JEE Advanced · 2019 · 27 May 2019 · Paper 1 · Practical and Analytical Chemistry

The green colour produced in the borax bead test of a chromium(III) salt is due to -

(A) $Cr (BO_{2})_{3}$ ✓ Correct
(B)CrB
(C) $Cr_{2} (B_{4} O_{7})_{3}$
(D) $Cr_{2} O_{3}$

Answer: (A)

Solution

In the borax bead test, chromium(III) salt decomposes to

Cr_{2} O_{3}

. Borax on heating gives

B_{2} O_{3}

. The chromium oxide reacts with boric oxide:

2 Cr_{2} O_{3} + 6 B_{2} O_{3} \to 4 Cr (BO_{2})_{3}

The green-colored bead is due to chromium metaborate

Cr (BO_{2})_{3}

, which is option (A).

JEE Advanced · 2019 · 27 May 2019 · Paper 1 · Single correct option

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