sulphydryl group See thiols.
superacid An *acid that has a proton-donating ability equal to or
greater than that of anhydrous sulphuric acid. A superacid is a particular 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 as Ûuorosulphuric acid
(HSO 3 F) or hydrogen Ûuoride (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 applications include the protonation of very
weak bases in organic chemistry and
the abstraction of hydrogen from saturated hydrocarbons to produce carbonium ions. An equimolar mixture
of HSO 3 F and S6F 5 is known by the
tradename Magic acid. Very strong
bases, such as lithium diisopropylamide, are sometimes known as superbases.
superbase See superacid.
superconductivity The property
possessed by some substances below
a certain temperature, the transition
point, of zero resistance. Until recently, 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 example, 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 crystal. 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 vibration, or by rough surfaces. 2. The
analogous cooling of a vapour to
make it supersaturated.
superÛuidity The exhibition by a
Ûuid at extremely low temperatures,
e.g. liquid helium at 2.186 K, of very
high thermal conductivity and frictionless Ûow. The temperature at
which superÛuidity occurs is called
the lambda point.
superheating The heating of a liquid to above its normal boiling point
by increasing the pressure.
superionic conductor An ionic
solid in which the electrical conductivity 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.
superlattice See solid solution.
supernatant liquid The clear liquid remaining when a precipitate has
settled.
superoxides A group of inorganic
compounds that contain the O 2
– ion.
They are formed in signiÜcant quantities only for sodium, potassium, rubidium, 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 paramagnetic and coloured (orange).
superphosphate A commercial
phosphate mixture consisting mainly
of monocalcium phosphate. Singlesuperphosphate is made by treating
phosphate rock with sulphuric acid;
the product contains 16–20% ‘available’ P 2 O 5 :
513
superphosphate
s
www.AzShimi.ir www.AzShimi.com
superacid An *acid that has a proton-donating ability equal to or
greater than that of anhydrous sulphuric acid. A superacid is a particular 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 as Ûuorosulphuric acid
(HSO 3 F) or hydrogen Ûuoride (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 applications include the protonation of very
weak bases in organic chemistry and
the abstraction of hydrogen from saturated hydrocarbons to produce carbonium ions. An equimolar mixture
of HSO 3 F and S6F 5 is known by the
tradename Magic acid. Very strong
bases, such as lithium diisopropylamide, are sometimes known as superbases.
superbase See superacid.
superconductivity The property
possessed by some substances below
a certain temperature, the transition
point, of zero resistance. Until recently, 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 example, 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 crystal. 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 vibration, or by rough surfaces. 2. The
analogous cooling of a vapour to
make it supersaturated.
superÛuidity The exhibition by a
Ûuid at extremely low temperatures,
e.g. liquid helium at 2.186 K, of very
high thermal conductivity and frictionless Ûow. The temperature at
which superÛuidity occurs is called
the lambda point.
superheating The heating of a liquid to above its normal boiling point
by increasing the pressure.
superionic conductor An ionic
solid in which the electrical conductivity 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.
superlattice See solid solution.
supernatant liquid The clear liquid remaining when a precipitate has
settled.
superoxides A group of inorganic
compounds that contain the O 2
– ion.
They are formed in signiÜcant quantities only for sodium, potassium, rubidium, 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 paramagnetic and coloured (orange).
superphosphate A commercial
phosphate mixture consisting mainly
of monocalcium phosphate. Singlesuperphosphate is made by treating
phosphate rock with sulphuric acid;
the product contains 16–20% ‘available’ P 2 O 5 :
513
superphosphate
s
www.AzShimi.ir www.AzShimi.com