GROUP VII: THE HALOGENS 347trifluoride, C1F 3 , cobalt(III) fluoride, CoF 3 , silver difluoride, AgF 2.
Hydrofluoric acid is used to etch glass, to remove sand from pre-
cision castings, in the manufacture of synthetic cryolite, NaAlF 6 ,
and as a preservative for yeast and anatomical specimens. Hydrogen
fluoride is a catalyst in the alkylation of butane to give higher
hydrocarbons, and in the presence of a catalyst is itself used to
prepare fluorocarbons. A wide variety of iluorocarbons are known
and used extensively as refrigerants, lubricants and as aerosol
propellants. Tetrafluoroethene (tetrafluoroethylene), C 2 F 4 , is readily
polymerised to give polytetrafluoroethylene, PTFE, a plastic of
high thermal stability and one not subject to chemical attack by
most reagents which finds considerable use not only in the chemical
industry but also in the manufacture of knon-stick' pans and oven
ware. Calcium fluoride, and other fluorides, are used as fluxes in
making vitreous enamels.
CHLORINE
World production of chlorine in 1965 was 14 million tons and the
production has risen steadily each year since. Most of it is now used
for chemical processes involving the introduction of chlorine into
organic compounds, for example the ehlorination of olefins, manu-
facture of carbon tetrachloride, ehlorination of paraffins to make
grease solvents, and the manufacture of plastics and synthetic
rubber. Hydrogen chloride is the by-product of many of these
processes. Much goes into use for sterilising water and sewage, and
it is used directly or indirectly as a bleaching agent. The use of
soluble chlorates(I) is replacing bleaching powder for such purposes
as bleaching paper pulp and cotton.
Chlorine is also used in the manufacture of hydrochloric acid, the
extraction of titanium, and the removing of tin from old tinplate
Cde- tinning').
BROMINE
Bromine is used in the manufacture of many important organic
compounds including 1,2-dibromoethane (ethylene dibromide),
added to petrol to prevent lead deposition which occurs by de-
composition of the "anti-knock'—lead tetraethyl; bromomethane
(methyl bromide), a fumigating agent, and several compounds used
to reduce flammability of polyester plastics and epoxide resins.
Silver(I) bromide is used extensively in the photographic industry