Silicon and its compounds

Crystalline silicon is a grey solid with a metallic lustre, melting at 1420 °C. On heating it combines with oxygen, halogens and carbon: Si + O₂ → SiO₂, Si + 2Cl₂ → SiCl₄, Si + C → SiC (carborundum); with metals it forms silicides: Si + 2Mg → Mg₂Si, and acids then give silane: Mg₂Si + 4HCl → 2MgCl₂ + SiH₄↑ (which ignites spontaneously in air). It does not react with acids but dissolves in alkali: Si + 2NaOH + H₂O → Na₂SiO₃ + 2H₂↑. SiO₂ is an acidic oxide with an atomic crystal lattice — hard, hard to melt and insoluble in water; CO₂ has a molecular lattice, which is why it is a gas while SiO₂ is a solid. Fused with alkalis or oxides it gives silicates: SiO₂ + 2NaOH → Na₂SiO₃ + H₂O, SiO₂ + CaO → CaSiO₃, and among acids it reacts only with hydrofluoric acid: SiO₂ + 4HF → SiF₄↑ + 2H₂O. H₂SiO₃ is an insoluble, very weak acid, weaker even than carbonic acid: Na₂SiO₃ + CO₂ + H₂O → Na₂CO₃ + H₂SiO₃↓; on heating H₂SiO₃ → SiO₂ + H₂O. Only Na₂SiO₃ and K₂SiO₃ are soluble, and their solution is called liquid glass.