light-dependent reaction See
photosynthesis.light-independent reaction See
photosynthesis.ligninA complex organic polymer
that is deposited within the cellulose
of plant cell walls during secondary
thickening. LigniÜcation makes the
walls woody and therefore rigid.ligniteSee coal.limeSee calcium oxide.
limestoneA sedimentary rock that
is composed largely of carbonate
minerals, especially carbonates of
calcium and magnesium. *Calcite
and *aragonite are the chief miner-
als; *dolomite is also present in the
dolomitic limestones. There are
many varieties of limestones but
most are deposited in shallow water.
Organic limestones (e.g. *chalk) are
formed from the calcareous skele-
tons of organisms; precipitated lime-
stonesinclude oolite, which is
composed of ooliths – spherical bod-
ies formed by the precipitation of
carbonate around a nucleus; and clas-
tic limestones are derived from frag-
ments of pre-existing calcareous
rocks.limewaterA saturated solution of
*calcium hydroxide in water. When
carbon dioxide gas is bubbled
through limewater, a ‘milky’ precipi-
tate of calcium carbonate is formed:
Ca(OH) 2 (aq) + CO 2 (g) →CaCO 3 (s) +
H 2 O(l)
If the carbon dioxide continues to be
bubbled through, the calcium car-
bonate eventually redissolves to form
a clear solution of calcium hydrogen-
carbonate:
CaCO 3 (s) + CO 2 (g) + H 2 O(g) →
Ca(HCO 3 ) 2 (aq)
If cold limewater is used the original
calcium carbonate precipitated has acalcite structure; hot limewater
yields an aragonite structure.limit cycleSee attractor.limoniteA generic term for a
group of hydrous iron oxides,
mostly amorphous. *Goethite and
*haematite are important con-
stituents, together with colloidal
silica, clays, and manganese oxides.
Limonite is formed by direct precipi-
tation from marine or fresh water in
shallow seas, lagoons, and bogs (thus
it is often called bog iron ore) and by
oxidation of iron-rich minerals. It is
used as an ore of iron and as a pig-
ment.Lindemann–Hinshelwood mech-
anismA mechanism for unimolecu-
lar chemical reactions put forward by
the British physicist Frederick Linder-
mann (1886–1957) in 1921 and exam-
ined in more detail by the British
chemist Sir Cyril Hinshelwood
(1897–1967) in 1927. The mechanism
postulates that a molecule of A be-
comes excited by colliding with an-
other molecule of A, and that having
been excited there is a possibility
that it undergoes unimolecular
decay. If the process of unimolecular
decay is sufÜciently slow, the reac-
tion has aÜrst-order rate law, in
agreement with experiment. The Lin-
demann–Hinshelwood mechanism
predicts that if the concentration of
A is reduced, the reaction kinetics be-
come second order. This change from
Ürst to second order agrees with ex-
periment qualitatively, although it
does not do so quantitatively. The
mechanism fails quantitatively be-
cause the molecule has to be excited
in a speciÜc way for a reaction to
take place. The RRK and RRKM theo-
ries improve on this deÜciency of the
Lindemann–Hinshelwood mecha-
nism.light-dependent reaction 326l