Aluminium trifluoride is insoluble in anhydrous $HF$ but dissolves on addition of $NaF$. Aluminium trifluoride precipitates out of the resulting solution when gaseous $BF_3$, is bubbled through. Give reasons.

Hydrogen fluoride ($HF$) is a covalent compound and has a very strong intermolecular hydrogen-bonding. Thus, it does not provide ions and aluminium fluoride ($AlF$) does not dissolve in it.

Sodium fluoride (NaF) is an ionic compound and when it is added to the mixture, AlF dissolves. This is because of the availability of free $\mathrm{F}^{-}$. The reaction involved in the process is:

$3 \mathrm{NaF}+\mathrm{AlF}_{3} \rightarrow \mathrm{Na}_{3}\left[\mathrm{AlF}_{6}\right]$

Soluble complex

When boron trifluoride $\left(\mathrm{BF}_{3}\right)$ is added to the solution, aluminium fluoride precipitates out of the solution. This happens because the tendency of boron to form complexes is much more than that of aluminium. Therefore, when $\mathrm{BF}_{3}$ is added to the solution, $B$ replaces Al from the complexes according to the following reaction.

$\mathrm{Na}_{3}\left[\mathrm{AlF}_{6}\right] 3 \mathrm{BF}_{3} \rightarrow 3 \mathrm{Na}\left[\mathrm{BF}_{6}\right]+\mathrm{AlF}_{3}$