How you can justify the acidic nature of BI3 ( boron triiodide) ?
The slightly polar Boron tri iodide when reacting with water forms hydroiodic acid and boric acid. These are string acids with Ph below 4. Since the compound is capable of giving one proton , that is a hydrogen ion it is justified as an acid. Also when dipped in litmus paper it turns litmus into red.
Hope this helps.