What happen when:
(a). `CO_2` gas is passed through aqueous solution of `Na_2CrO_4`.
(b). Ammonium dichromate is heated strongly.

Explanation

(a). Sodium dichromate `Na_2Cr_2O_7` is formed
`H_2O+CO_2tounderset(carbonic acid)(H_2CO_3)`
`2Na_2CrO_4+H_2CO_3toNa_2CO_3+Na_2Cr_2O+H_2O`
(b). `N_2` and `Cr_2O` are formed.
`(NH_4)_2Cr_2O_7overset(Delta)toN_2+Cr_2O_3+4H_2O`
(c). `A` and `B` and `H_2S`.
`K_2Cr_2O_7+4H_2SO_4toK_2SO_4+Cr_2(SO_4)_3+4H_2O+3[O]`
`underline(H_2S+[O]toH_2O+Sxx3` )
`underline(K_2Cr_2O_7+4H_2SO_4toK_2SO_4+Cr(SO_4)_3+7H_2O+3S)`
`K_2Cr_2O_7+H_2SO_4+2SO_2toK_2SO_4+Cr_2(SO_4)_3+H_2O`
When both `SO_2` and `H_2S` are passed simultaneously through water than colloidal sulphur is precipitated.
`2H_2S+SO_2to2H_2O+3S`