The name is also used for the synthetic compound, which is used as a
pigment.
ortho- 1. PreÜx indicating that a
benzene compound has two substituted groups in the 1,2 positions (i.e.
on adjacent carbon atoms). The abbreviation o- is used; for example
o-dichlorobenzene is 1,2-dichlorobenzene. Compare meta-; para-.
2. PreÜx formerly used to indicate
the most hydrated form of an acid.
For example, phosphoric(V) acid,
H 3 PO 4 , was called orthophosphoric
acid to distinguish it from the lower
metaphosphoric acid, HPO 3 (which is
actually (HPO 3 ) n ). 3. PreÜx denoting
the form of diatomic molecules in
which nuclei have parallel spins, e.g.
orthohydrogen (see hydrogen). Compare para-.
orthoboric acid See boric acid.
orthoclase See feldspars.
orthohelium A form of helium
once thought to exist as of one of
two species of the element. Because
the spectrum of atomic helium consists of transitions between singlet
states (including transitions from the
ground state) and transitions between triplet states but not transitions between singlet and triplet
states, early spectroscopists thought
that two species of helium existed.
The other form was called parahelium.
orthohydrogen See hydrogen.
orthophosphoric acid See phosphoric(v) acid.
orthoplumbate See plumbate.
orthorhombic See crystal system.
orthosilicate See silicate.
orthostannate See stannate.
oscillating reaction (clock reaction) A type of chemical reaction in
which the concentrations of the
products and reactants change periodically, either with time or with position in the reacting medium. Thus,
the concentration of a component
may increase with time to a maximum, decrease to a minimum, then
increase again, and so on, continuing
the oscillation over a period of time.
Systems are also known in which spirals and other patterns spread
through the reacting medium,
demonstrating a periodic spatial variation. Oscillating chemical reactions
have certain features in common.
They all occur under conditions far
from chemical equilibrium and all involve *autocatalysis, i.e. a product of
a reaction step acts as a catalyst for
that step. This autocatalysis drives
the oscillation by a process of positive feedback. Moreover, oscillating
chemical reactions are associated
with the phenomenon known as
bistability. In this, a reaction may be
in a steady-state condition, with reactants Ûowing into a reaction zone
while products are Ûowing out of it.
Under these conditions, the concentrations in the reaction zone may not
change with time, although the reaction is not in a state of chemical equilibrium. Bistable systems have two
possible stable steady states. Interaction with an additional substance in
the reaction medium causes the system to oscillate between the states as
the concentrations change. Oscillating chemical reactions are thought to
occur in a number of biochemical
processes. For example, they occur in
glycolysis, in which ATP is produced
by enzyme-catalysed reactions. They
are also known to regulate the
rhythm of the heartbeat. Most have
highly complex reaction mechanisms. See also lotka–volterra mechanism; brusselator; oregonator;
chaotic reaction.
ortho390
o
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pigment.
ortho- 1. PreÜx indicating that a
benzene compound has two substituted groups in the 1,2 positions (i.e.
on adjacent carbon atoms). The abbreviation o- is used; for example
o-dichlorobenzene is 1,2-dichlorobenzene. Compare meta-; para-.
2. PreÜx formerly used to indicate
the most hydrated form of an acid.
For example, phosphoric(V) acid,
H 3 PO 4 , was called orthophosphoric
acid to distinguish it from the lower
metaphosphoric acid, HPO 3 (which is
actually (HPO 3 ) n ). 3. PreÜx denoting
the form of diatomic molecules in
which nuclei have parallel spins, e.g.
orthohydrogen (see hydrogen). Compare para-.
orthoboric acid See boric acid.
orthoclase See feldspars.
orthohelium A form of helium
once thought to exist as of one of
two species of the element. Because
the spectrum of atomic helium consists of transitions between singlet
states (including transitions from the
ground state) and transitions between triplet states but not transitions between singlet and triplet
states, early spectroscopists thought
that two species of helium existed.
The other form was called parahelium.
orthohydrogen See hydrogen.
orthophosphoric acid See phosphoric(v) acid.
orthoplumbate See plumbate.
orthorhombic See crystal system.
orthosilicate See silicate.
orthostannate See stannate.
oscillating reaction (clock reaction) A type of chemical reaction in
which the concentrations of the
products and reactants change periodically, either with time or with position in the reacting medium. Thus,
the concentration of a component
may increase with time to a maximum, decrease to a minimum, then
increase again, and so on, continuing
the oscillation over a period of time.
Systems are also known in which spirals and other patterns spread
through the reacting medium,
demonstrating a periodic spatial variation. Oscillating chemical reactions
have certain features in common.
They all occur under conditions far
from chemical equilibrium and all involve *autocatalysis, i.e. a product of
a reaction step acts as a catalyst for
that step. This autocatalysis drives
the oscillation by a process of positive feedback. Moreover, oscillating
chemical reactions are associated
with the phenomenon known as
bistability. In this, a reaction may be
in a steady-state condition, with reactants Ûowing into a reaction zone
while products are Ûowing out of it.
Under these conditions, the concentrations in the reaction zone may not
change with time, although the reaction is not in a state of chemical equilibrium. Bistable systems have two
possible stable steady states. Interaction with an additional substance in
the reaction medium causes the system to oscillate between the states as
the concentrations change. Oscillating chemical reactions are thought to
occur in a number of biochemical
processes. For example, they occur in
glycolysis, in which ATP is produced
by enzyme-catalysed reactions. They
are also known to regulate the
rhythm of the heartbeat. Most have
highly complex reaction mechanisms. See also lotka–volterra mechanism; brusselator; oregonator;
chaotic reaction.
ortho390
o
www.AzShimi.ir www.AzShimi.com