the hydrogencarbonate, which make
the mixture rise.
baking soda See sodium hydrogencarbonate.
balance An accurate weighing device. The simple beam balance consists of two pans suspended from a
centrally pivoted beam. Known
masses are placed on one pan and
the substance or body to be weighed
is placed in the other. When the
beam is exactly horizontal the two
masses are equal. An accurate laboratory balance weighs to the nearest
hundredth of a milligram. Specially
designed balances can be accurate to
a millionth of a milligram. More
modern substitution balances use the
substitution principle. In this calibrated weights are removed from the
single lever arm to bring the single
pan suspended from it into equilibrium with a Üxed counter weight.
The substitution balance is more accurate than the two-pan device and
enables weighing to be carried out
more rapidly. In automatic electronic
balances, mass is determined not by
mechanical deÛection but by electronically controlled compensation
of an electric force. A scanner monitors the displacement of the pan
support generating a current proportional to the displacement. This current Ûows through a coil forcing the
pan support to return to its original
position by means of a magnetic
force. The signal generated enables
the mass to be read from a digital
display. The mass of the empty container can be stored in the balance’s
computer memory and automatically
deducted from the mass of the container plus its contents.
Balmer series See hydrogen spectrum.
banana bond Informal name for
the type of electron-deÜcient threecentre bond holding the B–H–B
bridges in *boranes and similar compounds.
band spectrum See spectrum.
band theory See energy bands.
bar A c.g.s. unit of pressure equal
to 10
6 dynes per square centimetre
or 10
5 pascals (approximately
750 mmHg or 0.987 atmosphere).
The millibar (100 Pa) is commonly
used in meteorology.
Barbier–Wieland degradation
The stepwise degradation of a carboxylic acid to the next lower homologue. First the ester is converted
into a tertiary alcohol using a Grignard reagent (PhMgX) and acid (HX):
RCH 2 COOCH 3 → RCH 2 C(OH)Ph 2 .
The secondary alcohol is then dehydrated using ethanoic anhydride
(CH 3 COOCOCH 3 ) to give an alkene:
RCH 2 C(OH)Ph 2 → RCH=CPh 2 .
The alkene is oxidized with chromic
acid:
RCH=CPh 2 → RCOOH + Ph 2 CO.
The result is conversion of an acid
RCH 2 COOH to the lower acid
RCOOH.
barbiturate Any one of a group of
drugs derived from barbituric acid
that have a depressant effect on the
central nervous system. Barbiturates
were originally used as sedatives and
sleeping pills but their clinical use is
now limited due to their toxic sideeffects; prolonged use can lead to addiction. SpeciÜc barbiturates in
clinical use include phenobarbitone,
for treating epilepsy, and methohexitane sodium, used as an anaesthetic.
Barfoed’s test A biochemical test
to detect monosaccharide (reducing)
sugars in solution, devised by the
Swedish physician Christen Barfoed
(1815–99). Barfoed’s reagent, a mixture of ethanoic (acetic) acid and copbaking soda
56
b
www.AzShimi.ir www.AzShimi.com
the mixture rise.
baking soda See sodium hydrogencarbonate.
balance An accurate weighing device. The simple beam balance consists of two pans suspended from a
centrally pivoted beam. Known
masses are placed on one pan and
the substance or body to be weighed
is placed in the other. When the
beam is exactly horizontal the two
masses are equal. An accurate laboratory balance weighs to the nearest
hundredth of a milligram. Specially
designed balances can be accurate to
a millionth of a milligram. More
modern substitution balances use the
substitution principle. In this calibrated weights are removed from the
single lever arm to bring the single
pan suspended from it into equilibrium with a Üxed counter weight.
The substitution balance is more accurate than the two-pan device and
enables weighing to be carried out
more rapidly. In automatic electronic
balances, mass is determined not by
mechanical deÛection but by electronically controlled compensation
of an electric force. A scanner monitors the displacement of the pan
support generating a current proportional to the displacement. This current Ûows through a coil forcing the
pan support to return to its original
position by means of a magnetic
force. The signal generated enables
the mass to be read from a digital
display. The mass of the empty container can be stored in the balance’s
computer memory and automatically
deducted from the mass of the container plus its contents.
Balmer series See hydrogen spectrum.
banana bond Informal name for
the type of electron-deÜcient threecentre bond holding the B–H–B
bridges in *boranes and similar compounds.
band spectrum See spectrum.
band theory See energy bands.
bar A c.g.s. unit of pressure equal
to 10
6 dynes per square centimetre
or 10
5 pascals (approximately
750 mmHg or 0.987 atmosphere).
The millibar (100 Pa) is commonly
used in meteorology.
Barbier–Wieland degradation
The stepwise degradation of a carboxylic acid to the next lower homologue. First the ester is converted
into a tertiary alcohol using a Grignard reagent (PhMgX) and acid (HX):
RCH 2 COOCH 3 → RCH 2 C(OH)Ph 2 .
The secondary alcohol is then dehydrated using ethanoic anhydride
(CH 3 COOCOCH 3 ) to give an alkene:
RCH 2 C(OH)Ph 2 → RCH=CPh 2 .
The alkene is oxidized with chromic
acid:
RCH=CPh 2 → RCOOH + Ph 2 CO.
The result is conversion of an acid
RCH 2 COOH to the lower acid
RCOOH.
barbiturate Any one of a group of
drugs derived from barbituric acid
that have a depressant effect on the
central nervous system. Barbiturates
were originally used as sedatives and
sleeping pills but their clinical use is
now limited due to their toxic sideeffects; prolonged use can lead to addiction. SpeciÜc barbiturates in
clinical use include phenobarbitone,
for treating epilepsy, and methohexitane sodium, used as an anaesthetic.
Barfoed’s test A biochemical test
to detect monosaccharide (reducing)
sugars in solution, devised by the
Swedish physician Christen Barfoed
(1815–99). Barfoed’s reagent, a mixture of ethanoic (acetic) acid and copbaking soda
56
b
www.AzShimi.ir www.AzShimi.com