`AlF_(3)` is insoluble in anhydrous `HF` but dissolves on addition of `NaF. AlF_(3)` precipitates out of the resulting solution when gaseous `BF_(3)` is bubbled through. Give reasons.

Explanation

(i) Anhydrous `HF` is a covalent compound and is strongly `H`-bonded. Therefore , it does not give `F^(-)` ions and hence `AlF_(3)` does not dissolve in `HF`. In contrast, `NaF` being an ionic compound contains `F^(-)` ions and hence combines with `AlF_(3)` to form the soluble complex.
`3NaF + AlF_(3) rarr underset((" soluble complex "))underset(" Sod. hetrafluoroborate (III) ")(3Na [BF_(4)]) + AlF_(3) (s)`
(ii) Because of smaller size and higher electronegativity, has high much higher tendency to form complexes than Al, therefore when `BF_(3)` is added to the above solution. `AlF_(3)` gets precipitated.