Question

The compound formed in the borax bead test of $C{u}^{2+}$ ion in oxidising flame is

• A. $Cu$
• B. $CuBO$
• C. $Cu(B{O}_2{)}_2$
• D. None of these

Answer 1

The correct option is C $Cu(B{O}_2{)}_2$

Borax bead test is used to identify the metal cation such as $N{i}^{2+},C{o}^{2+},C{r}^{3+},C{u}^{2+},M{n}^{2+}$ etc.
Borax reacts with certain metal salts such as$N{i}^{2+},C{o}^{2+},C{r}^{3+},C{u}^{2+},M{n}^{2+}$ to form colored metaborates.

On heating, borax loses water of crystallisation and swells up to form fluffy mass. On further heating, it melts to give a clear liquid which solidifies to a transparent glassy bead consisting of sodium metaborate and boric anhydride.
$N{a}_2{B}_4{O}_7\cdot 10H_{2}O\stackrel{△}{\to }N{a}_2{B}_4{O}_7+10H_{2}O\stackrel{{740}^{o}C}{⟶}\underset{\text{Sodium Metaborate}}{2NaB{O}_2}+B_2{O}_3$
$CuO+B_2{O}_3\to Cu(B{O}_2{)}_2\text{blue bead}.$
Hence, $Cu(B{O}_2{)}_2$ is formed in oxidising flame in borax bead test.