Alcohols containing CH3CH(OH)− group and carbonyl compounds containing CH3CO− group give yellow precipitate with iodine and mathrmNaOH solution. This reaction is called iodoform test.Thus, CH3CHO due to presence of CH3CO group gives yellow ppt with I2 and NaOH. CH3CHO+3I2+4NaOH→CHI3→+HCOONa+3NaI+2H2O