Silicon Dioxide (Quartz / Sand) (SiO₂) — Properties, Uses & Safety

Overview

Silicon dioxide (SiO2), also known as silica, is a covalent network solid with a molar mass of 60.08 g/mol. It is the most abundant mineral in Earth's crust, appearing in numerous forms including quartz crystals, sand, flint, and diatomaceous earth. In its pure crystalline form (quartz), SiO2 is a colorless, transparent solid, while common sand is an impure form colored by iron oxide and other mineral impurities. The structure of silicon dioxide consists of each silicon atom tetrahedrally bonded to four oxygen atoms, with each oxygen atom bridging two silicon atoms, creating an extensive three-dimensional covalent network. This continuous network of strong Si-O bonds (bond energy approximately 452 kJ/mol) gives SiO2 an extremely high melting point of 1,713 °C and makes it very hard (7 on the Mohs scale). When SiO2 is melted and cooled rapidly, it forms amorphous silica glass — the basis of window glass, optical fibers, and laboratory glassware. The glass-making process, one of humanity's oldest chemical technologies, typically involves melting sand with sodium carbonate and calcium oxide. Silicon dioxide is also the starting material for producing elemental silicon used in semiconductor chips and solar cells, connecting it directly to the digital technology revolution. The GHS hazard code H373 (may cause damage to organs through prolonged or repeated exposure) applies to crystalline silica dust, which can cause silicosis, a serious occupational lung disease. For chemistry students, SiO2 illustrates covalent network solids (contrasting with molecular solids like CO2), tetrahedral bonding geometry, the relationship between structure and properties, amorphous versus crystalline solids, and the chemistry of glass-making. Comparing SiO2 with CO2 — both containing group 14 elements bonded to oxygen but with vastly different structures and properties — is one of the most instructive comparisons in introductory chemistry.

IUPAC Name

silicon dioxide

CAS Number

14808-60-7

PubChem CID

24261

Molecular Formula

SiO₂

Molar Mass

60.085 g/mol

State at STP

solid

Appearance

Colorless to white solid; sand is impure form