used as a bleaching agent. With
acids, hydrogen peroxide is formed
and the reaction is used in the labo-
ratory preparation of hydrogen per-
oxide.barium sulphateAn insoluble
white solid, BaSO 4 , that occurs natu-
rally as the mineral *barytes (or
heavy spar) and can be prepared as a
precipitate by adding sulphuric acid
to barium chloride solution; r.d. 4.50;
m.p. 1580°C. The rhombic form
changes to a monoclinic form at
1149 °C. It is used as a raw material
for making other barium salts, as a
pigment extender in surface coating
materials (called blancÜxe), and
in the glass and rubber industries.
Barium compounds are opaque to
X-rays, and a suspension of the sul-
phate in water is used in medicine to
provide a contrast medium for X-rays
of the stomach and intestine. Al-
though barium compounds are ex-
tremely poisonous, the sulphate is
safe to use because it is very insolu-
ble.barrel A measurement of volume,
widely used in the chemical industry,
equal to 35 UK gallons (approxi-
mately 159 litres).Bartlett, Neil(1932– ) British in-
organic chemist who prepared oxy-
gen hexachloroplatinate in 1961 and,
in 1962, xenon hexachloroplatine –
theÜrst compound of a noble gas.Barton, Sir Derek (Harold
Richard) (1918–98) British organic
chemist who worked on steroids and
other natural products. He is noted
for his investigations into the confor-
mation of organic compounds and its
effect on chemical properties. Barton
was awarded the 1969 Nobel Prize
for chemistry (with O. Hassell
(1897–1981)).barytaSee barium hydroxide.barytes(barite)An orthorhombic
mineral form of *barium sulphate,
BaSO 4 ; the chief ore of barium. It is
usually white but may also be yellow,
grey, or brown. Large deposits occur
in Andalusia, Spain, and in the USA.basalSee apical.basaltAÜne-grained basic igneous
rock. It is composed chieÛy of cal-
cium-rich plagioclase feldspar and
pyroxene; other minerals present
may be olivine, magnetite, and ap-
atite. Basalt is the commonest type of
lava.baseA compound that reacts with a
protonic acid to give water (and a
salt). The deÜnition comes from the
Arrhenius theory of acids and bases.
Typically, bases are metal oxides, hy-
droxides, or compounds (such as am-
monia) that give hydroxide ions in
aqueous solution. Thus, a base may
be either: (1) An insoluble oxide or
hydroxide that reacts with an acid,
e.g.
CuO(s) + 2HCl(aq) →CuCl 2 (aq) +
H 2 O(l)
Here the reaction involves hydrogen
ions from the acid
CuO(s) + 2H+(aq) →H 2 O(l) +
Cu2+(aq)
(2) A soluble hydroxide, in which
case the solution contains hydroxide
ions. The reaction with acids is a re-
action between hydrogen ions and
hydroxide ions:
H++ OH–→H 2 O
(3) A compound that dissolves in
water to produce hydroxide ions.
For example, ammonia reacts as fol-
lows:
NH 3 (g) + H 2 O(l) ˆNH 4 +(aq) + –OH
Similar reactions occur with organic
*amines (see also nitrogenous base;
amine salts). A base that dissolves in
water to give hydroxide ions is calledbarium sulphate 58b