$A l F_{3}$ in insoluble unhydrous(HF) but dissolves on addition of $N a F$ , $A l F_{3}$ precipitates out when gaseous $B F_{3}$ is bubbled through it. Give reason.

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Solution

Hydrogen fluoride is a is a very a covalent compound and have very strong intermolecular hydrogen bonding. Thus it does not provide ions and aluminium fluoride does not dissolve in it. sodium fluoride is an ionic compound and when it is added to a mixture, aluminium fluoride dissolves due to the availability of $F^{-}$ ions. this reaction involved in the process
$A l F_{3} + 3 N a F \to N a_{3} [A l F_{6}]$
When $B F_{3}$ is added to the solution $A l F_{3}$ precipitates out of he solution. This is due to more tendency of boron to form complexes than aluminium,therefore when $B F_{3}$ is added to the solution, $B$ replaces $A l$ from the complexes and follow reaction
$N a_{3} [A l F_{6}] + 3 B F_{3}$ $\to 3 N a [B F_{4}] + A l F_{3}$

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