Identify the correct statements from the following:
$(a)$ In orthoboric acid,boron is in planar geometry.
$(b)$ In $BCl_3 \cdot NH_3$,boron has tetrahedral geometry.
$(c)$ Aqueous solution of borax is acidic.

Two gases when mixed give white dense fumes,the gases are

Conc. $HNO_3$ can be stored in a container of

Which of the following boron compounds is the hardest?

Which of the following processes will not yield anhydrous $AlCl_3$?

In the following sets of reactants,which two sets best exhibit the amphoteric character of $Al_2O_3 \cdot xH_2O$?
Set $1$: $Al_2O_3 \cdot xH_2O_{(s)}$ and $OH^{-}_{(aq)}$
Set $2$: $Al_2O_3 \cdot xH_2O_{(s)}$ and $H_2O_{(l)}$
Set $3$: $Al_2O_3 \cdot xH_2O_{(s)}$ and $H^{+}_{(aq)}$
Set $4$: $Al_2O_3 \cdot xH_2O_{(s)}$ and $NH_{3(aq)}$