`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

Anhydrous `HF` is a covalent compound and is strongly `H-bonded`. Therefore, it does not give `F^(ө)` ions and `AlF_(3)` does not dissolve in `HF`.
However, `NaF` being a ionic compound, gives `F^(ө)` ions and hence combines with `AlF_(3)` to form soluble compound, `Na_(3)[AlF_(6)]`.
`3NaF+AlF_(3)to underset("Soluble complex")underset("sodium hexafluoride aluminate(III)")(Na_(3)[AlF_(6)])`
When `BF_(3)` is bubbled through the resulting solution, `AlF_(3)` precipitates out. This is because `B` has higher tendency to form complex as compared to `Al` due to its smaller size and high (charge/radius) ratio.
`Na_(3)[AlF_(6)]+3BF_(3)to underset("Soluble complex")underset("sodium tetrafluoroborate (III)") (3Na[BF_(4)]+AlF_(3(S)))`