stanniteSee stannate.
stannous compoundsCom-
pounds of tin in its lower (+2) oxida-
tion state; e.g. stannous chloride is
tin(II) chloride.
starch A *polysaccharide consisting
of various proportions of two glucose
polymers, *amylose and *amy-
lopectin. 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 in-
soluble 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 titra-
tions.
Stark effect The splitting of lines
in the *spectra of atoms due to the
presence of a strong electricÜeld. It
is named after the German physicist
Johannes Stark (1874–1957), who dis-
covered it in 1913. Like the normal
*Zeeman effect, the Stark effect can
be understood in terms of the classi-
cal electron theory of Lorentz. The
Stark effect for hydrogen atoms was
also described by the Bohr theory of
the atom. In terms of quantum me-
chanics, the Stark effect is described
by regarding the electricÜeld as a
perturbation on the quantum states
and energy levels of an atom in the
absence of an electricÜeld. This ap-
plication of perturbation theory was
itsÜrst use in quantum mechanics.
Stark–Einstein lawThe law stat-
ing that in a photochemical process
(such as a photochemical reaction)
one photon is absorbed by each mol-
ecule causing the main photochemi-
cal process. In some circumstances,
one molecule, having absorbed a
photon, initiates a process involv-ing several molecules. The Stark–
Einstein law is named after Johannes
Stark and Albert *Einstein.state of matterOne 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 phaseSee chromatog-
raphy.stationary stateA state of a sys-
tem when it has an energy level per-
mitted 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 mechanicsThe branch
of physics in which statistical meth-
ods 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 proper-
ties of gases on the basis of the statis-
tical properties of large assemblies of
molecules.
In classical statistical mechanics,
each particle is regarded as occupy-
ing a point in phase space, i.e. to
have an exact position and momen-
tum at any particular instant. The
probability that this point will oc-
cupy any small volume of the phase
space is taken to be proportional to
the volume. The Maxwell–Boltzmann
law gives the most probable distribu-
tion 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 hf,
where h is the Planck constant and fstannite 502s