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.

(N/A) Hydrogen fluoride $(HF)$ is a covalent compound with strong intermolecular hydrogen bonding,so it does not provide ions for $AlF_3$ to dissolve.
When $NaF$ is added,it provides free $F^-$ ions,which react with $AlF_3$ to form a soluble complex,sodium hexafluoroaluminate $(III)$:
$AlF_3 + 3NaF \to Na_3[AlF_6]$
When gaseous $BF_3$ is bubbled through,it reacts with the complex because boron has a higher tendency to form complexes than aluminium. $BF_3$ displaces $Al$ from the complex,causing $AlF_3$ to precipitate:
$Na_3[AlF_6] + 3BF_3 \to 3Na[BF_4] + AlF_3$