which of the following is not hydrolysed by water and why?
BF_​3,BCl_{3 and} BBr₃
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BBr₃ does not undergo hydrolysis as it does not form hydrogen bond with water. As the electronegativity of Br is less, it does not form hydrogen bond
BCl₃ and BF₃ undergoes hydrolysis to form the product boric acid, as they form hydrogen bond with water. 
BX₃ + 3H₂O ---> H₃BO₃ + 3HX
Where X is Cl and F.