sulphydryl group See thiols.
superacidAn *acid that has a pro-
ton-donating ability equal to or
greater than that of anhydrous sul-
phuric acid. A superacid is a particu-
lar type of Brønsted acid. Those that
are much stronger than sulphuric
acids can be made by adding certain
pentaÛuorides and their derivatives
to such acids asÛuorosulphuric acid
(HSO 3 F) or hydrogenÛuoride (HF).
The pentaÛuorides, such as antimony
pentaÛuoride (SbF 5 ), are very strong
Lewis acids. The mixtures HF-SbF 5
and HSO 3 F-SbF 5 are among the
strongest acids known; their applica-
tions include the protonation of very
weak bases in organic chemistry and
the abstraction of hydrogen from sat-
urated hydrocarbons to produce car-
bonium ions. An equimolar mixture
of HSO 3 F and S6F 5 is known by the
tradename Magic acid. Very strong
bases, such as lithium diisopropyl-
amide, are sometimes known as su-
perbases.
superbaseSee superacid.
superconductivity The property
possessed by some substances below
a certain temperature, the transition
point, of zero resistance. Until re-
cently, the known superconducting
materials had very low transition
points. However, synthetic organic
conductors and certain metal oxide
ceramics have now been produced
that become superconducting at
much higher temperatures. For ex-
ample, much work has been done on
ytterbium–barium–copper oxides,
which have transition temperatures
of about 100 K.
supercooling 1.The cooling of a
liquid to below its freezing point
without a change from the liquid to
solid state taking place. In this
metastable state the particles of the
liquid lose energy but do not fall into
place in the lattice of the solid crys-
tal. If the liquid is seeded (see seed)
crystallization usually takes place
and the liquid returns to its normal
freezing point. Crystallization can
also be induced by the presence of
particles of dust, by mechanical vi-
bration, or by rough surfaces. 2.The
analogous cooling of a vapour to
make it supersaturated.superÛuidityThe exhibition by a
Ûuid at extremely low temperatures,
e.g. liquid helium at 2.186 K, of very
high thermal conductivity and fric-
tionlessÛow. The temperature at
which superÛuidity occurs is called
the lambda point.superheatingThe heating of a liq-
uid to above its normal boiling point
by increasing the pressure.superionic conductorAn ionic
solid in which the electrical conduc-
tivity due to the motion of ions is
similar to that of a molten salt, i.e. a
much higher conductivity than is
usually observed in ionic solids.superlatticeSee solid solution.
supernatant liquid The clear liq-
uid remaining when a precipitate has
settled.
superoxides A group of inorganic
compounds that contain the O 2 – ion.
They are formed in signiÜcant quan-
tities only for sodium, potassium, ru-
bidium, and caesium. They are very
powerful oxidizing agents and react
vigorously with water to give oxygen
gas and OH–ions. The superoxide ion
has an unpaired electron and is para-
magnetic and coloured (orange).
superphosphateA commercial
phosphate mixture consisting mainly
of monocalcium phosphate. Single-
superphosphate is made by treating
phosphate rock with sulphuric acid;
the product contains 16–20% ‘avail-
able’ P 2 O 5 :513 superphosphate
s