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Introduction :
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula ?SiO?_2. It is most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and most abundant families of materials, existing as a compound of several minerals and as synthetic product. In food production as an anti-coagulant. It is also used in the manufacturing of cast iron and ceramics. Notable examples include fused quartz, fumed silica, silica gel, and aerogels. It is used in structural materials, microelectronics, and as components in the food and pharmaceutical industries.

Physical Properties :
Silica is a group IV metal oxide, which has good abrasion resistance, electrical insulation and high thermal stability. It is insoluble in all acids with the exception of hydrogen fluoride (HF).
Some important physical properties of silica are :
Appearance Transparent solid (Amorphous) White/Whitish Yellow (Powder/Sand)
Odor Odorless
Atomic weight 60.08g/mole
Density 2.648 (?-quartz),
2.196 (amorphous) ?g·cm?^(-3)
Molecules 2.3 10 /cm
Specific heat 1.0J/g-K
Melting point 1710 C
Boiling point 2230 C

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The silicon dioxide molecule can be described as a three-dimensional network of tetrahedra cells, with four oxygen atoms surrounding each silicon ion. The tetrahedral form is the basic unit from which a ?SiO?_2 structure is formed, even though ?SiO?_2 can exist in a crystalline structure. The reason for the amorphous oxide structure is the absence of any crystalline form of ?SiO?_2 whose lattice size closely matches the silicon lattice.
Silica has a low but non-negligible solubility in water. At pH’s above 8 and below 3, the solubility of silica increases rapidly. In dilute concentration, the silica can dissolve from the surface until equilibrium is reached.

Specific Chemical Properties :
Silica is another name for the chemical compound silicon dioxide. Each unit of silica includes one atom of silicon and two atoms of oxygen.
Some important chemical properties of silica are:
Flammability Not flammable
Toxicity Silica ingested orally is essentially nontoxic. Inhaling finely divided crystalline silica is toxic and can lead to silicosis, bronchitis, lung cancer, and systemic autoimmune diseases, such as lupus and rheumatoid arthritis .
Heat of combustion Very high
Reactivity with water Doesn’t react with water. But may react in high temperature
Reactivity with acids Very weakly acidic, reacting with strong bases
Oxidation Not a typical oxidation catalyst. But, an effective catalyst for the ammoximation of ketons to oximes
Radioactivity Gamma irradiation
Corrosion Dissolution, Selective leaching, and stress-corrosion cracking can degrade the performance of silica
Pharmaceutical and Biological uses :
Hydrophobic silica is used as a defoamer component.
In its capacity as a refractory, it is useful in fiber form as a high-temperature thermal protection fabric.
Silica is used in the extraction of DNA and RNA due to its ability to bind to the nucleic acids under the presence of chaotropes.
A silica-based aerogel was used in the Stardust spacecraft to collect extraterrestrial particles.
In cosmetics, its useful for its light-diffusing properties and natural absorbency.
Hydrated silica is used in toothpaste as a hard abrasive to remove tooth plaque.
In pharmaceutical products, silica aids powder flow when tablets are formed
Silica is a common additive in food production, where it is used primarily as a flow agent in powdered foods, or to adsorb water in hygroscopic applications.
Colloidal silica is also used as a wine, beer, and juice fining agent.
Silica sand is used in buildings and roads in the form of portland cement, concrete, and mortar, as well as sandstone.
Silica gel is often used as a desiccant to remove moisture.
It is often used in glassware for laboratories.

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