Which of the following is an amphoteric oxide?

$Na_2B_4O_7 \cdot 10H_2O \xrightarrow{\Delta} X + NaBO_2 + H_2O$
$X + Cr_2O_3 \xrightarrow{\Delta} Y$ (Green coloured)
$X$ and $Y$ are

Which of the following reactions will yield the crystalline variety of boron?

Aluminium $(III)$ chloride forms a dimer because ........

White fumes appear around the bottle of anhydrous aluminium chloride. Give reason.

Which of the following statements about $BF_4^{-}$ and $AlF_6^{3-}$ are correct?
$(i)$ $B$ and $Al$ differ in their oxidation states.
$(ii)$ $B$ and $Al$ differ in their covalency.
$(iii)$ $B$ obeys the octet rule.
$(iv)$ $B$ and $Al$ are in diagonal relationship.