stannite See stannate.
stannous compounds Compounds of tin in its lower (+2) oxidation state; e.g. stannous chloride is
tin(II) chloride.
starch A *polysaccharide consisting
of various proportions of two glucose
polymers, *amylose and *amylopectin. It occurs widely in plants,
especially in roots, tubers, seeds, and
fruits, as a carbohydrate storage
product and energy source. Starch is
therefore a major energy source for
animals. When digested it ultimately
yields glucose. Starch granules are insoluble in cold water but disrupt if
heated to form a gelatinous solution.
This gives an intense blue colour
with iodine solutions and starch is
used as an *indicator in certain titrations.
Stark effect The splitting of lines
in the *spectra of atoms due to the
presence of a strong electric Üeld. It
is named after the German physicist
Johannes Stark (1874–1957), who discovered it in 1913. Like the normal
*Zeeman effect, the Stark effect can
be understood in terms of the classical electron theory of Lorentz. The
Stark effect for hydrogen atoms was
also described by the Bohr theory of
the atom. In terms of quantum mechanics, the Stark effect is described
by regarding the electric Üeld as a
perturbation on the quantum states
and energy levels of an atom in the
absence of an electric Üeld. This application of perturbation theory was
its Ürst use in quantum mechanics.
Stark–Einstein law The law stating that in a photochemical process
(such as a photochemical reaction)
one photon is absorbed by each molecule causing the main photochemical process. In some circumstances,
one molecule, having absorbed a
photon, initiates a process involving several molecules. The Stark–
Einstein law is named after Johannes
Stark and Albert *Einstein.
state of matter One of the three
physical states in which matter can
exist, i.e. *solid, *liquid, or *gas.
Plasma is sometimes regarded as the
fourth state of matter.
stationary phase See chromatography.
stationary state A state of a system when it has an energy level permitted by *quantum mechanics.
Transitions from one stationary state
to another can occur by the emission
or absorption of an appropriate
quanta of energy (e.g. in the form of
photons).
statistical mechanics The branch
of physics in which statistical methods are applied to the microscopic
constituents of a system in order to
predict its macroscopic properties.
The earliest application of this
method was Boltzmann’s attempt to
explain the thermodynamic properties of gases on the basis of the statistical properties of large assemblies of
molecules.
In classical statistical mechanics,
each particle is regarded as occupying a point in phase space, i.e. to
have an exact position and momentum at any particular instant. The
probability that this point will occupy any small volume of the phase
space is taken to be proportional to
the volume. The Maxwell–Boltzmann
law gives the most probable distribution of the particles in phase space.
With the advent of quantum
theory, the exactness of these
premises was disturbed (by the
Heisenberg uncertainty principle). In
the *quantum statistics that evolved
as a result, the phase space is divided
into cells, each having a volume h
f
,
where h is the Planck constant and f
stannite
502
s
www.AzShimi.ir www.AzShimi.com
stannous compounds Compounds of tin in its lower (+2) oxidation state; e.g. stannous chloride is
tin(II) chloride.
starch A *polysaccharide consisting
of various proportions of two glucose
polymers, *amylose and *amylopectin. It occurs widely in plants,
especially in roots, tubers, seeds, and
fruits, as a carbohydrate storage
product and energy source. Starch is
therefore a major energy source for
animals. When digested it ultimately
yields glucose. Starch granules are insoluble in cold water but disrupt if
heated to form a gelatinous solution.
This gives an intense blue colour
with iodine solutions and starch is
used as an *indicator in certain titrations.
Stark effect The splitting of lines
in the *spectra of atoms due to the
presence of a strong electric Üeld. It
is named after the German physicist
Johannes Stark (1874–1957), who discovered it in 1913. Like the normal
*Zeeman effect, the Stark effect can
be understood in terms of the classical electron theory of Lorentz. The
Stark effect for hydrogen atoms was
also described by the Bohr theory of
the atom. In terms of quantum mechanics, the Stark effect is described
by regarding the electric Üeld as a
perturbation on the quantum states
and energy levels of an atom in the
absence of an electric Üeld. This application of perturbation theory was
its Ürst use in quantum mechanics.
Stark–Einstein law The law stating that in a photochemical process
(such as a photochemical reaction)
one photon is absorbed by each molecule causing the main photochemical process. In some circumstances,
one molecule, having absorbed a
photon, initiates a process involving several molecules. The Stark–
Einstein law is named after Johannes
Stark and Albert *Einstein.
state of matter One of the three
physical states in which matter can
exist, i.e. *solid, *liquid, or *gas.
Plasma is sometimes regarded as the
fourth state of matter.
stationary phase See chromatography.
stationary state A state of a system when it has an energy level permitted by *quantum mechanics.
Transitions from one stationary state
to another can occur by the emission
or absorption of an appropriate
quanta of energy (e.g. in the form of
photons).
statistical mechanics The branch
of physics in which statistical methods are applied to the microscopic
constituents of a system in order to
predict its macroscopic properties.
The earliest application of this
method was Boltzmann’s attempt to
explain the thermodynamic properties of gases on the basis of the statistical properties of large assemblies of
molecules.
In classical statistical mechanics,
each particle is regarded as occupying a point in phase space, i.e. to
have an exact position and momentum at any particular instant. The
probability that this point will occupy any small volume of the phase
space is taken to be proportional to
the volume. The Maxwell–Boltzmann
law gives the most probable distribution of the particles in phase space.
With the advent of quantum
theory, the exactness of these
premises was disturbed (by the
Heisenberg uncertainty principle). In
the *quantum statistics that evolved
as a result, the phase space is divided
into cells, each having a volume h
f
,
where h is the Planck constant and f
stannite
502
s
www.AzShimi.ir www.AzShimi.com