A salt with few drops of conc. HCl gives apple green colour in...

An apple green colour in the flame test indicates the presence of $Ba^{2+}$ ion.

The group precipitate of $Ba^{2+}$ is $BaCO_3$, which dissolves in acetic acid to form barium acetate. Upon treatment with $K_2CrO_4$, it gives a yellow precipitate of barium chromate ($BaCrO_4$), confirming the presence of $Ba^{2+}$.

When the sodium carbonate extract is heated with conc. $HNO_3$ and ammonium molybdate, the formation of a canary yellow precipitate indicates the presence of phosphate ($PO_4^{3-}$) ion. The canary yellow precipitate is ammonium phosphomolybdate, $(NH_4)_3PO_4 \cdot 12MoO_3$.

Therefore, the cation is $Ba^{2+}$ and the anion is $PO_4^{3-}$.

Answer: $Ba^{2+}$ and $PO_4^{3-}$