Give two tests for aldehydes.

Explanation

(i) Tollens Reagent Test : Tollens reagent is an ammonical silver solution. When an aldehyde is warmed with Tollens reagent a bright silver mirror is produced due to the formation of silver metal. This reaction is also called silver mirror test for aldehydes.
`CH_(3)CHO+2[Ag(NH_(3))_(2)]^(+)+3OH^(-)toCH_(3)COO^(-)+underset("Silver mirror")(4NH_(3)+2Ag+2H_(2)O)`
(ii) Fehlings solution Test : (a) Fehlings solution is prepared by mixing equal volumes of Fehlings solution 'A' containing aqueous copper sulphate and Fehlings solution 'B' containing alkaline solution of sodium potassium tartarate (Rochelle salt)
(b) When aldehyde is warmed with Fehlings sodium deep blue colour solution is changed to red precipitate of cuprous oxide.
(iii) Schiffs' reagent Test : Dilute solution of aldehydes when added to schiff's reagent (Rosaniline hydrochloride dissolved in water and its red colour decolourised by passing `SO_(2)`) yields its red colour. This is known as Schiff's test for aldehydes. Ketones do not give this test. Acetone however gives a positive test but slowly.