Question
Aluminium trifluoride is insoluble in anhydrous HF but dissolves on the addition of NaF. Aluminium trifluoride precipitates out of the resulting solution when gaseous BF3 is bubbled through. Give reasons.
Solution
(i) Anhydrous HF, being a covalent compound and is strongly H-bonded, therefore it does not give ions. Hence AlF3 does not dissolve in HF. On the other hand, NaF is an ionic compound and gives F– ions and hence AlF3 dissolves in NaF forming soluble complex
(ii) Borax has much higher tendency to form complexes than aluminium because of its smaller size and higher electronegativity. Hence when gaseous BF3 is bubbled through the resulting solution. AlF3 gets precipitated. 
