A sodium salt on treatment with ${\mathrm{MgCl}}_2$ gives white precipitate only on heating. The anion of the sodium salt is : 

With ${\mathrm{MgCl}}_2$, a sodium salt of an unidentified anion produces a white precipitate $\left({\mathrm{MgCO}}_3\right)$ that only dissolves when boiled. ${\mathrm{HCO}}^{-}3$ ion is the anion.

$\begin{array}{l}{\mathrm{MgCl}}_2+2{\mathrm{NaHCO}}_3⟶\underset{\text{ soluble }}{\mathrm{Mg}{\left({\mathrm{HCO}}_3\right)}_2+2\mathrm{NaCl}}\\ \mathrm{Mg}{\left({\mathrm{HCO}}_3\right)}_2\stackrel{\text{ Boiling }}{⟶}\underset{\text{ white ppt. }}{{\mathrm{MgCO}}_3\downarrow }+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2\end{array}$