to test paper, acid calcium carbonate is neutral, while acid sodium carbonate is alkaline. A third class of salts is formed by the association of one or more molecules of normal salt, with one or more additional molecules of the base: these are known as basic salts. Thus, carbonic acid and the base lead hydroxide form such a salt known as basic lead carbonate. CHAPTER IV CHEMICAL SYMBOLS CHEMISTS are agreed in adopting certain symbols to denote the atoms of the various elementary forms of matter. The table on page 22 contains the names of the elements at present recognised, and in the second column are given the symbols which are employed to represent their atoms. The names of the rare elements are printed in italics. In a number of instances the atomic symbol is the initial letter of the ordinary name of the element: thus Boron, B; Carbon, C ; Fluorine, F; Hydrogen, H; Oxygen, O; Sulphur, S. When more than one element has the same initial, either the first two letters of the name, or the first and another that is prominently heard in pronouncing the word are employed, as Bromine, Br; Cobalt, Co; Chlorine, Cl; Platinum, Pt. In some cases letters taken from the Latin names for the elements are used, such as Antimony (Stibium), Sb ; Gold (Aurum), Au ; Silver (Argentum), Ag; Lead (Plumbum), Pb; and Iron (Ferrum), Fe. These symbols are not intended to be employed as mere shorthand signs, to be substituted as abbreviations for the full names of the elements, but in every case they denote one atom of the element. The symbol H stands for one atom of hydrogen, the symbol O stands for one atom of oxygen; Cl means one atom of chlorine, and Ag represents one atom of silver. No other use of these symbols is legitimate. It has been already mentioned (page 8) that the molecules of the different elements are composed of different numbers of atoms; for example, the molecule of hydrogen consists of two atoms, and ordinary oxygen also forms diatomic molecules. These facts are expressed in chemical notation by the use of small numerals placed immediately after the symbol of the atom, thus H2 denotes a molecule of hydrogen, O, a molecule of oxygen. The molecule of ozone consists of an aggregation of three atoms of oxygen, and is represented by the symbol O3, while the tetr-atomic character of the phosphorus molecule is expressed in the symbol P4. The composition of compound molecules is expressed by placing the symbols of the atoms which compose such molecules in juxtaposition thus a molecule consisting of one atom of sodium (symbol Na) and one atom of chlorine (symbol Cl) is represented by the united symbols of these two elements, NaCl; a compound consisting of one atom of carbon and one atom of oxygen by the symbols of these two atoms, CO. Such arrangements of symbols representing molecules are termed molecular formula, or, simply, formula. When the molecule contains more than one atom of any particular element, this fact is indicated by the use of numerals placed immediately after the symbol to be multiplied: thus, a molecule of water consists of two atoms of hydrogen and one atom of oxygen, the formula for water is therefore H2O. One molecule of ammonia, consisting of an atom of nitrogen with three atoms of hydrogen, is represented by the formula NH; and a molecule of sulphuric acid, which is an aggregation of two atoms of hydrogen, one atom of sulphur, and four atoms of oxygen, has the formula H2SO It is sometimes necessary to represent the presence in a molecule of certain groups of atoms, groups which seem to hold together and often to function as a single atom. This is accomplished by the use of brackets: thus (NH4)2SO4 is the formula for a molecule containing one atom of sulphur, four atoms of oxygen, eight atoms of hydrogen, and two atoms of nitrogen; the nitrogen and hydrogen atoms being present as two groups, in each of which one nitrogen atom is associated with four hydrogen atoms. Such groups of atoms are termed compound radicals. When it is required to indicate more than one molecule of the same substance, numerals are placed immediately in front of the formula thus 2H2O signifies two molecules of water, and 3NH3 expresses three molecules of ammonia. By means of these symbols and formulæ, chemists are enabled to represent, in a concise manner, the various chemical changes which it is the province of chemistry to examine. Such changes are usually termed chemical reactions, and they are represented in the form of equations in which the symbols and formulæ of the reacting substances as they are before the change are placed on the left, and those of the substances which result from the change upon the right, thus H2+Cl2 2HCI The sign has a different significance as used on the left side of the equation to that which it bears upon the right. On the left hand it implies that chemical action takes place between the substances, while on the opposite side it has the simple algebraic meaning. Thus, the second of the above equations is understood to mean, that when the compounds, mercuric chloride and potassium iodide, are brought together in such a way that chemical action results, a redistribution of the atoms will take place, resulting in the formation of mercury iodide and also potassium chloride. As further illustrations of the use of chemical symbols, the following three examples may be given as exemplifying the three modes of chemical action mentioned on page 13 : (1) NH3 + HCl = NH,CI. Ammonia combines with hydrochloric acid, and gives ammonium chloride. (2) H2SO4 + Na2CO3 = Na„SO + CO2 + H2O. Sulphuric acid combines with normal sodium carbonate, and gives normal sodium sulphate, carbon dioxide, and water. (3) (CN)O(NH,) = (NH,),CO. Ammonium cyanate is converted into urea. In all cases where the nature of the chemical change is understood, it is capable of expression by such equations, and as matter is indestructible, every atom present in the interacting molecules upon the left of the expression reappears on the right-hand side in some fresh association of atoms.* * See also Chemical Notation, chapter vii. |