Question

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

Answer 1

`underbrace(SO_2(g))_"sulfur dioxide" + H_2O(l) rarr underbrace(H_2SO_3(aq))_"sulfurous acid"`

Explanation:

Sulfur dioxide is the acid anhydride of sulfurous acid, i.e. `H_2SO_3-=SO_2+H_2O`. While sulfurous acid can be deprotonated, often we can treat it as water saturated with sulfur dioxide, i.e. as `H_2O*SO_2` (which is of course formally `H_2SO_3`), The smell is sulfurous, but many people (including me!) find it pleasant, and antiseptic.