When hydrogen sulphide (H2S) gas is passed into solution of K2Cr2O7, H2S is oxidised to a pale yellow solid (precipitate) of sulphur. Orange coloured solution becomes green due to formation of chromic sulphate (green coloured).
In the reaction, H2S is oxidised to S and K2Cr2O7 is reduced to Cr2(SO4)3.
