Question

Which of the following salt is used in borax bead test?

• A. $K_{2}S{O}_{4}A{l}_2(S{O}_4{)}_3.24H_{2}O$
• B. $N{a}_2{B}_4{O}_7.10H_{2}O$
• C. $CaS{O}_4.2H_{2}O$
• D. None of these

Answer 1

The correct option is B $N{a}_2{B}_4{O}_7.10H_{2}O$

The salt which gives borax bead best is the Borax $N{a}_2{B}_4{O}_7.10H_{2}O$

Borax-bead Test:
Borax reacts with certain metal salts such as $N{i}^2+,C{o}^{2+},C{r}^{3+},C{u}^{2+},M{n}^{2+}$ basically transition metal cations, etc., to form colored metaborates. $B{O}_2^{-}$ is known as metaborate. $B_2{O}_3$ is glassy transparent bead.
$Cr(B{O}_2{)}_2$ is green in colour while $Cu(B{O}_2{)}_2$ is blue in colour.
The colour of the metaborates can be used to identify the metallic ions (cations) in its salts.
$N{a}_2{B}_4{O}_7.10H_{2}O\stackrel{△}{\underset{-10H_{2}O}{\to }}N{a}_2{B}_4{O}_7\stackrel{{740}^{\circ }C}{\to }\underset{\text{Glassy mass}}{2NaB{O}_2+B_2{O}_3}$
$CuO+B_2{O}_3\to \underset{\text{Blue bead}}{Cu(B{O}_2{)}_2}$