The hardest substance amongst the following is

The chemical name of borax is

Dilute aqueous solution of $.......$ acid is used as an antibiotic.

In the structure of borax,the numbers of boron atoms and $B-O-B$ units,respectively,are

What are the correct statements about the elements of group $13$ given below?
$(I)$ The stability of $+1$ oxidation state follows the order $Tl > In > Ga$
$(II)$ Boron has the lowest melting point and boiling point as it is a non-metal
$(III)$ Boron shows a maximum covalency of $4$ in its compounds
$(IV)$ The order of atomic radius is $Ga > Al > In$
$(V)$ Aluminium is passive to concentrated nitric acid

Give the physical and chemical properties of orthoboric acid $(H_3BO_3)$.