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.

Explanation

AlF3 does not dissolve in anhydrous HF because HF is a covalent compound and is strongly hydrogen bonded. NaF is an ionic compound and provides `F^-` ions which combine with `AlF_3` to form a soluble complex, `Na_3 [AlF_6]`. Therefore, `AlF_3` dissolves in presence of NaF.
`3NaF + AlF_3 to underset("Sodium hexafluoroaluminate (III)")(Na_3 [ AlF_6])`
When gaseous `BF_3` is bubbled through the resulting solution, boron because of its smaller size and higher electronegativity, enters into the complex and expels aluminium. Thus, `AlF_3`gets precipitated.
`Na_3 [AlF_6] + 3BF_3 to underset("Sod. tetrafluoroborate (III) (soluble)")(3Na[BF_4]) + underset("insoluble")(AlF_3) darr`