Which of the following ions give blue colour in the borax bead test?

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Solution

The correct option is A
Cu

Borax bead test: On heating, borax first swells up due to elimination of water molecules. On, heating further, it melts to a liquid which then solidifies to a transparent glassy mass.

$N a_{2} B_{4} O_{7} .10 H_{2} O h e a t - ---- \to - 10 H_{2} O N a_{2} B_{4} O_{7} A n h y d r o u s 740^{\circ} C - -- \to 2 N a B O_{2} + B_{2} O_{3}      S o d i u m m e t a b o r a t e B o r i c a n h y d r i d e G l a s s y M a s s$

When hot glassy mass is brought in contact with a coloured salt & heated again in flame, $B_{2} O_{3}$ displaces the volatile oxides and combines with basic oxides to form metaborates. These metaborates have characteristic colours.

$C u S O_{4} + B_{2} O_{3} \to C u O . B_{2} O_{3} + S O_{3}$
$(C u (B O_{2})_{2})$

Blue

Ni - Brown
Cu - Blue
Fe - Green
Co - Blue
Cr - Green

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