Ethanol is oxidised with PCC in $\mathrm{CH}{}_2\mathrm{Cl}{}_2$ to give A. A is warmed with $\mathrm{I}{}_2/\mathrm{NaOH}$ to give a yellow precipitate B. B is

PCC oxidises ethanol to ethanal (A), $\mathrm{CH}{}_3\mathrm{CHO}$. Ethanal contains the $\mathrm{CH}{}_3\mathrm{CO}{}^{-}$ group and gives the haloform reaction with $\mathrm{I}{}_2/\mathrm{NaOH}$, forming yellow iodoform, $\mathrm{CHI}{}_3$, and sodium methanoate.