silicon dioxide

Silica redirects here. For other uses, see Silica (disambiguation)

Silicon dioxide
General
Other names	Silica, Quartz, sand see text
Molecular formula	SiO2
Molar mass	60.1 g·mol−1
Appearance	white powdery substance solid (when pure)
CAS number	7631-86-9
Properties
Density and phase	2.2 g/cm³, solid
Solubility in water	0.012 g in 100g of water
Melting point	1650 (±75) °C
Boiling point	2230 °C
Structure
Coordination geometry	tetrahedral
Crystal structure	various
Hazards
MSDS	External MSDS
EU classification
NFPA 704	0 0 0
R-phrases	R42 R43 R49
S-phrases	S22 S36 S37 S45 S53
Flash point	non-flammable
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	Silicon sulfide
Other cations	Carbon dioxide Germanium dioxide Tin(IV) oxide Lead(IV) oxide
Related compounds	Silicic acid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

The chemical compound silicon dioxide, also known as silica or silox (from the Latin "silex"), is the oxide of silicon, chemical formula SiO₂, and has been known for its hardness since the 16th century. It is a principal component of most types of glass and substances such as concrete.

Siliceous is an adjective meaning "related to silica".

Manufactured forms

Silica is manufactured in several forms including:

• glass (a colorless, high-purity form is called fused silica)
• synthetic amorphous silica
• silica gel (used e.g. as desiccants in new clothes and leather goods)

It is used in the production of various products.

• Inexpensive soda-lime glass is the most common and typically found in drinking glasses, bottles, and windows.
• A raw material for many whiteware ceramics such as earthenware, stoneware and porcelain.
• A raw material for the production of Portland cement.
• A food additive, primarily as a flow agent in powdered foods, or to absorb water (see the ingredients list for).
• The natural ("native") oxide coating that grows on silicon is hugely beneficial in microelectronics. It is a superior electric insulator, possessing high chemical stability. In electrical applications, it can protect the silicon, store charge, block current, and even act as a controlled pathway to allow small currents to flow through a device. At room temperature, however, it grows extremely slowly, and so to manufacture such oxide layers on silicon, the traditional method has been the deliberate heating of silicon in high temperature furnaces within an oxygen ambient (thermal oxidation).
• Raw material for aerogel in the Stardust spacecraft
• Used in the extraction of DNA and RNA due to its ability to bind to the nucleic acids under the presence of chaotropes.
• Added to medicinal anti-foaming agent, like Simethicone, with a small portion to enhance defoaming activity.

Health effects

Manufactured silica fume at maximum surface area of 380m²/g

Inhaling finely divided crystalline silica dust in significant quantities can lead to silicosis or (much more rarely) cancer, as the dust becomes lodged in the lungs and continuously irritates them (silica does not dissolve over time). This effect can be an occupational hazard for people working with sandblasting equipment, products that contain powdered silica, and so on.

In all other respects, silicon dioxide is inert and harmless. When silica is ingested orally, it passes unchanged through the gastrointestinal tract, exiting in the feces, leaving no trace behind. Small pieces of silicon dioxide are equally harmless, as long as they are not large enough to mechanically obstruct the GI tract, or jagged enough to lacerate its lining. Silicon dioxide produces no fumes and is insoluble in vivo. It is indigestible, with zero nutritional value and zero toxicity.

Chemistry

Silicon dioxide is formed when silicon is exposed to oxygen (or air). A very thin layer (approximately 1 nm or 10 Å) of so-called 'native oxide' is formed on the surface when silicon is exposed to air under ambient conditions. Higher temperatures and alternate environments are used to grow well-controlled layers of silicon dioxide on silicon.

Silicon dioxide has covalent bonding and forms a network structure (also known as lattice or continuous).

Silicon dioxide is attacked by hydrofluoric acid (HF). HF is used to remove or pattern silicon dioxide in the semiconductor industry.

References

• R. K. Iler, The Chemistry of Silica (ISBN 0-471-02404-X)

See also

• Silicon
• Amorphous carbonia
• Fused silica
• Quartz
• Glass
• Sand
• Silicon carbide
• Mesoporous silica

External links

• International Chemical Safety Card 0807 (Tridymite)
• International Chemical Safety Card 0808 (Quartz)
• International Chemical Safety Card 0809 (Cristobalite)
• NIOSH Pocket Guide to Chemical Hazards (amorphous)
• NIOSH Pocket Guide to Chemical Hazards (crystalline, as respirable dust)
• Quartz SiO2 piezolelctric properties

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