Give simple chemical tests to distinhuish between the following pairs of compounds
Formic acid and acetic acid

Explanation

Formic acid (HCOOH) and Acetic acid `(CH_(3)COOH)`. Out of these two acids formic acid can easily be oxidised with weak oxidising agents (Tollens reagent or mercuric chloride) while it is not possible to oxidise acetic acid with these weak oxidising agents.
(a) Out of these two acids only formic acid gives silver mirror test with Tollen.s reagent.
`underset("Formic acid")(HCOOH) + underset("Tollen.s reagent")(2[Ag(NH_(3))_(2)]OH) rarr underset("Silver mirror")(2Ag) + 2H_(2)O + CO_(2) + 4NH_(3)`
(b) Out of these two acids only formic acid reduces mercuric chloride to mercuric chloride. thus forming a white ppt.
`underset("Formic acid")(HCOOH) + 2HgCl_(2) rarr underset("White ppt.")(HgCl_(2)) + CO_(2) + 2HCl`