cal compound that rotates the plane
of polarization of plane-polarized
light to the right (clockwise as observed by someone facing the oncoming radiation). See optical
activity.
dextrose See glucose.
diagonal relationship A relationship within the periodic table by
which certain elements in the second
period have a close chemical similarity to their diagonal neighbours in
the next group of the third period.
This is particularly noticeable with
the following pairs.
Lithium and magnesium:
(1) both form chlorides and bromides
that hydrolyse slowly and are soluble
in ethanol;
(2) both form colourless or slightly
coloured crystalline nitrides by direct
reaction with nitrogen at high temperatures;
(3) both burn in air to give the normal oxide only;
(4) both form carbonates that decompose on heating.
Beryllium and aluminium:
(1) both form highly refractory oxides with polymorphs;
(2) both form crystalline nitrides that
are hydrolysed in water;
(3) addition of hydroxide ion to solutions of the salts gives an amphoteric
hydroxide, which is soluble in excess
hydroxide giving beryllate or aluminate ions [Be(OH) 4 ]
2– and [Al(OH) 4 ]
– ;
(4) both form covalent halides and covalent alkyl compounds that display
bridging structures;
(5) both metals dissolve in alkalis.
Boron and silicon:
(1) both display semiconductor properties;
(2) both form hydrides that are unstable in air and chlorides that hydrolyse in moist air;
(3) both form acidic oxides with covalent crystal structures, which are
readily incorporated along with
other oxides into a wide range of
glassy materials.
The reason for this relationship is a
combination of the trends to increase
size down a group and to decrease
size along a period, and a similar, but
reversed, effect in electronegativity,
i.e. decrease down a group and increase along a period.
dialysis A method by which large
molecules (such as starch or protein)
and small molecules (such as glucose
or amino acids) in solution may be
separated by selective diffusion
through a semipermeable membrane. For example, if a mixed solution of starch and glucose is placed in
a closed container made of a semipermeable substance (such as Cellophane), which is then immersed in a
beaker of water, the smaller glucose
molecules will pass through the
membrane into the water while the
starch molecules remain behind. The
cell membranes of living organisms
are semipermeable, and dialysis
takes place naturally in the kidneys
for the excretion of nitrogenous
waste. An artiÜcial kidney (dialyser)
utilizes the principle of dialysis by
taking over the functions of diseased
kidneys.
diamagnetism See magnetism.
diaminobenzene (phenylenediamine) Any one of three isomeric
aromatic compounds, C 6 H 4 (NH 2 ) 2 ,
with strong basic properties. 1,2Diaminobenzene is a yellow-brown
crystalline solid, m.p. 104°C, used as
a photographic developer and in
making dyes. 1,3-Diaminobenzene is
a colourless crystalline solid, m.p.
63°C, which turns brown on standing
in air. It is made by reducing 1,3dinitrobenzene with iron and hydrochloric acid, and is used for making dyes. 1,4-Diaminobenzene is a
white crystalline solid, m.p. 147°C,
dextrose
172
d
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of polarization of plane-polarized
light to the right (clockwise as observed by someone facing the oncoming radiation). See optical
activity.
dextrose See glucose.
diagonal relationship A relationship within the periodic table by
which certain elements in the second
period have a close chemical similarity to their diagonal neighbours in
the next group of the third period.
This is particularly noticeable with
the following pairs.
Lithium and magnesium:
(1) both form chlorides and bromides
that hydrolyse slowly and are soluble
in ethanol;
(2) both form colourless or slightly
coloured crystalline nitrides by direct
reaction with nitrogen at high temperatures;
(3) both burn in air to give the normal oxide only;
(4) both form carbonates that decompose on heating.
Beryllium and aluminium:
(1) both form highly refractory oxides with polymorphs;
(2) both form crystalline nitrides that
are hydrolysed in water;
(3) addition of hydroxide ion to solutions of the salts gives an amphoteric
hydroxide, which is soluble in excess
hydroxide giving beryllate or aluminate ions [Be(OH) 4 ]
2– and [Al(OH) 4 ]
– ;
(4) both form covalent halides and covalent alkyl compounds that display
bridging structures;
(5) both metals dissolve in alkalis.
Boron and silicon:
(1) both display semiconductor properties;
(2) both form hydrides that are unstable in air and chlorides that hydrolyse in moist air;
(3) both form acidic oxides with covalent crystal structures, which are
readily incorporated along with
other oxides into a wide range of
glassy materials.
The reason for this relationship is a
combination of the trends to increase
size down a group and to decrease
size along a period, and a similar, but
reversed, effect in electronegativity,
i.e. decrease down a group and increase along a period.
dialysis A method by which large
molecules (such as starch or protein)
and small molecules (such as glucose
or amino acids) in solution may be
separated by selective diffusion
through a semipermeable membrane. For example, if a mixed solution of starch and glucose is placed in
a closed container made of a semipermeable substance (such as Cellophane), which is then immersed in a
beaker of water, the smaller glucose
molecules will pass through the
membrane into the water while the
starch molecules remain behind. The
cell membranes of living organisms
are semipermeable, and dialysis
takes place naturally in the kidneys
for the excretion of nitrogenous
waste. An artiÜcial kidney (dialyser)
utilizes the principle of dialysis by
taking over the functions of diseased
kidneys.
diamagnetism See magnetism.
diaminobenzene (phenylenediamine) Any one of three isomeric
aromatic compounds, C 6 H 4 (NH 2 ) 2 ,
with strong basic properties. 1,2Diaminobenzene is a yellow-brown
crystalline solid, m.p. 104°C, used as
a photographic developer and in
making dyes. 1,3-Diaminobenzene is
a colourless crystalline solid, m.p.
63°C, which turns brown on standing
in air. It is made by reducing 1,3dinitrobenzene with iron and hydrochloric acid, and is used for making dyes. 1,4-Diaminobenzene is a
white crystalline solid, m.p. 147°C,
dextrose
172
d
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