Silicon dioxide

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Silicon dioxide
Silicon dioxide acid washed beach sand.jpg
Ground silica and acid washed beach sand; SiO2 at different purity levels
IUPAC name
Silicon dioxide
Other names
Crystalline silica
Silicic oxide
Silicon(IV) oxide
Jmol-3D images Image
Molar mass 60.08 g/mol
Appearance White solid
Odor Odorless
Density 2.648 g/cm3 (α-quartz)
2.196 g/cm3 (amorphous)
Melting point 1,713 °C (3,115 °F; 1,986 K)
Boiling point 2,950 °C (5,340 °F; 3,220 K)
Completely insoluble
Solubility Reacts with alkali, fluoroboric acid, hydrogen fluoride
Insoluble in all solvents
Vapor pressure ~0 mmHg
42 J·mol−1·K−1
−911 kJ/mol
Safety data sheet Sigma-Aldrich
Flash point Non-flammable
Related compounds
Related compounds
Carbon dioxide
Germanium dioxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Silicon dioxide, or silica, is the compound with chemical formula SiO2. It exists in nature as the mineral quartz and is also the primary compound found in beach sand. The colloquial term "glass" usually refers to glasses predominantly composed of silicon dioxide, with other trace compounds that determine the precise use of the glass variant. Silicon dioxide(and therefore glass) is notably very chemically inert, as very few substances react with it, making it ideal for most lab glassware.



It is important to note that while silicon dioxide is largely unreactive with most substances, it is incompatible with a few very reactive chemicals, including hydrofluoric acid, which readily dissolves most kinds of glass. Very hot or concentrated solutions of sodium hydroxide, or any alkali hydroxide, can also dissolve and react with silicon dioxide, forming an aqueous solution of sodium silicate (waterglass). Molten sodium hydroxide, or any alkali hydroxide, will very quickly attack silica, meaning it must be contained in stainless steel or another medium for use in molten state.


Silicon dioxide is a white compound, insoluble in water and acids. Silicon dioxide has many allotrope forms. It melts at 1600 °C and boils at 2,230 °C.


Relatively pure silicon dioxide can be obtained from beach sand by treating the sand with hot concentrated acid for an extended period of time, dissolving most impurities. While ordinary glass is mostly silica, it also contains calcium and or sodium oxides, that are nearly impossible to remove. Powdered glass likely contains other impurities that cannot be removed through the above process. One way to obtain pure silica from gall is to dissolve it in molten sodium hydroxide and after cooling, the resulting sodium and calcium, zinc ions from the glass are dissolved in acid, leaving behind relative pure silica, which can be purified via filtration. However, since molten sodium hydroxide is extremely dangerous to handle, this should not be attempted unless you're a professional.

A more pure form of silicon dioxide can be found in the small packs of silica gel placed in the containers of items to absorb any moisture that may otherwise affect the product. While each pack only contains a small amount, an opportunity such as working in a warehouse may arise where a very substantial supply of silica gel packs can be found.

Natural quartz crystals are a very good source of silicon dioxide, as it has few impurities (the colorless ones are purer). The windows from old CPU's and EPROMs are made of fused quartz. Several other electronic devices that contain quartz are crystal oscillators, quartz clocks, quartz heat lamps.

Ceramics stores often sell quite pure silicon dioxide that is already reduced to a fine powder, making this perhaps the best source to obtain it. Amounts as high as 30 pounds can often be bought, or as little as one pound, for a reasonable price.


Silicon dioxide can be prepared by oxidizing elemental silicon, for obtaining high purity silica. It can also be made by calcinating silica gel obtained by neutralization or decomposition of sodium silicate. Fumed silica can be obtained by burning silicon tetrachloride in air.

Less pure silicon dioxide is cheaper to purchase than to make.


  • Extracting elemental phosphorus from phosphates
  • Make elemental silicon



While silica is inert to most forms of life, inhaling fine silica particles, especially the crystalline form, can lead to silicosis, bronchitis, or cancer, as the dust becomes lodged in the lungs and continuously irritates them, reducing lung capacities. It is recommended to use a gas mask and goggles or to work outside when handling finely divided silica.


Silicon dioxide does not require special storage as it is inert, but porous forms of silica such as the gel form absorb water and other contaminants from the air, so these types need to be sealed off from the air.


Silicon dioxide can be disposed of in any setting, as it is already one of the most common compounds in the earth's crust and soils. Care should be taken when disposing of finely divided or powdered silica, as this poses a serious health risk if inhaled.


See also


Relevant Sciencemadness threads