the reduction in luminous intensity
of light passing through a material to
the length of the light’s path through
the material: i.e.
log(I/I 0 ) = –ε[J]l,where εis the molar *absorption
coefÜcient, I is the intensity after
passing through a sample of length l,
I 0 is the incident intensity, and [J] is
the molar concentration of species J.
The Beer–Lambert law was formed
empirically; however, it can be de-
rived on the basis that the loss in in-
tensity dI is proportional to the
thickness dl of the sample, the con-
centration [J], and the intensity I
(since the rate of absorption is pro-
portional to the intensity). The
Beer–Lambert law means that the in-
tensity of light (or any other form of
electromagnetic radiation) passing
through a sample diminishes expo-
nentially with the concentration and
the thickness of the sample (for a
given wave number).
beet sugarSee sucrose.
Beilstein’s test A test for the pres-
ence of a halogen (chlorine, bromine,
or iodine) in an organic compound. A
piece of copper wire or gauze is pre-
heated strongly in the oxidizing
Ûame of a Bunsen burner (until the
Ûame is no longer green) and the test
substance placed on the wire or
gauze, which is re-heated. A green
Ûame indicates the presence of a
halogen.
bell metalA type of *bronze used
in casting bells. It consists of 60–85%
copper alloyed with tin, often with
some zinc and lead included.
Belousov–Zhabotinskii reaction
See b–z reaction.
Bénard cell A structure associated
with a layer of liquid that is conÜned
by two horizontal parallel plates, in
which the lateral dimensions are
much larger than the width of the
layer. Before heating the liquid is ho-
mogeneous. However, if after heating
from below the temperatures of the
plates are T 1 and T 2 , at a critical value
of the temperature gradient ∆T = T 1 –
T 2 the liquid abruptly starts to con-
vect. The liquid spontaneously orga-
nizes itself into a set of convection
rolls, i.e. the liquid goes round in a
series of ‘cells’, called Bénard cells.Benedict’s test A biochemical test
to detect reducing sugars in solution,
devised by the US chemist S. R. Bene-
dict (1884–1936). Benedict’s reagent –
a mixture of copper(II) sulphate and a
Ültered mixture of hydrated sodium
citrate and hydrated sodium carbon-
ate – is added to the test solution and
boiled. A high concentration of re-
ducing sugars induces the formation
of a red precipitate; a lower concen-
tration produces a yellow precipitate.
Benedict’s test is a more sensitive al-
ternative to *Fehling’s test.beneÜciation(ore dressing) The
separation of an ore into the valuable
components and the waste material
(gangue). This may be achieved by a
number of processes, including
crushing, grinding, magnetic separa-
tion, frothÛotation, etc. The dressed
ore, consisting of a high proportion
of valuable components, is then
ready for smelting or some other
reÜning process.bent sandwichSee sandwich com-
pound.benzaldehydeSee benzenecar-
baldehyde.benzeneA colourless liquid hydro-
carbon, C 6 H 6 ; r.d. 0.88; m.p. 5.5°C;
b.p. 80.1°C. It is now made from
gasoline from petroleum by catalytic
reforming (formerly obtained from
coal tar). Benzene is the archetypal
*aromatic compound. It has an un-61 benzene
b