CaWO 4. The ore is heated with con-
centrated sodium hydroxide solution
to form a soluble tungstate. The
oxide WO 3 is precipitated from this
by adding acid, and is reduced to the
metal using hydrogen. It is used in
various alloys, especially high-speed
steels (for cutting tools) and in lamp
Ülaments. Tungsten forms a protec-
tive oxide in air and can be oxidized
at high temperature. It does not dis-
solve in dilute acids. It forms com-
pounds in which the oxidation state
ranges from +2 to +6. The metal was
Ürst isolated by Juan d’Elhuyer and
Fausto d’Elhuyer (1755–1833) in
1783.
A- Information from the WebElements site
tungsten carbideA black powder,
WC, made by heating powdered
tungsten metal with lamp black at
1600 °C. It is extremely hard (9.5 on
Mohs’ scale) and is used in dies and
cutting tools. A ditungsten carbide,
W 2 C, also exists.
tunnel effect An effect in which
electrons are able to tunnel through
a narrow potential barrier that would
constitute a forbidden region if the
electrons were treated as classical
particles. That there is aÜnite proba-
bility of an electron tunnelling from
one classically allowed region to an-
other arises as a consequence of
*quantum mechanics. The effect is
made use of in the tunnel diode.
turpentineAn oily liquid extracted
from pine resin. It contains pinene,
C 10 H 16 , and other terpenes and is
mainly used as a solvent.turquoise A mineral consisting of a
hydrated phosphate of aluminium
and copper, CuAl 6 (PO 4 ) 4 (OH) 8 .4H 2 O),
that is prized as a semiprecious
stone. It crystallizes in the triclinic
system and is generally blue in col-
our, the ‘robin’s egg’ blue varietybeing the most sought after. It usu-
ally occurs in veinlets and as masses
and is formed by the action of sur-
face waters on aluminium-rich rocks.
TheÜnest specimens are obtained
from Iran.
twinningThe growth of crystals in
such a way that two distinct crystals
form sharing a common plane of
atoms.twistSee ring conformations.
Tyndall effect The scattering of
light as it passes through a medium
containing small particles. If a poly-
chromatic beam of light is passed
through a medium containing parti-
cles with diameters less than about
one-twentieth of the wavlength of
light, the scattered light appears
blue. This accounts for the blue ap-
pearance of tobacco smoke. At
higher particle diameters, the scat-
tered light remains polychromatic.
The effect is seen in suspensions and
certain colloids. It is named after
John Tyndall (1820–93).type A and B metalsA classiÜca-
tion of metal ions according to the
stability of their complexes for a
given ligand. Type A metals cations
include the ions of group 1 (Li+to
Cs+), the ions of group 2 (Be2+to
Ba2+), and ions of lighter transition
metals in high oxidation states (e.g.
Co3+, Ti4+, Fe3+). The type B metal
cations are those of heavier transi-
tion metals in lower oxidation states
(e.g. Ag+, Cu+, Ni2+, Pd2+, Pt2+). Certain
ligands tend to form more stable
complexes with type A metals; others
form more stable complexes with
type B. For example, the tendency of
halide anions to complex with type A
metals is in the sequence
F–>Cl–>Br–>I–.
Their tendency to complex with type
B metals is the opposite sequence.
This led to a classiÜcation of ligandstungsten carbide 538t