elementary atoms can be accounted for by supposing that each atom has the power of fixing to itself a certain number of other atoms. It is as though a carbon atom, for example, had four arms by which it could grasp and be grasped by four hydrogen, or chlorine atoms, or two oxygen, or sulphur atoms, each of the latter having two arms of its own. It must, of course, be understood that this is a mere illustration, for no one believes that the atoms have real arms projecting from them. But the illustration gives a lively idea of the hypothesis; and atoms are sometimes figured as circles with arms (called bonds) proceeding from them. The following figures represent a few of these atoms and their compounds: The variations of atomicity by pairs, which have been described before (page 85), are explained by supposing that the bonds of an atom have the power, under certain circumstances, of neutralizing one another in pairs. To carry out the simile used before, the carbon atom has four arms, and is therefore a tetrad; but if two of those arms are clasped together, they will not so easily grasp other atoms, and the carbon atom will therefore act as a diad. The hypothesis, put in this form, appears somewhat fanciful, but it harmonizes well with known facts, and has the merit of assisting wonderfully in the comprehension of the complex compounds of organic chemistry. EQUATIONS, OR FORMULE OF CHEMICAL CHANGE. Chemical changes of all kinds can be very conveniently represented in the form of equations, the formula for the substances concerned being written in the first half, and the formulæ for the new substances produced in the other. Thus the combination of zinc and chlorine is thus expressed : Zinc. Chlorine. Zinc Chloride. Zn" +Cl2 = Zn′′ Cl2; which may be read in the following manner. One molecule of zinc added to one molecule (two atoms) of chlorine is equal to, or rather produces one molecule of zinc chloride. EQUATIONS, OR FORMULE OF CHEMICAL CHANGE. 93 Such equations are often called formula, which introduces a little confusion, since we have already seen that the term formula is also applied to the aggregate of symbols which denotes the molecule of a single element or compound. In making out an equation of this kind, it is necessary, as with algebraical equations, to take care that every atom which appears in the first half shall duly appear and be accounted for in the second. The following equation, which expresses the oxidation of ethylene gas, illustrates this: Two atoms of carbon, four of hydrogen, and six of oxygen are concerned in the change, and it will be seen that though differently arranged, they are all accounted for in the second half of the equation. The large figures in the above and in similar equations refer, it must be remembered, to the whole molecule. 2CO2, for instance, means two molecules of carbonic anhydride, containing together two atoms of carbon and four of oxygen. A few more equational formulæ, one or two of them rather complex ones, may, with advantage, be studied in this place. They represent chemical changes of several kinds. 2 Potassium Iodide. = Mercuric Potassium Hg" Cl2+ 2 KI = Hg" I1⁄2 + 2 K Cl. The brackets in the two last examples will readily be understood. Cu" (NO3)2 expresses that one atom of the diad metal copper is united with two units of the monad radical NO.. One great advantage of these equations is that they afford us the means of calculating the respective quantities by weight in which bodies act on one another. Take, for example, the above-described action of mercuric chloride and potassium iodide. HgCl, means one atom of mercury weighing 200 combined with two atoms of chlorine weighing 35.5×2=71, total 271. KI means one atom of potassium 39 and one atom of iodine 127, total 166; or, as 2KI is employed, 332. We therefore know that 271 parts (pounds, grammes, or tons) of mercuric chloride will act upon 332 parts of potassium iodide, and in a similar manner we can calculate that 454 parts of mercuric iodide (HgI, = mercury, 200+ iodine, 127 × 2 254), and 149 parts of potassium chloride will be produced during the change. Now suppose that a chemist has 100 grains of mercuric chloride, and wishes to know how much potassium iodide he must add to convert all the mercury into iodide. He knows that 271 parts of the chloride will require 332 of the iodide, and he has therefore only to perform a simple proportion sum. = 271: 332 :: 100 : x = 122 grains. In like manner he can readily find out that he ought to obtain as the result of the action 167 grains of mercuric iodide and 55 grains of potassium chloride, for as 271 parts of mercuric chloride yield 454 parts of mercuric iodide and 149 parts of potassium iodide, 271 454 : 100 : x = 167 grains and 271: 149: 100: x = 55 In short, to sum up the whole reaction, 100 grains of mercuric chloride, with 122 grains of potassium iodide, will yield 167 grains of mercuric iodide and 55 grains of potassium chloride. It is of course equally easy to find out how much mercuric chloride and potassium iodide must be employed to yield a certain weight, say 100 grains of mercuric iodide. When the equations refer to gases they have another advantage. As every single formula for an element or compound denotes two volumes of gas, the volumes of different gases concerned in a reaction can at once be inferred from the equation. In the equation given above C2 H1+302 = 2 CO2+ 2 H2 0, 2 C2 H4 means 2 volumes of ethylene, and 30, means 3×2=6 volumes of oxygen; so that 2 volumes of ethylene require 6 volumes of oxygen for their complete oxidation. There will be produced during the action, 2CO2, that is, 4 volumes of carbonic anhydride and 2H2O, that is, 4 volumes of steam. For if CO2 represents 2 volumes, 2CO, must evidently represent 4 volumes. It must not, however, be forgotten that these relations of volume are only correct if the pressure and temperature remain unaltered. If otherwise, a correction must be made by the methods already given (pages 34, 57). ACIDS, BASES, SALTS. Allusion has already been made (pages 49, 50) to certain substances which have long been known under the respective names of acids and bases. Acids are sour and redden litmus, and bases, when soluble, have what is called an alkaline taste, and turn the colour of reddened litmus back again to blue. They have a kind of antagonistic function, and will neutralize one another. When an acid acts on a base, a new compound called a salt is produced, which commonly has no action on either blue or red litmus. are But although in well-marked cases acids, bases, and salts so different from one another in properties, modern chemistry has taught us that the compounds usually known by those names bear so much resemblance to one another and to other compounds in structure, that it is impossible to |