c. Salts. These are formed by the union of acids and bases, or by replacing the hydrogen of an acid by metal and the hydrogen of a basic hydrate by a negative group of atoms.

A salt is named by placing the name of the positive atom first and then that of the negative group, ending in either ate or ite, according as the acid or acid-forming oxide ended in ic or ous. Thus, from sodium oxide and nitric acid is formed sodium nitrate, and from potassium hydrate and hypochlorous acid is formed potassium hypochlorite.

In considering the formulas of salts, we must bear in mind the basicity of the acid and the valence of the positive atom. Thus, if we have a monad metal like sodium and a monobasic acid like nitric acid they will exactly balance, and sodium nitrate, NaNO,, is the result. If with a dyad metal like calcium we unite nitric acid, we must take two molecules of the latter to form the salt Ca(NO3)2. Similarly, if we take sodium and a dibasic acid like sulphuric, we must take two atoms of the metal to neutralize one molecule of the acid and form the salt Na,SO. In general, to form a neutral salt such number of atoms of metal and molecules of acid must be taken as will furnish exactly the same number of bonds. If a lesser number of atoms of metal be taken than demanded by the basicity of the acid, the salt will still partake somewhat of the character of an acid; if the acidity of the base be not completely satisfied by acid, the salt will have something of the character of a base. Hence we distinguish three classes of salts: normal (or neutral) salts, in which the base and acid exactly counterbalance each other; acid salts, in which the hydrogen of the acid is not completely replaced by metal; and basic salts, in which the hydrogen of the base is not completely replaced by acid-forming groups. Thus, Na,SO, is neutral sodium sulphate, while NaHSO, is acid sodium sulphate; HNa,PO, and H„NaPO are both acid salts, while Na,PO, is a neutral salt; Ca(NO3), is neutral calcium nitrate, while Ca(OH)NO, is basic calcium nitrate. III. LAWS OF COMBINATION BY WEIGHT AND VOLUME. 1. Relation of Molecules and Volumes.-According to the law of Avogadro, equal volumes of all substances in the gaseous state contain the same number of molecules, and therefore, as a consequence, the molecules of all substances when in the gaseous state are of the same size. We are thus able to compare equal volumes of gaseous bodies just as if we were comparing single molecules of the respective substances.

2

In

this way we can experimentally determine the relative molecular weights, taking the weight of the hydrogen molecule as standard. 2. Laws of Combination by Weight.-We owe to the English chemist John Dalton two important laws, that of combination in constant proportions and that of combination in multiple proportions. According to the first, each element combines with the others in a fixed and definite proportion by weight, and according to the second, when two elements unite in several different proportions these are simple multiples of each other. Based upon these two laws we have the atomic hypothesis of Dalton, which furnishes the explanation of these relations.

3. Laws of Combination by Volume.-The French chemist Gay-Lussac has stated these relations also in two laws. According to the first, the ratio in which gases combine by volume is always a simple one, and according to the second, the volume of the gaseous product obtained in a combination bears a simple ratio to the volumes of its constituents.

These laws of combination by volume are illustrated in the formation of certain typical hydrogen compounds.

One volume of hydrogen and one volume of chlorine combine to form two volumes of hydrogen chloride.

Two volumes of hydrogen and one volume of oxygen combine to form two volumes of water vapor.

Three volumes of hydrogen and one volume of nitrogen combine to form two volumes of ammonia.

Four volumes of hydrogen and one volume of carbon vapor combine to form two volumes of methane.

Ha

Ha

+ Ca = HAC H4C

H2

H2

4. Relation of Density to Molecular Weight.-As stated, atomic weights are relative numbers based on the weight of the hydrogen atom as unity. Molecular weights are obtained by adding together the weights of the constituent atoms in a molecule. Thus, the hydrogen molecule containing two atoms would weigh 2, and as the molecule of hydrogen chloride containing one hydrogen atom and one chlorine atom weighs 1 + 35.5 or 36.5, the weights of the two molecules are as 36.5 to 2 or as 18.25 Hence the rule usually given, that the density of any body in the state of gas is half the molecular weight. Conversely it is possible by experimentally determining the density of any gaseous body to arrive at its molecular weight.

IV. CHEMICAL REACTIONS AND EQUATIONS.

1. Meaning of Reactions and their Expression in Equations.--A reaction is a chemical change taking place within a molecule or between two or more molecules. The several substances taking part in the change are called reagents. Moreover, as each molecule concerned has its proper chemical formula, we can state the reaction by the use of these formulas. If the several reagents taking part in the reaction are placed together connected by the sign plus and made equal to the several compounds which result also connected by the sign plus, we have a chemical equation. In this case the substances entering into the reaction are called the factors, and the substances issuing from the reaction are called the products.

Moreover, as every atom has its proper atomic weight, which is not altered through all the changes and rearrangement in new

molecules, the chemical equation must be capable of change into a numerical equation in which both sides must sum up alike.

2. Conditions favoring Reactions.-As we have learned in considering the several states of matter, the molecules are freer to move, and so come into the sphere of action, when in the liquid or gaseous state than when solid. Hence fusion or solution of solids and vaporization of liquids are steps which facilitate chemical reaction. In many cases the mixing of two solids produces no reaction, but the same solids will react if fused together, or, if soluble in the same menstruum, when their solutions are admixed.

Two other points should be noted as favoring chemical change. If two substances are taken in solution and then mixed, a product may form which is insoluble in the menstruum employed. This is called a precipitate. If such precipitate can form by the rearrangement of the molecules taken in any case, we have an immediate formation and the reaction goes on to completion. Again, the reaction of two substances may cause the liberation of a gas. In such case the reaction is a ready one and proceeds steadily to completion.

3. Classification of Reactions.-The simplest kind of reaction is that which simply represents the breaking up of a complex molecule into simpler ones. Such change is frequently brought about by heat. Thus, calcium carbonate under the influence of heat breaks up into calcium oxide and carbon dioxide, thus expressed, CaCO, CaO + CO2, or potassium chlorate is decomposed into potassium chloride and oxygen, 2KCIO, = 2KCl+ (O2)3

The reverse of this is found when two simple molecules unite to form a more complex one. Thus, hydrogen and chlorine unite to form hydrogen chloride, H2+ Cl2 = 2HCl, or calcium oxide unites with water to form calcium hydrate, CaO + H2O= Ca(OH)2.

The great majority of reactions, however, involve two factors which by their reaction yield two products, an interchange of atoms taking place. In such cases it must be remembered that this interchange must be according to the laws of valency, and each atom or group of atoms transferred from one molecule to another must be replaced by its exact equivalent.

Thus, silver nitrate and sodium chloride when in solution react with each other to produce silver chloride and sodium nitrate, AgNO,+NaCl = AgCl + NaNO3. Again, sodium chloride and

sulphuric acid react to form sodium sulphate and hydrogen chloride, 2NaCl + H2SO4 = Na2SO + 2HCl.

4. Calculations from Equations.-As before stated, every atomic symbol stands for a definite amount of an element known as its atomic weight. Hence every molecular formula stands for a definite molecular weight, and every chemical equation is capable of translation into numbers.

This makes it possible to calculate readily a variety of numerical relations both from molecular formulas and from chemical equations. Thus, the percentage composition of any chemical compound is readily calculated from its formula. If m represents the molecular weight, a the weight of any constituent atom, n the number of atoms of this constituent, and x its percentage amount, we have the proportion man :: 100: x. This expression by the well-known "rule of three" will enable us to ascertain the values of m, a, n, or x, the other terms being known.

The most generally required calculations, however, are those of quantities entering into or produced in chemical reactions. For these the molecular weights of the factors and the products of a reaction are needed. Thus, if we take the reaction 2NaNO, + H2SO, Na,SO, + 2HNO,, and put it into figures, it becomes 170 +98= 142 + 126. It now becomes possible to calculate how much sodium sulphate or how much nitric acid can be produced from a given weight of sodium nitrate, or how much sulphuric acid it will take to decompose a given weight of sodium nitrate. Thus, 170: 126: 100: 74.12 shows that 100 parts by weight of sodium nitrate will yield about 74.12 parts by weight of nitric acid. It is equally obvious that if the reaction be well understood it is possible for the experimenter to ascertain by calculation how much of any factor is needed to furnish a definite amount of a product.