silicon

Name silicon
Symbol Si
Atomic Weight 28.0855
Atomic Number 14
CAS ID 7440-21-3
Group Number 14
Group Name (none)
Period Number 3
Origin of Name Latin, silicis = "flint."
Year Discovered 1824
Discovery Credits Isolated by J. J. Berzelius at Stockholm, Sweden.
Class Semi-Metallic
Color Dark Gray with a Bluish Tinge
Standard State Solid at 77 F & 298 K
Description Black amorphous Si obtained by reduction of sand (SiO2) with carbon; ultrapure semiconductor grade crystals are blue-gray metallic. Bulk Si unreactive towards oxygen, water, acids (except HF), but dissolves in hot alkali. Used in semiconductors, alloys, polymers.

Diagnostic tests: 
The surest method of detecting a silicate is to get the mineral in solution and obtain gelatinous silica by evaporation.

If a silicate is dissolved in boiling acid, HCl or HNO3, the silica gel separates upon evaporation. At this point, the other elements in the mineral are in solution and can be tested for additional unknowns.

If a silicate is decomposed in boiling acid, HCl or HNO3, the silica separates as an insoluble residue without the formation of a jelly. This residue, if due to silica, is soluble in boiling KOH.

Fusion in Na2CO3 is necessary if the mineral is unaffected by acid digestion. Mix the powdered mineral with equal parts of sodium carbonate into a paste and fuse on charcoal with the blowpipe. Dissolve the residue in HCl, the silicate will separate as gelatinous mass upon evaporation. Note, most silicates are soluble in a borax flux and will give the same gelatinous reaction as the sodium carbonate.

Silicates are partially soluble in a Na2(NH4)(PO4) bead. When a powdered silicate is fused in the bead, the insoluble silicate skeleton or translucent mass is detectable with a hand lens.

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