Chemistry Reference
In-Depth Information
Alloys of mercury are called amalgams. Dentists use silver-tin amalgams for
dental fillings: they fill the tooth with the freshly prepared ductile mixture, where it
hardens within 1 h and forms crystals, which enclose the Hg atoms in the special
crystal structure. Gold amalgam plays an important role for gold diggers, mercury is
used for the separation of gold dust and sand: a generous amount of mercury has to
be added to the suspension of gold dust and sand in water and then shaken; gold
dissolves in mercury. The gold amalgam has to be washed out and vaporized later in
the camp fire: gold crystals remain.
Alloys are compounds with overall metallic character. Smelts of pure metals
have to be mixed and crystallized by cooling to produce mostly solid alloy crystals.
Some important alloys are listed with their compositions and densities; steel is an
alloy that contains up to 1.7% of carbon (see Table
10.2
).
If the atoms have more or less the same size, they form substitutional mixed
crystals. If they have different sizes, the small atoms occupy sites between the
bigger atoms: interstitial mixed crystals (see Fig.
10.13
). Two kinds of metals with
the same structure and more or less the same atomic radii cause the metals to form a
continuous solid solution. For example, gold and copper, both metals with a cubic
face-centered structure and nearly same atomic radii, can alloy in every possible
Table 10.2 Familiar alloys, their compositions and densities
Alloys
Density (g cm
1
)
Components (%)
Steel
Fe, C (bis 1.7%)
7.8
Chromium-nickel steel
55 Fe, 25 Cr, 20 Ni, 0,5 Si
7.9
Inox steel
71 Fe, 20 Cr, 8 Ni, 0.2 Si/Co/Mn
7.3
...
7.4
Brass, white
50 Cu, 50 Zn
8.2
Brass, yellow
70 Cu, 30 Zn
8.4
Brass, red
90 Cu, 10 Zn
8.8
Bell bronze
80 Cu, 20 Sn
8.7
Duralumin
92 Al, 5 Cu, 2 Mg, 1 Mn
2.8
Electron
95,7 Mg, 4 Al, 0.3 Mn
1.8
Constantan
60 Cu, 40 Ni
8.9
Nickel-silver
64 Cu, 30 Zn, 6 Ni
8.3
Platinum-iridium
5 Pt, 95 Ir
22.4
Substitutional crystal
Interstitial crystal
Fig. 10.13 Models of substitutional (
left
) or interstitial (
right
) metal alloy structures
