Dissolves silicon dioxide


SiO2 is not a molecule, but the empirical formula of a group of inorganic polymers in which each Si atom is bonded to 4 oxygen atoms.

Surname Silicon dioxide
other names Silicon oxide, silicon dioxide
Molecular formulaSiO2
CAS number various, e.g. B. 7631-86-9, 112945-52-5, 112926-00-8, 14808-60-7
Brief description -
molar mass 60.1 g / mol
Physical state firmly
density depending on the modification between 1.9 and 4.29 g / cm³, mostly 2.2 (amorphous) to 2.65 (crystalline) g / cm³
Refractive index 1.47 ± 0.015 (with amorphous thin film)
Breakdown field strength 4 - 10 MV / cm
(depending on the manufacturing process, e.g. with wet thermal oxidation 4 - 6 MV / cm with dry higher.)
Melting point 1723 ° C
boiling point 2230 ° C
Vapor pressure - Pa (x ° C)
solubility -
safety instructions
Hazard symbols
R and S phrases

S: 22

MAK Burnt kieselguhr 0.3 mg / m³ A, precipitated silica 4 mg / m³ E
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions.

Silicon dioxide (often also: Silicon dioxide) is the collective term for chemical compounds with the empirical formula SiO2. In German, the term silicic acid is incorrectly used for silicon dioxide, although silicon dioxide is only the anhydride of the ortho-silicic acid Si (OH)4 or H4SiO4 is formed from silicon dioxide by the addition of two molecules of water.

Silicon dioxide is the main component of all types of glass.

Mineralogy and Occurrence

Non-crystalline (amorphous) SiO2 occurs naturally in largely pure form - also in volcanic glasses and tektites - which are very inhomogeneous and inconsistent in their composition:

  • biogenic: skeletons of radiolarians, diatoms and sponges made of opal, diagenetically solidified to form rock, for example silica slate
  • Geyserite: amorphous sintered products from hot springs
  • Tachylite: volcanic glass of basaltic composition, which in addition to SiO2 larger contents of FeO, MgO, CaO and Al2O3 contains
  • Obsidian: volcanic glass of granitic composition
  • Tektite: rock glasses formed by melting rocks as a result of meteorite impacts
  • Lechatelierite: pure natural SiO2-Glass as it z. B. occurs in tektites or in quartz sands when lightning strikes (fulgurite)
  • opal
  • SiO2-Melting: at temperatures above 1727 ° C (at 1 bar)

In contrast to the amorphous SiO2 the crystalline forms only have a very low tolerance to impurities. They only differ in their structure.

  • Moganite (chalcedony):
  • α-quartz (deep quartz): Formation conditions: temperature T < 573 °c,="" druck="" p="">< 20 kbar="">
  • β-quartz (high quartz): 573 ° C < t="">< 867 °c,="" p="">< 30 kbar="">
  • Tridymite: 867 ° C < 1470 °c,="" p="">< 5 kbar="">
  • Cristobalite: 1470 ° C < 1727 °c="">
  • Coesit: 20 kbar < p="">< 75 kbar="">
  • Stishovite: 75 kbar < p="">< ? kbar>

As part of silicates such as e.g. B. feldspar, clay minerals or in free form as quartz the main component of the earth's crust and thus also the most common silicon compound.

Chemical properties

Water and acids are capable of SiO2 practically insoluble, with the exception of hydrofluoric acid (HF), which forms gaseous silicon tetrafluoride (SiF4) is attacked. Alkali melts and - to a lesser extent - aqueous alkali lye dissolve particularly amorphous silicon dioxide.

Technical manufacturing

Synthetic SiO2, which is mostly amorphous, is produced in large quantities in various processes on an industrial scale.

The large-scale production of synthetic SiO2 mainly takes place via precipitation processes starting from water glass, which can be obtained by breaking up quartz sand with sodium carbonate or potassium carbonate. SiO produced in this way2 Depending on the process conditions, they are called precipitated silicas or silica gels. Another important production variant is the production of so-called pyrogenic SiO2 in an oxyhydrogen flame, based on liquid chlorosilanes such as silicon tetrachloride (SiCl4). Important manufacturers of synthetic silicas are Degussa, Wacker-Chemie, Rhodia, Grace and others.

Technical application

Synthetic SiO2 plays a major role in everyday life, usually unnoticed. It is just as important in paints and varnishes, plastics and adhesives as it is in modern manufacturing processes in semiconductor technology or as a pigment in inkjet paper coatings. As a non-toxic substance, it is found in pharmaceutical articles as well as in cosmetic products, is used in food processes (e.g. beer clarification) and as a cleaning aid in toothpaste. In terms of quantity, the main applications include the use as a filler for plastics and sealing compounds, especially in rubber articles. Modern car tires benefit from the reinforcement by a special SiO2System and save about 5% fuel compared to rubber compounds traditionally only filled with soot, while at the same time improving safety.

In terms of quantity, silicon dioxide is of course in the form of glass. It is mostly mixed with substances such as aluminum oxide, boron oxide, calcium and sodium oxide to lower the melting temperature, to make processing easier or to improve the properties of the end product. Pure silicon dioxide is quartz glass that is difficult to melt, but it is particularly robust.

Quartz glass is used in optics in the form of lenses, prisms, etc. In microsystem technology or nanotechnology, they are often used as mask carriers, as gate oxide in MOSFETs and as insulating layers in ICs (integrated circuits). In the chemical laboratory, quartz glass is used as device glass as soon as particularly high resistance to temperature changes, chemical resistance or UV permeability are required. Borosilicate glass, which is also relatively resistant, is usually used in the laboratory.

Also is SiO2 increasingly used in the food industry. So you can find it z. B. in spices and spice mixtures (mostly paprika mixtures).

Another use for silicon dioxide can be found in pyrotechnics, where it is used to e.g. B. Manufacture Brandgels. Or as a means of combating grain beetles (which hollow out the grain, generate heat and thus promote mold infestation (etc.)).

See also

Categories: Silicon Compound | Oxide | Pharmaceutical excipient