Fehling's test is used to detect the presence of aliphatic aldehydes and reducing sugars. Fehling's reagent consists of two separate solutions: Fehling solution A and Fehling solution B.

1. Fehling solution A is an aqueous solution of copper sulphate ($CuSO_4$).
2. Fehling solution B is an alkaline solution of sodium potassium tartrate, also known as Rochelle salt. These two solutions are mixed in equal amounts just before conducting the test. Therefore, Fehling solution 'A' corresponds to aqueous copper sulphate.