Silica: Preparation, Properties, Bonding, Structure, Comparison and Uses

Silicon Dioxide (SiO₂) - Silica

Silicon dioxide, commonly known as silica, is one of the most abundant compounds on Earth, making up about 59% of the Earth's crust. It exists in amorphous and three crystalline forms each having low and hight temperature modifications.

Major Forms of Silica

• Crystalline Silica: Quartz, Cristobalite, Tridymite
• Amorphous Silica:
  • Fumed silica (Aerosil®)
  • Precipitated silica
  • Silica gel
  • Colloidal silica
  • Fused silica

Methods of Preparation

• From Sand (Natural Quartz): High-purity quartz sand is purified and melted at >1700 °C to produce fused silica.
• Thermal Decomposition of Silicon Compounds:
  SiCl₄ + 2H₂O → SiO₂ + 4HCl (fumed silica/pyrogenic silica)
• Precipitation Method:
  Neutralization of sodium silicate solution with acid:
  Na₂SiO₃ + H₂SO₄ → SiO₂↓ + Na₂SO₄ + H₂O (precipitated silica)
• Sol-Gel Process: Hydrolysis and condensation of tetraethoxysilane (TEOS):
  Si(OC₂H₅)₄ + 2H₂O → SiO₂ + 4C₂H₅OH (high-purity silica gel/aerogel)
• From Rice Husk Ash: Burning rice husks under controlled conditions yields high-purity amorphous silica.

Physical Properties

Property	Value/Description
Chemical Formula	SiO2
Molar Mass	60.08 g/mol
Appearance	White or colorless solid (crystalline or amorphous)
Density	2.2 – 2.6 g/cm³ (varies with form)
Melting Point	~1713 °C (quartz)
Boiling Point	~2950 °C
Solubility in Water	Practically insoluble
Crystal Forms	Quartz, Cristobalite, Tridymite (crystalline); Fused silica, Silica gel, Precipitated silica (amorphous)
Hardness (Mohs)	7 (quartz)
Refractive Index	1.45 – 1.55

Chemical Properties

1. Due to acidic nature, it dissolves in alkali.
   2NaOH + SiO₂ = Na₂SiO₃ + H₂O
2. It resists the action of all acids but dissolves in HF due to complex formation.
   SiO₂ + 4HF = 2H₂O + SiF₄
   SiF₄ + 2HF = H₂[SiF₆]
3. When SiO₂ is fused with microcosmic salt, Na(NH₄)HPO₄.4H₂O on a Pt-loop, insoluble silica is seen swimming in the bead-
   Na(NH₄)HPO₄ → NaPO₃ + NH₃ + H₂O
   NaPO₃ + SiO₂ = NaPO₃.SiO₂
4. Silica slightly dissolves in methanol at high temperature and pressure.
   SiO₂ + 4CH₃OH = Si(OCH₃)₄ + 2H₂O
5. Silica displaces volatile acid oxides from their salts.
   Na₂CO₃ + SiO₂ = Na₂SiO₃ + CO₂ ↑
   Na₂SO₄ + SiO₂ = Na₂SiO₃ + SO₃ ↑
   Ca₃(PO₄)₂ + 3SiO₂ = 3CaSiO₃ + P₂O₅ ↑
   

Structure and Bonding in Silicon Dioxide (SiO₂)

Key Fact: SiO₂ is a giant covalent network solid (macromolecular structure) consisting of tetrahedral SiO₄ units linked by bridging oxygen atoms.

Fundamental Bonding

• Type of bonding: Primarily covalent with ~50–55% ionic character (due to electronegativity< difference: Si = 1.8, O = 3.5)
• Each silicon atom is bonded to four oxygen atoms → tetrahedral arrangement
• Each oxygen atom bridges two silicon atoms (Si–O–Si linkage)
• Coordination numbers:
  • Silicon: 4 (tetrahedral)
  • Oxygen: 2 (bridging)
• Si–O bond energy ≈ 452 kJ/mol → extremely strong bonds
• No discrete SiO₂ molecules → continuous 3D network

Crystalline Polymorphs of SiO₂

Polymorph	Structure	Si Coordination	Stability	Occurrence
α-Quartz	Hexagonal, spiral chains of tetrahedra	4	Room temp → 573 °C	Most common (sand, granite)
β-Quartz	Higher symmetry form of quartz	4	573–870 °C	High-temperature quartz
Cristobalite	Cubic/tetragonal (diamond-like with Si–O–Si links)	4	>1470 °C	Volcanic rocks
Tridymite	Hexagonal plates, most open structure	4	High-temperature	Rare
Coesite	Dense, distorted tetrahedra	4	High pressure	Impact craters
Stishovite	Rutile (TiO₂)-type structure	6 (octahedral)	>10 GPa	Meteorite impacts

Amorphous Silica (Fused Silica / Silica Glass)

• No long-range order, but short-range order is preserved
• Each Si still surrounded by 4 O in tetrahedral geometry
• Si–O–Si bond angles vary widely (average ≈ 144°)
• Random network of corner-sharing tetrahedra
• Contains rings of 5-, 6-, 7-membered SiO₄ units

Comparison: Crystalline vs Amorphous SiO₂

Feature	Crystalline (e.g., Quartz)	Amorphous (Fused Silica)
Appearance	Transparent, sparkling crystals	transparent glass or translucent/opalescent
Long-range order	Yes	No
Short-range order	Perfect tetrahedral	Same tetrahedral units
Si coordination	4 (except stishovite = 6)	4
Si–O–Si angle	Fixed for each polymorph	Variable (120°–180°)
Density	Higher (especially high-P forms)	Lower (~2.2 g/cm³)
Melting behavior	Sharp melting point	Softens gradually
Mechanical properties	Anisotropic	Isotropic
Thermal expansion	Varies by polymorph	Extremely low and smooth thermal expansion
Refractive index	1.54-1.55	1.458
Examples in nature	Quartz crystals, sandstone, granite	Opal, volcanic glass, diatom shells, fulgurite

Uses of Silica (SiO₂)

Form	Major Applications
Quartz	Glass manufacturing, optical fibers, piezoelectric devices, watches, semiconductors
Fumed Silica	Thickening agent in paints, coatings, adhesives, cosmetics; reinforcing filler in silicone rubber
Precipitated Silica	Reinforcing filler in tires (green tires), toothpaste (abrasive), food additives (anti-caking agent E551)
Silica Gel	Desiccant packets, chromatography, cat litter
Fused Silica	Crucibles, telescope mirrors, UV lamps, high-temperature windows, semiconductor industry
Colloidal Silica	Polishing agent (CMP in semiconductors), binder in precision casting, paper coating
Aerogel	Super-insulation material, space applications (NASA Stardust mission), thermal insulation

Health and Safety Note

Crystalline silica (especially respirable quartz dust) is classified as a carcinogen (IARC Group 1) and can cause silicosis, lung cancer, and other diseases upon prolonged inhalation. Amorphous silica is generally considered much safer.

Why SiO₂ has High Melting Point and Extreme Stability

• Extensive 3D covalent network (similar to diamond)
• Very strong Si–O bonds throughout the structure
• No weak intermolecular forces — must break strong covalent bonds to melt
• Contrast with CO₂: molecular solid with weak van der Waals forces → sublimes at –78 °C

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