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 and exhibits strong intermolecular hydrogen bonding. Consequently,it does not provide free fluoride ions,and $AlF_3$ remains insoluble in it.
Sodium fluoride $(NaF)$ is an ionic compound. Upon adding $NaF$ to the mixture,$AlF_3$ dissolves due to the availability of free $F^-$ ions,forming a soluble complex:
$3 NaF + AlF_3 \rightarrow Na_3[AlF_6]$
When gaseous boron trifluoride $(BF_3)$ is bubbled through the solution,$AlF_3$ precipitates out. This occurs because boron has a higher tendency to form complexes with fluoride ions compared to aluminium. Thus,$BF_3$ displaces $Al$ from the complex:
$Na_3[AlF_6] + 3 BF_3 \rightarrow 3 Na[BF_4] + AlF_3$