CHAPTER XXIV.

FLUORINE, BROMINE, IODINE, AND THEIR

COMPOUNDS.

315. Halogens. The elements fluorine, chlorine, bromine, and iodine are called "the halogens," from hals, Greek for "salt," and the suffix gen, meaning "a constituent of," as in "hydrogen," etc. Their binary compounds, e. g., chlorides, bromides, etc., are called halides.

Fluorine and chlorine are gaseous at the ordinary temperature; bromine is a liquid boiling at about 59° C.; while iodine is a black solid which gives off, even at ordinary temperatures, a beautiful, violet vapor.

316. Fluorine. The element fluorine was known in its compounds long before it was obtained in the free condition. The most common of its compounds is calcium fluoride, or fluorspar, CaF2. Fluorspar derives its name from fluo, Latin for "to flow," and spar, meaning "a rock." The name is applied to this substance owing to the use of fluorspar as a flux in metallurgy.

A flux is an easily fusible substance added to the mixture of an ore and a reducing agent to promote fusion of the mixtureThe substance resulting from the union of the flux with the impurities present is usually called "slag " (cf. § 482).

HYDROFLUORIC ACID.

295

Another important natural fluorine compound is cryolite. This is a double fluoride of aluminum and sodium; its formula is AlF3. 3 NaF, or NaзAIF6. Fluorine was prepared by Moissan (1886) by the electrolysis of anhydrous hydrogen fluoride containing potassium fluoride, KHF2 (Fig. 76).

Fluorine unites with hydrogen explosively, even in the dark (cf. § 118), to form hydrogen fluoride. It does not unite with oxygen (cf. § 24). Silicon and antimony form the fluorides, SiF4, and SbF3. Liquid fluorine boils at -187° C. under ordinary

pressure.

318. Hydrofluoric Acid. Hydrogen fluoride is prepared by heating calcium fluoride with concentrated sulphuric acid. The simplest formula (HF) must be doubled.

CaF2+H2SO1CaSO1+H2F2 ↑ .

4

Anhydrous hydrogen fluoride boils at 19° C. The solution is a strong acid. Being dibasic, it forms both acid and normal salts, e. g., KHF2 and K2F2. Both hydrogen fluoride vapor and its solution are very poisonous. The aqueous solution reacts with almost all the metals, forming fluorides and hydrogen, and with their oxides, forming fluorides and water.

Silicon dioxide (quartz, sand, etc.) and hydrofluoric acid give silicon fluoride (a gas) and water. SiF41+2 H2O.

SiO2+2 H2F2

Glass a mixture of silicates (cf. § 394)

reacts

with hydrofluoric acid like a mixture of metal oxides and silicon dioxide. With calcium silicate, CaSiO3, the equation is,

CaSiO3+3 H2F2·

CaF2+SiF4 ↑ +3 H2O.

Hence, when glass is treated with hydrofluoric acid, the silicon present in the glass escapes as SiF4, leaving a depression in the glass. This fact is made use of in the operation of etching glass. The glass is first covered with a thin layer of paraffin, and a design is drawn in the paraffin by means of a sharp point. When the exposed glass is wet with the solution of the acid (a swab of cotton attached to a stick may be used to apply the solution) or is left in the vapor of the acid, the design is etched into the glass.

Hydrofluoric acid is commonly kept in bottles of paper, covered inside and out with a thick layer of paraffin. Vessels of lead, platinum, or rubber may also be used.

PREPARATION OF BROMINE.

297

BROMINE.

319. Preparation of Bromine. - Bromine is found in nature in the combined form, chiefly as bromides. The most common bromides are those of sodium (NaBr), of potassium (KBr), and of magnesium (MgBr2).

Bromides occur in sea-water and in salt deposits.

Bromine is prepared by heating a bromide with manganese dioxide and dilute sulphuric acid. The bromine vapor evolved is condensed in cold receivers. With sodium bromide the complete equation is,

MnO2+2NaBr+3 H,SO — MnSO +2 NaHSO4+Br2+

2 H2O.

This reaction is like that for the preparation of chlorine (§ 115) from common salt, manganese dioxide, and hydrochloric acid. There are three stages:

(1) Sulphuric acid and sodium bromide give hydrogen bromide and sodium hydrogen sulphate.

2 NaBr+2 HSO4 2 NaHSO4+(2 HBr).

(2) Sulphuric acid and manganese dioxide give manganous sulphate and oxygen.

MnO2+H2SO4 →→→→→ MnSO4+H2O+(0).

(3) Hydrogen bromide and oxygen give free bromine.

Bromine may also be prepared by conducting the proper amount of chlorine into the solution of a bromide. With magnesium bromide the equation is,

MgBr2+Cl2 MgCl2+Br2.

320. Properties of Bromine. Bromine is a brown liquid about 3.2 times as heavy as water. Its vapor has an odor much like that of chlorine, and affects the eyes. Bromine boils at about 59° C.

The density of bromine vapor shows that the molecule is diatomic; its formula is, therefore, Br2. At about 1000° C. the molecule begins to dissociate into molecules containing only one atom each (cf. § 142).

Bromine dissolves in water, carbon disulphide, and other solvents. The aqueous solution is called "bromine water."

In the presence of some substance capable of taking up oxygen, bromine reacts with water energetically, according to the equation,

The oxygen is in the nascent state (cf. § 123). Bromine water is thus a good oxidizing agent.

The same action goes on more slowly when no oxidizable substance is present; bromine water thus becomes converted into a dilute solution of hydrobromic acid, HBr.

Bromine is less active than chlorine, but, like chlorine, it unites with hydrogen and with metals. Most bromides are soluble (cf. § 130).

321. Hydrobromic Acid. Hydrogen bromide cannot be made in a pure state from a bromide and concentrated sulphuric acid, because some of it is oxidized by the sulphuric acid. The product is,