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 benzene compound in which two substituents 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 1,4dimethylbenzene. Compare ortho-;
meta-. 2. PreÜx denoting the form of
diatomic molecules in which the nuclei have opposite spins, e.g. parahydrogen (see hydrogen). Compare
ortho-.
parafÜn See petroleum.
parafÜns See alkanes.
parafÜn wax See petroleum.
paraformaldehyde See methanal.
parahelium See orthohelium.
parahydrogen See hydrogen.
paraldehyde See ethanal.
parallel spins Neighbouring spinning electrons in which the *spins,
and hence the magnetic moments, of
the electrons are aligned in the same
direction.
paramagnetism See magnetism.
Paraquat Tradename for an organic 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 pesticide.
parent See daughter.
partially permeable membrane
A membrane that is permeable to the
small molecules of water and certain
solutes but does not allow the passage of large solute molecules. This
term is preferred to semipermeable
membrane when describing biological membranes. See osmosis.
partial pressure See dalton’s law.
particle in a box A system in
quantum mechanics used to illustrate 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 between the walls and inÜnite outside
the walls. The time-independent
*Schrödinger equation is exactly soluble for this problem, with the energies E n being given by n
2
h
2
/8mL
2
, n =
1,2,…, and the wave functions ψ n
being given by ψ n = (2/L)
½ sin(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 molecule 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, enabling 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,
para398
p
www.AzShimi.ir www.AzShimi.com
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 benzene compound in which two substituents 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 1,4dimethylbenzene. Compare ortho-;
meta-. 2. PreÜx denoting the form of
diatomic molecules in which the nuclei have opposite spins, e.g. parahydrogen (see hydrogen). Compare
ortho-.
parafÜn See petroleum.
parafÜns See alkanes.
parafÜn wax See petroleum.
paraformaldehyde See methanal.
parahelium See orthohelium.
parahydrogen See hydrogen.
paraldehyde See ethanal.
parallel spins Neighbouring spinning electrons in which the *spins,
and hence the magnetic moments, of
the electrons are aligned in the same
direction.
paramagnetism See magnetism.
Paraquat Tradename for an organic 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 pesticide.
parent See daughter.
partially permeable membrane
A membrane that is permeable to the
small molecules of water and certain
solutes but does not allow the passage of large solute molecules. This
term is preferred to semipermeable
membrane when describing biological membranes. See osmosis.
partial pressure See dalton’s law.
particle in a box A system in
quantum mechanics used to illustrate 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 between the walls and inÜnite outside
the walls. The time-independent
*Schrödinger equation is exactly soluble for this problem, with the energies E n being given by n
2
h
2
/8mL
2
, n =
1,2,…, and the wave functions ψ n
being given by ψ n = (2/L)
½ sin(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 molecule 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, enabling 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,
para398
p
www.AzShimi.ir www.AzShimi.com