Which of the following oxides is strongly basic?

When orthoboric acid $(H_3BO_3)$ is subjected to strong heating, the residue left is

Anhydrous $AlCl_3$ is obtained from

$A$ certain salt $X$ gives the following results:
$(i)$ Its aqueous solution is alkaline to litmus.
$(ii)$ It swells up to a glassy material $Y$ on strong heating.
$(iii)$ When conc. $H_{2}SO_{4}$ is added to a hot solution of $X,$ white crystals of an acid $Z$ separate out.
Write equations for all the above reactions and identify $X, Y,$ and $Z$.

Which of the following statements is not true about borax?

Boron is unable to form $BF_{6}^{3-}$ ion. Explain.