${\mathrm{CH}}_3\mathrm{CHO}\text{ and }{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{CH}}_2\mathrm{CHO}$ can be distinguished chemically by

CH₃​CHO and C₆​H₅​CH₂​CHO react with Tollen's reagent, Fehling's solution and Benedict's solution. But, CH₃​CHO reacts with NaOH and I₂​ to give yellow crystals of Iodoform, while C₆​H₅​CH₂​CHO doesn't react with it. So, iodoform test is used.

CH₃​CHO + 3I₂ ​+ 4NaOH ⟶ CHI₃​ + HCOONa + 3NaI + 3H₂​O

C₆​H₅​CH₂​CHO + I₂ ​+ 4NaOH ⟶ No reaction.