osmiridium A hard white naturally
occurring alloy consisting principally
of osmium (17–48%) and iridium
(49%). It also contains small quantities of platinum, rhodium, and ruthenium. It is used for making small
items subject to wear, e.g. electrical
contacts or the tips of pen nibs.
osmium Symbol Os. A hard bluewhite metallic *transition element;
a.n. 76; r.a.m. 190.2; r.d. 22.57; m.p.
3045°C; b.p. 5027°C. It is found associated with platinum and is used in
certain alloys with platinum and iridium (see osmiridium). Osmium forms
a number of complexes in a range of
oxidation states. It was discovered by
Smithson Tennant (1761–1815) in
1804.
A
• Information from the WebElements site
osmium(IV) oxide (osmium tetroxide) A yellow solid, OsO 4 , made by
heating osmium in air. It is used as
an oxidizing agent in organic chemistry, as a catalyst, and as a Üxative in
electron microscopy.
osmometer See osmosis.
osmosis The passage of a solvent
through a semipermeable membrane
separating two solutions of different
concentrations. A semipermeable
membrane is one through which the
molecules of a solvent can pass but
the molecules of most solutes cannot. There is a thermodynamic tendency for solutions separated by such
a membrane to become equal in concentration, the water (or other solvent) Ûowing from the weaker to the
stronger solution. Osmosis will stop
when the two solutions reach equal
concentration, and can also be
stopped by applying a pressure to the
liquid on the stronger-solution side
of the membrane. The pressure required to stop the Ûow from a pure
solvent into a solution is a characteristic of the solution, and is called the
osmotic pressure (symbol Π). Osmotic
pressure depends only on the concentration of particles in the solution, not on their nature (i.e. it is a
*colligative property). For a solution
of n moles in volume V at thermodynamic temperature T, the osmotic
pressure is given by ΠV = nRT, where
R is the gas constant. Osmoticpressure measurements are used in
Ünding the relative molecular masses
of compounds, particularly macromolecules. A device used to measure
osmotic pressure is called an osmometer.
osmotic pressure See osmosis.
Ostwald, Friedrich Wilhelm
(1853–1932) German physical
chemist. In 1887 he went to Leipzig
where he worked on a wide range of
topics including hydrolysis, viscosity,
ionization, and catalysis. Ostwald was
highly inÛuential in the development
of physical chemistry as a subject. He
was awarded the 1909 Nobel Prize
for chemistry.
Ostwald ripening A process used
in crystal growth in which a mixture
of large and small crystals are both in
contact with a solvent. The large
crystals grow and the small crystals
disappear. This occurs because there
is a higher energy associated with
the smaller crystals. When they dissolve the heat associated with this
higher energy is released, enabling
recrystallization to occur on the large
crystals. Ostwald ripening is used in
such applications as photography, requiring crystals with speciÜc properties.
Ostwald’s dilution law An expression for the degree of dissociation of a weak electrolyte. For
example, if a weak acid dissociates in
water
HA ˆ H
+ + A
–
391
Ostwald’s dilution law
o
www.AzShimi.ir www.AzShimi.com
occurring alloy consisting principally
of osmium (17–48%) and iridium
(49%). It also contains small quantities of platinum, rhodium, and ruthenium. It is used for making small
items subject to wear, e.g. electrical
contacts or the tips of pen nibs.
osmium Symbol Os. A hard bluewhite metallic *transition element;
a.n. 76; r.a.m. 190.2; r.d. 22.57; m.p.
3045°C; b.p. 5027°C. It is found associated with platinum and is used in
certain alloys with platinum and iridium (see osmiridium). Osmium forms
a number of complexes in a range of
oxidation states. It was discovered by
Smithson Tennant (1761–1815) in
1804.
A
• Information from the WebElements site
osmium(IV) oxide (osmium tetroxide) A yellow solid, OsO 4 , made by
heating osmium in air. It is used as
an oxidizing agent in organic chemistry, as a catalyst, and as a Üxative in
electron microscopy.
osmometer See osmosis.
osmosis The passage of a solvent
through a semipermeable membrane
separating two solutions of different
concentrations. A semipermeable
membrane is one through which the
molecules of a solvent can pass but
the molecules of most solutes cannot. There is a thermodynamic tendency for solutions separated by such
a membrane to become equal in concentration, the water (or other solvent) Ûowing from the weaker to the
stronger solution. Osmosis will stop
when the two solutions reach equal
concentration, and can also be
stopped by applying a pressure to the
liquid on the stronger-solution side
of the membrane. The pressure required to stop the Ûow from a pure
solvent into a solution is a characteristic of the solution, and is called the
osmotic pressure (symbol Π). Osmotic
pressure depends only on the concentration of particles in the solution, not on their nature (i.e. it is a
*colligative property). For a solution
of n moles in volume V at thermodynamic temperature T, the osmotic
pressure is given by ΠV = nRT, where
R is the gas constant. Osmoticpressure measurements are used in
Ünding the relative molecular masses
of compounds, particularly macromolecules. A device used to measure
osmotic pressure is called an osmometer.
osmotic pressure See osmosis.
Ostwald, Friedrich Wilhelm
(1853–1932) German physical
chemist. In 1887 he went to Leipzig
where he worked on a wide range of
topics including hydrolysis, viscosity,
ionization, and catalysis. Ostwald was
highly inÛuential in the development
of physical chemistry as a subject. He
was awarded the 1909 Nobel Prize
for chemistry.
Ostwald ripening A process used
in crystal growth in which a mixture
of large and small crystals are both in
contact with a solvent. The large
crystals grow and the small crystals
disappear. This occurs because there
is a higher energy associated with
the smaller crystals. When they dissolve the heat associated with this
higher energy is released, enabling
recrystallization to occur on the large
crystals. Ostwald ripening is used in
such applications as photography, requiring crystals with speciÜc properties.
Ostwald’s dilution law An expression for the degree of dissociation of a weak electrolyte. For
example, if a weak acid dissociates in
water
HA ˆ H
+ + A
–
391
Ostwald’s dilution law
o
www.AzShimi.ir www.AzShimi.com