The Foundations of Chemistry

heat

2Ca 3 (PO 4 ) 2 
6SiO 2 
10C888n 6CaSiO 3 
10CO
P 4

calcium phosphate calcium silicate

(phosphate rock) (slag)

Vaporized phosphorus is condensed to a white solid (mp44.2°C, bp280.3°C)
under H 2 O to prevent oxidation. Even when kept under H 2 O, white phosphorus slowly
converts to the more stable red phosphorus allotrope (mp597°C; sublimes at 431°C).
Red phosphorus and tetraphosphorus trisulfide, P 4 S 3 , are used in matches. They do not
burn spontaneously, yet they ignite easily when heated by friction. Both white and red
phosphorus are insoluble in water.
The largest use of phosphorus is in fertilizers. Phosphorus is an essential nutrient, and
nature’s phosphorus cycle is very slow owing to the low solubility of most natural phos-
phates. Phosphate fertilizers are therefore essential. To increase the solubility of the natural
phosphates, they are treated with H 2 SO 4 to produce “superphosphate of lime,” a mixture
of two salts. This solid is pulverized and applied as a powder.

This reaction represents the most popular use of sulfuric acid, the industrial chemical
produced in largest quantity.

SILICON

Silicon is a shiny, blue-gray, high-melting, brittle metalloid. It looks like a metal, but it is
chemically more like a nonmetal. It is second only to oxygen in abundance in the earth’s
crust, about 87% of which is composed of silica (SiO 2 ) and its derivatives, the silicate
minerals. The crust is 26% Si, compared with 49.5% O. Silicon does not occur free in
nature. Pure silicon crystallizes with a diamond-type structure, but the Si atoms are less
closely packed than C atoms. Its density is 2.4 g/cm^3 compared with 3.51 g/cm^3 for
diamond.

Ca 3 (PO 4 ) 2 


  2H 2 SO 4 4H 2 O [Ca(H 2 PO 4 ) 2 2(CaSO 4 2H 2 O)]

evaporate

phosphate rock calcium

dihydrogen

phosphate

calcium

sulfate

dihydrate

superphosphate of lime

24-17 Phosphorus 965

The tips of “strike anywhere” matches contain

tetraphosphorus trisulfide and red phosphorus.

Friction converts kinetic energy into heat, which

initiates a spontaneous reaction.

P 4 S 3 (s)
8O 2 (g)88nP 4 O 10 (s)
3SO 2 (g)

Superphosphate fertilizer.

Pure silicon is used in solar cells to

collect energy from the sun.