an alkali. Ammonia and sodium hy-
droxide are common examples.
The original Arrhenius deÜnition of
a base has been extended by the
Lowry–Brønsted theory and by the
Lewis theory. See acid.
base dissociation constantSee
dissociation.
base metal A common relatively
inexpensive metal, such as iron or
lead, that corrodes, oxidizes, or tar-
nishes on exposure to air, moisture,
or heat, as distinguished from pre-
cious metals, such as gold and silver.
base pairingPairing of nucleotide
bases by hydrogen bonding. Base
pairing holds together the two
strands in the double helix structure
of *DNA. The purine adenine pairs
with the pyrimidine thymine and the
purine guanine pairs with the pyrim-
idine cytosine. See also chargaff’s
rule.
base unitA unit that is deÜned ar-
bitrarily rather than being deÜned by
simple combinations of other units.
For example, the ampere is a base
unit in the SI system deÜned in terms
of the force produced between two
current-carrying conductors, whereas
the coulomb is a derived unit,
deÜned as the quantity of charge
transferred by one ampere in one
second.
basic 1.Describing a compound
that is a base. 2.Describing a solu-
tion containing an excess of hydrox-
ide ions; alkaline.
basic dyeSee dyes.
basicity constantSee dissocia-
tion.
basic-oxygen process (BOP
process) A high-speed method of
making high-grade steel. It originated
in the Linz–Donawitz(L–D) process.
Molten pig iron and scrap are
charged into a tilting furnace, similar
to the Bessemer furnace except that
it has no tuyeres. The charge is con-
verted to steel by blowing high-
pressure oxygen onto the surface of
the metal through a water-cooled
lance. The excess heat produced en-
ables up to 30% of scrap to be incor-
porated into the charge. The process
has largely replaced the Bessemer
and open-hearth processes.basic saltA compound that can be
regarded as being formed by replac-
ing some of the oxide or hydroxide
ions in a base by other negative ions.
Basic salts are thus mixed salt–oxides
(e.g. bismuth(III) chloride oxide,
BiOCl) or salt–hydroxides (e.g. lead(II)
chloride hydroxide, Pb(OH)Cl).basic slag*Slag formed from a
basicÛux (e.g. calcium oxide) in a
blast furnace. The basicÛux is used
to remove acid impurities in the ore
and contains calcium silicate, phos-
phate, and sulphide. If the phospho-
rus content is high the slag can be
used as a fertilizer.bathochromic shift A shift of a
spectral band to longer wavelengths
as a result of substitution in a mol-
ecule or a change in the conditions.
Compare hypsochromic shift.batteryA number of electric cells
joined together. The common car
battery, or *accumulator, usually
consists of six secondary cells con-
nected in series to give a total e.m.f.
of 12 volts. A torch battery is usually
a dry version of the *Leclanché cell,
two of which are often connected in
series. Batteries may also have cells
connected in parallel, in which case
they have the same e.m.f. as a single
cell, but their capacity is increased,
i.e. they will provide more total
charge. The capacity of a battery is
usually speciÜed in ampere-hours,59 battery
b