Correct statements from the following are
A. Potassium dichromate is an oxidising agent and it oxidises $FeSO_4$ to $Fe_2(SO_4)_3$ in acidic medium.
B. Sodium dichromate can be used as primary standard in volumetric estimation.
C. $CrO_4^{2-}$ and $Cr_2O_7^{2-}$ are interconvertible in aqueous solution by varying the $pH$ of the solution.
D. $Cr$-$O$-$Cr$ bond angle in $Cr_2O_7^{2-}$ is $126^\circ$.

Choose the correct answer from the options given below:

Statement A is correct. Potassium dichromate acts as a strong oxidising agent in acidic medium and oxidises $Fe^{2+}$ to $Fe^{3+}$, converting $FeSO_4$ to $Fe_2(SO_4)_3$.

Statement B is incorrect. Sodium dichromate is deliquescent (absorbs moisture from the atmosphere), which prevents it from being weighed accurately. Therefore, it cannot be used as a primary standard in volumetric estimation. Potassium dichromate is used instead.

Statement C is correct. Chromate ($CrO_4^{2-}$) and dichromate ($Cr_2O_7^{2-}$) ions are interconvertible in aqueous solution depending on the $pH$ of the solution. In acidic medium, chromate changes to dichromate, and in basic medium, dichromate changes to chromate: $2CrO_4^{2-} + 2H^+ \rightleftharpoons Cr_2O_7^{2-} + H_2O$.

Statement D is correct. The structure of the dichromate ion consists of two tetrahedral $CrO_3$ groups joined by a bridging oxygen atom, and the $Cr-O-Cr$ bond angle is $126^\circ$.

Therefore, statements A, C, and D are correct.

Answer: A, C and D Only