Question

Pick the correct statement
(i) Borax is white crystalline solid containing
$\left[ B _{4} O _{5}( OH )_{4}\right]^{2-}$ units.
(ii) Aqueous solution of borax is acidic in nature.
(iii) Cobalt gives blue colour in borax bead test.

• A. (i) and (ii)
• B. (i) and (iii)
• C. (ii) only
• D. (ii) and (iii)

Answer 1

Answer: (B)

1. Structural formula of borax $\left( Na _{2} B _{4} O _{7} \cdot 10 H _{2} O \right)$ is
$Na _{2}\left[ B _{4} O _{5}( OH )_{4}\right] \cdot 8 H _{2} O \Rightarrow $ correct statement.
II. Aqueous solution of borax is basic in nature, because borax in water dissociates into $H _{3} BO _{3}$ (weak acid) and $NaOH$ (strong base).
$Na _{2} B _{4} O _{7}+7 H _{2} O \rightarrow 4 H _{3} BO _{3}+2 NaOH \Rightarrow $ Incorrect statement.
III. Cobalt (Co) gives blue colour of cobalt metaborate $\left[ Co \left( BO _{2}\right)_{2}\right]$ in borax bead $\left( NaBO _{2}+ B _{2} O _{3}\right)$ test $\Rightarrow $ Correct statement.
So, the correct statements are $(I)$ and $(|I|)$.