violet radiation or by spraying with a
substance that reacts to give a
coloured spot (e.g. ninhydrin gives a
blue coloration with amino acids).
The components can be identiÜed by
the distance they move in a given
time.
para-1.PreÜx designating a ben-
zene compound in which two sub-
stituents are in the 1,4 positions, i.e.
directly opposite each other, on the
benzene ring. The abbreviation p- is
used; for example, p-xylene is 1,4-
dimethylbenzene. Compare ortho-;
meta-. 2.PreÜx denoting the form of
diatomic molecules in which the nu-
clei have opposite spins, e.g. parahy-
drogen (see hydrogen). Compare
ortho-.parafÜnSee petroleum.
parafÜnsSee alkanes.parafÜn waxSee petroleum.paraformaldehydeSee methanal.
paraheliumSee orthohelium.parahydrogen See hydrogen.
paraldehydeSee ethanal.parallel spinsNeighbouring spin-
ning electrons in which the *spins,
and hence the magnetic moments, of
the electrons are aligned in the same
direction.paramagnetismSee magnetism.
ParaquatTradename for an or-
ganic herbicide (see dipyridyl) used
to control broadleaved weeds and
grasses. It is poisonous to humans,
having toxic effects on the liver,
lungs, and kidneys if ingested.
Paraquat is not easily broken down
and can persist in the environment
adsorbed to soil particles. See also pes-
ticide.
parent See daughter.partially permeable membraneA membrane that is permeable to the
small molecules of water and certain
solutes but does not allow the pas-
sage of large solute molecules. This
term is preferred to semipermeable
membrane when describing biologi-
cal membranes. See osmosis.partial pressure See dalton’s law.
particle in a boxA system in
quantum mechanics used to illus-
trate important features of quantum
mechanics, such as quantization of
energy levels and the existence of
*zero-point energy. A particle with a
mass m is allowed to move between
two walls having the coordinates x =
0 and x = L. The potential energy of
the particle is taken to be zero be-
tween the walls and inÜnite outside
the walls. The time-independent
*Schrödinger equation is exactly sol-
uble for this problem, with the ener-
gies Enbeing given by n^2 h^2 /8mL^2 , n =
1,2,..., and the wave functions ψn
being given by ψn= (2/L)½sin(nπx/L),
where n are the quantum numbers
that label the energy levels and h is
the *Planck constant. The particle in
a box can be used as a rough model
for delocalized electrons in a mol-
ecule or solid. Thus it can be used to
explain the colours of molecules
with conjugated double bonds. It can
also explain the colour imparted to
crystals by F-centres. The problem of
a particle in a box can also be solved
in two and three dimensions, en-
abling relations between symmetry
and degenerate energy levels to be
seen.particulate matter (PM)Matter
present as small particles. Particulate
matter, also known as particulates, is
produced as an airborne pollutant in
many processes. It may be inorganic,
e.g. silicate or carbon, or organic, e.g.
polyaromatic hydrocarbons (PAHs).
Particulates are classiÜed by size; e.g.
PM10, with particles less than 10 μm,para- 398p