Formic acid behaves as reducing agent as it is oxidised to an unstable acid, carbonic acid, which decomposes into CO₂ and H₂O. Acetic acid remains unaffected by mercuric chloride, as it does not show reducing properties.

In acetic acid, methyl group is present which exerts +I effect and thus increases negative charge on the carboxylate ion and destabilise it. The loss of proton becomes comparatively difficult in comparison to formic acid. Hence, acetic acid is weaker acid than formic acid.