二氧化硫Sulfur dioxide

SO₂ 为 V 形分子，O–S–O 键角约 119°，S–O 键长 143 pm，中心硫原子有一对孤对电子。分子有极性（偶极矩 1.63 D），易溶于水。

SO₂ is bent, with an O–S–O angle of about 119° and S–O bonds of 143 pm; the sulfur carries one lone pair. The molecule is polar (1.63 D) and dissolves readily in water.

溶于水生成亚硫酸：SO₂ + H₂O ⇌ H₂SO₃；与碱反应生成亚硫酸盐。作还原剂可使酸性高锰酸钾和溴水褪色。在 V₂O₅ 催化下被氧化为 SO₃：2SO₂ + O₂ ⇌ 2SO₃，这是接触法制硫酸的关键步骤。与 H₂S 反应时作氧化剂：SO₂ + 2H₂S → 3S + 2H₂O。SO₂ 能与某些有色物质化合使其褪色，但这种漂白可逆，加热后恢复颜色。

In water it forms sulfurous acid, SO₂ + H₂O ⇌ H₂SO₃, and with alkalis sulfites. As a reducing agent it decolourises acidified permanganate and bromine water. Over V₂O₅ it is oxidised to SO₃, 2SO₂ + O₂ ⇌ 2SO₃, the key step of the Contact process. With H₂S it acts as an oxidising agent: SO₂ + 2H₂S → 3S + 2H₂O. It bleaches some dyes by combining with them, but reversibly; heating restores the colour.

化石燃料（尤其是煤）中的硫燃烧和金属硫化物矿的冶炼会释放 SO₂，在大气中被氧化、溶于雨水形成酸雨，因此电厂需进行烟气脱硫。少量 SO₂ 和亚硫酸盐用作食品防腐剂和抗氧化剂（E220）。SO₂ 刺激呼吸道，哮喘患者尤其敏感。

Burning sulfur in fossil fuels, especially coal, and smelting sulfide ores release SO₂, which is oxidised in the air and dissolves in rain to form acid rain; power stations therefore use flue-gas desulfurisation. Small amounts of SO₂ and sulfites preserve food and act as antioxidants (E220). SO₂ irritates the airways, and people with asthma are especially sensitive.