二氧化硅Silicon dioxide

SiO₂ 中不存在单个分子，化学式只表示硅氧原子数之比为 1 : 2。熔化时要破坏大量强的 Si–O 共价键，所以熔点约 1710 °C。由于没有可自由移动的电子或离子，固态和熔融态都不导电。这与 CO₂ 形成鲜明对比：CO₂ 由分子构成，常温为气体。

SiO₂ contains no discrete molecules; the formula only gives the 1 : 2 ratio of silicon to oxygen. Melting breaks many strong Si–O covalent bonds, so the melting point is about 1710 °C. With no mobile electrons or ions it does not conduct electricity, solid or molten. This contrasts sharply with CO₂, which is molecular and a gas at room temperature.

SiO₂ 是酸性氧化物，不溶于水也不与水反应，但能与热的浓碱溶液或熔融碱反应生成硅酸盐：SiO₂ + 2NaOH → Na₂SiO₃ + H₂O，因此盛放碱液的试剂瓶不用玻璃塞。它在高温下与碱性氧化物反应，高炉中用来除去杂质：CaO + SiO₂ → CaSiO₃（炉渣）。SiO₂ 与氢氟酸反应：SiO₂ + 4HF → SiF₄ + 2H₂O，可用于刻蚀玻璃。

SiO₂ is an acidic oxide. It neither dissolves in nor reacts with water, but reacts with hot concentrated or molten alkali to form silicates: SiO₂ + 2NaOH → Na₂SiO₃ + H₂O, which is why bottles of alkali do not have glass stoppers. At high temperature it reacts with basic oxides, removing impurities in the blast furnace: CaO + SiO₂ → CaSiO₃ (slag). It reacts with hydrofluoric acid, SiO₂ + 4HF → SiF₄ + 2H₂O, which is used to etch glass.

石英砂是制造玻璃、水泥和陶瓷的主要原料。高纯石英用于制造光导纤维和石英玻璃器皿。石英晶体有压电性，用于钟表和电子设备的振荡器。多孔的硅胶用作干燥剂和色谱固定相。

Quartz sand is the main raw material for glass, cement and ceramics. High-purity silica is drawn into optical fibres and made into fused-silica glassware. Quartz crystals are piezoelectric and serve as oscillators in clocks and electronics. Porous silica gel is used as a desiccant and as a chromatographic stationary phase.