Silicon Dioxide
Properties
| State | Solid (crystalline or amorphous) |
| Color | Colorless (crystal); white (powder) |
| Solubility | Insoluble in water and most acids; soluble in HF and hot alkali |
| Melting Point | 1713°C |
| Boiling Point | 2950°C |
About Silicon Dioxide
Silicon dioxide makes up roughly 60 percent of Earth's crust by mass and forms the giant covalent network we call quartz, sand, and ordinary glass. The formula SiO2 is misleading because there are no discrete SiO2 molecules — each silicon sits in a tetrahedron of four oxygens, each oxygen bridges two silicons, and the empirical formula falls out of that 1:2 ratio across the infinite network. The polymorphs are rich: alpha-quartz at room temperature, beta-quartz above 573 °C, tridymite, cristobalite, the high-pressure forms coesite and stishovite (the latter found at meteorite impact craters), and the disordered amorphous forms vitreous silica and fused quartz. Quartz is piezoelectric, which is why a thin AT-cut quartz slab vibrating at 32.768 kHz keeps time in essentially every quartz wristwatch and computer real-time clock on the planet. SiO2 is the upstream raw material for the semiconductor industry: high-purity sand is reduced with carbon in an electric arc furnace to metallurgical-grade silicon, which is then converted to trichlorosilane and purified by distillation to the nine-nines silicon used for chip wafers. Chemically, SiO2 is amphoteric — slowly soluble in hot concentrated NaOH to give silicate, and uniquely attacked by HF to give SiF4 and water, which is why HF is the only common acid that etches glass.
Where you'll encounter it
If you've ever poured a column of silica gel for flash chromatography, watched the bands separate, and wondered why your amine product streaked while your ester moved cleanly — that's SiO2's surface acidity at work, with silanol groups (Si-OH, pKa ~ 7) hydrogen-bonding strongly to basic functional groups. Switching to neutral or basic alumina is the standard fix. In a semiconductor cleanroom, SiO2 grown thermally on silicon wafers (dry oxidation in O2 at 1000 °C, or wet oxidation with steam) forms the gate dielectric in MOSFETs — the layer that separates the gate electrode from the channel and makes the entire transistor switch work. The third place you encounter SiO2 daily is fused quartz cuvettes for UV-vis spectroscopy: ordinary borosilicate glass cuts off below 320 nm, but fused silica transmits down to 200 nm and lets you measure DNA absorbance at 260 nm and protein at 280 nm.
Common Uses
- Stationary phase for flash chromatography and HPLC silica columns
- Gate dielectric in MOSFET semiconductor manufacturing via thermal oxidation
- Fused-quartz cuvettes for UV-vis spectroscopy below 320 nm
- Float-glass and borosilicate glass manufacturing for windows and labware
- Piezoelectric crystal oscillators for quartz watches and 32.768 kHz timing circuits
- Anti-caking agent E551 in powdered foods, spices, and supplements at 1-2 percent
- Silica-gel desiccant packets for moisture control in pharmaceutical and electronics packaging
Safety Information
Crystalline silica respirable dust is an IARC Group 1 human carcinogen and the cause of silicosis, an irreversible progressive lung fibrosis with a long latency. OSHA PEL for respirable crystalline silica was tightened in 2016 to 50 micrograms/m3 (8-hr TWA), with an action level at 25 micrograms/m3. NIOSH REL is 50 micrograms/m3. Construction and stone-fabrication operations (engineered quartz countertops in particular) require water-fed cutting, HEPA-vacuumed grinding, and respirator programs. Amorphous silica (silica gel, fumed silica, food-grade E551) is not classified as carcinogenic but still warrants dust control. GHS for crystalline silica: STOT RE Category 1 lung (H372), Carcinogenicity Category 1A (H350i).
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.