# What is sulfurous acid, and how does it relate to SO_2?

${\underbrace{S {O}_{2} \left(g\right)}}_{\text{sulfur dioxide" + H_2O(l) rarr underbrace(H_2SO_3(aq))_"sulfurous acid}}$
Sulfur dioxide is the acid anhydride of sulfurous acid, i.e. ${H}_{2} S {O}_{3} \equiv S {O}_{2} + {H}_{2} O$. While sulfurous acid can be deprotonated, often we can treat it as water saturated with sulfur dioxide, i.e. as ${H}_{2} O \cdot S {O}_{2}$ (which is of course formally ${H}_{2} S {O}_{3}$), The smell is sulfurous, but many people (including me!) find it pleasant, and antiseptic.