Silicon dioxide

 

Silicon dioxide
aka Silica

Chemical formula	SiO2
OTP appearance	clear crystal
Molar Mass(g/mol)	60.08
Density(g/cc)	2.648
Melting Point(°C)	1713
Boiling Point(°C)	2950
NFPA 704	0 0 0

Uses

• Primary component of glass
• Fused silicon dioxide is a high-temperature-resistant glass
• Primary source of pure silicon

Natural occurrence

Silicon dioxide occurs naturally, usually as sand and other silacious minerals.

Hazards

• GRAS

Purification

phase I

1. grind to a fine powder, breaking the sand crystals along faults and exposing impurities on the surface
2. Wash thoroughly with hot hydrochloric acid, converting calcium, magnesium, sodium, potassium, and iron compounds to water-soluble chlorides
3. Wash thoroughly with hot water

Phase II

1. melt N ubm of sodium hydroxide
2. add M ubm of acid-and-water-washed sand

   2 NaOH(l) + SiO₂(s) → Na₂SiO₃ + H₂O(v)

3. heat and stir, producing a mixture of silcon dioxide, sodium hydroxide, and sodium silicate
4. dissolve the mixture in water
5. cool to room temperature
6. filter
7. discard residue (unreacted sand)
8. heat and add hydrochloric acid until the solution is acidic

   Na₂SiO₃ + 2 HCl → 2 NaCl + H₂SiO₃ // excess sodium hydroxide becomes salt and water

9. cool to room temperature
10. filter
11. neutralize and discard filtrate
12. retain residue (gel of silicic acid)
13. wash with water
14. heat silicic acid to produce pure silicon dioxide.

    H₂SiO₃

    {

    →

    heat}

    SiO₂(s) + H₂O(v)

Disposal

Provided the crystals are coarse enough to fall out of the air, silicon dioxide can be disposed of into the environment.