and PM2.5, with particles less than
2.5 μm.
parting agentSee release agent.
partition If a substance is in con-
tact with two different phases then,
in general, it will have a different
afÜnity for each phase. Part of the
substance will be absorbed or dis-
solved by one and part by the other,
the relative amounts depending on
the relative afÜnities. The substance
is said to be partitionedbetween the
two phases. For example, if two im-
miscible liquids are taken and a third
compound is shaken up with them,
then an equilibrium is reached in
which the concentration in one sol-
vent differs from that in the other.
The ratio of the concentrations is
the partition coefÜcientof the sys-
tem. The partition law states that
this ratio is a constant for given liq-
uids.
partition coefÜcientSee parti-
tion.
partition function The quantity Z
deÜned by
Z = ∑exp(–Ei/kT),where the sum is taken over all
states i of the system. Eiis the energy
of the ith state, k is the Boltzmann
constant, and T is the thermody-
namic temperature. Z is a quantity of
fundamental importance in equilib-
rium statistical mechanics. For a sys-
tem in which there are nontrivial
interactions, it is very difÜcult to cal-
culate the partition function exactly.
For such systems it is necessary to
use approximation techniques. The
partition function links results at the
atomic level to thermodynamics,
since Z is related to the Helmholtz
free energy F by F = kTlnZ.
pascal The *SI unit of pressure
equal to one newton per square
metre. It is named after the French
mathematician Blaise Pascal
(1623–62).Paschen–Back effect An effect on
atomic line spectra that occurs when
the atoms are placed in a strong mag-
neticÜeld. Spectral lines that give
the anomalous *Zeeman effect when
the atoms are placed in a weaker
magneticÜeld have a splitting pat-
tern in a very strong magneticÜeld.
The Paschen–Back effect is named
after the German physicists Louis
Carl Heinrich Friedrich Paschen
(1865–1947) and Ernest E. A. Back
(1881–1959), who discovered it in- In the quantum theory of
atoms the Paschen–Back effect is ex-
plained by the fact that the energies
of precession of the electron’s orbital
angular momentum l and the spin
angular momentum s about the di-
rection of the magneticÜeld H are
greater than the energies of coupling
between l and s. In the Paschen–Back
effect the orbital magnetic moment
and the spin magnetic moment pre-
cess independently about the direc-
tion of H.
Paschen seriesSee hydrogen spec-
trum.
passiveDescribing a solid that has
reacted with another substance to
form a protective layer, so that fur-
ther reaction stops. The solid is said
to have been ‘rendered passive’. For
example, aluminium reacts sponta-
neously with oxygen in air to form a
thin layer of *aluminium oxide,
which prevents further oxidation.
Similarly, pure iron forms a protec-
tive oxide layer with concentrated ni-
tric acid and is not dissolved further.
Pasteur, Louis (1822–95) French
chemist and microbiologist, who
held appointments in Strasbourg
(1849–54) and Lille (1854–57), before
returning to Paris to the Ecole Nor-
male and the Sorbonne. From 1888399 Pasteur, Louis
p