The reversibility of this reaction may be shown by mixing concentrated solutions of sodium hydrogen sulphate and hydrochloric acid. Sodium chloride is precipitated.

In more dilute solutions, all the substances are soluble, and there is equilibrium:

NaCl +H2SO, NaHSO1+HCl.

In all the cases considered, double decomposition reactions become complete because one of the products is removed in an insoluble form. In neutralization this is not the case: the completeness of the reaction depends, rather, on the fact that water is only very slightly dissociated. Yet, slight as is the ionization represented by the equation,

H2OH+OH,

it is sufficient to produce hydrolysis (cf. § 184) of salts formed from any but the strong acids and bases.

246. Mass Action. The combination of particles (molecules or ions), whether in gaseous form or in solution, is influenced by the frequency with which the particles meet. If, therefore, we wish to stop or to diminish the dissociation of a substance AB, we see to it that an excess of one of the particles produced by the dissociation, e. g., A, is present. By thus increasing the active mass of A, we diminish the possibility that B can exist uncombined with A.

The action of an excess of one of the substances (or ions) produced in a reversible reaction is called Mass Action.

To illustrate: the molecular weight of phosphorus pentachloride, PC15, as determined by vapor density methods, is too low, owing to the dissociation of some of the molecules into phosphorus trichloride and chlorine.

PCI, PC13+Cl2.

The dissociation can be prevented if the vapor density determination is carried out in an atmosphere of phosphorus trichloride; because the chlorine molecules then meet molecules of phosphorus trichloride so frequently that practically no chlorine molecules remain free.

Similar effects occur in solution. Thus, if we wish to precipitate the sulphuric acid, i. e., the SO4 ions, contained in a solution, we use barium chloride solution. If we use exactly the theoretical amount of barium chloride, however, much barium sulphate remains in solution (as Ba and SO, ions). The reaction is only partly represented by the equation,

BaSO,+2 H+2 Cl,

Whatever will cause the SO4 ions to meet more Ba ions in a given time will increase the precipitation of the SO4 ions as barium sulphate. We obtain this result by adding a large excess of barium chloride solution.

247. Exercises.

a

1. How does the amount of steam necessary to "saturate given volume of air (cf. Fig. 53) compare with the amount that

would evaporate into this space if air were not present; that is, if the space were a vacuum "?

2. Write the equation showing the equilibrium between water at 0° C. and some ice floating in it, if the room is also at 0° C. What change would occur if the temperature of the room were raised slightly?

3. Write the equation for the equilibrium between salt in contact with a saturated salt solution. What result would follow if water were added? If concentrated hydrochloric acid were added? Why?

4. To precipitate all the manganese of manganous sulphate solution as the sulphide, MnS (cf. § 256), we use an excess of sodium sulphide. Why?

5. Write the equation for the equilibrium between ammonium hydroxide and its ions. For the equilibrium between ammonium chloride and its ions. If a concentrated solution of ammonium chloride were added to ammonium hydroxide, would the mixture be as strong a base as the ammonium hydroxide alone? Why?

CHAPTER XXI.

SULPHUR AND ITS COMPOUNDS.

248. Occurrence and Preparation of Sulphur. Sulphur occurs in nature in both a free and a combined form. In the free condition it is obtained chiefly from Sicily, Mexico, and, to some extent, from Louisiana. Natural sulphur is usually found mixed with much earthy material, from which it must be separated to prepare it for the market.

The first operation in the purification of sulphur usually consists in heating the natural product; the sulphur melts and flows away, leaving the infusible impurities behind.

In the second operation the partially purified sul

FIG. 54.

phur is distilled from large iron retorts (see Fig. 54), and is thus separated from less volatile impurities.

The melted sulphur in the reservoir A is allowed to flow from time to time into the retort B, in which the sulphur is vaporized.

The sulphur vapor which passes into the condenser collects either in the liquid state, at the bottom (C) of the condenser, or in a solid state upon the cold walls (D). The liquid sulphur is run into molds to crystallize, thus producing the "roll-sulphur," or "brimstone" of commerce; the sulphur which solidifies upon the walls appears in the form of fine meal and is called " flowers (more correctly, "flour ") of sulphur.

Sulphur,

249. Physical Properties; Allotropism. like many other elements, exists in several different physical forms; consequently, in giving the properties of sulphur we must specify the kind of sulphur to which we are referring. The several varieties of sulphur may be grouped into three classes:

FIG. 55.

(1) Ordinary, or rhombic, sulphur (cf. Fig. 55 and Appendix, xi). This is the natural form. All other forms revert to it at the ordinary temperature. It is yellow, odorless, tasteless, melts at 114.5° C., has a specific gravity of 2.06, and is very soluble in carbon disulphide 46 g. in 100 g. of CS2 at the ordinary temperature. In water it is insoluble. Roll sulphur is rhombic; so are the crystals that separate

from carbon disulphide solution.

(2) Prismatic, or monoclinic, sulphur (Fig. 56). This is formed by slow cooling of melted sulphur. As soon as a crust forms, it is broken, and the liquid sulphur is poured off. The crystals are long, almost colorless needles, that melt at 119° C., and dissolve in carbon disulphide. Their specific gravity is 1.96. If kept below 96° C., they change, in a few days, to the

FIG. 56.