JEE Main · 2022 · 29 Jul 2022 · Shift 2 · p-Block Elements

When borax is heated with CoO on a platinum loop, blue coloured bead formed is largely due to :

(A) $B_{2} O_{3}$
(B) $Co (BO_{2})_{2}$ ✓ Correct
(C) $CoB_{4} O_{7}$
(D) $Co [B_{4} O_{5} (OH)_{4}]$

Answer: (B)

Solution

In the borax bead test, borax (

Na_{2} B_{4} O_{7}

) is heated to form a glassy bead of

B_{2} O_{3}

. When CoO is added and heated, it reacts with

B_{2} O_{3}

to form cobalt(II) metaborate:

B_{2} O_{3} + CoO \to Co (BO_{2})_{2}

This compound gives a characteristic blue color.

Answer: (B)

Co (BO_{2})_{2}

JEE Main · 2022 · 29 Jul 2022 · Shift 2 · Single correct option

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