Question

Which of the following compounds is formed in borax bead test?
A. Metaborate
B. Tetraborate
C. Triborate
D. Orthoborate

Answer 1

Hint:In the borax bead test, you carry out the thermal decomposition of borax. First you lose water of crystallization. Then you obtain a mixture of two different products.

Complete answer:
The chemical formula of borax is \[{\text{N}}{{\text{a}}_{\text{2}}}{{\text{B}}_{\text{4}}}{{\text{O}}_{\text{7}}}{\text{.10}}{{\text{H}}_{\text{2}}}{\text{O}}\]. One molecule of borax contains 10 molecules of water of crystallization.
When you heat borax, you lose water of crystallization. You will get a fluffy mass due to swelling. When you continue heating, the fluffy mass will melt to form a clear liquid. This clear liquid will then solidify to a transparent glossy bead. Thus solid, transparent glossy beads contain sodium metaborate and boric anhydride. The chemical formula of sodium metaborate is \[{\text{NaB}}{{\text{O}}_{\text{2}}}\]. The chemical formula of boric anhydride is \[{{\text{B}}_{\text{2}}}{{\text{O}}_{\text{3}}}\]
Write the chemical equation for the above process as shown below:
\[{\text{N}}{{\text{a}}_{\text{2}}}{{\text{B}}_{\text{4}}}{{\text{O}}_{\text{7}}}{\text{.10}}{{\text{H}}_{\text{2}}}{\text{O }}\xrightarrow[{ - {\text{10}}{{\text{H}}_{\text{2}}}{\text{O}}}]{\Delta }{\text{N}}{{\text{a}}_{\text{2}}}{{\text{B}}_{\text{4}}}{{\text{O}}_{\text{7}}}\xrightarrow{\Delta }{\text{2NaB}}{{\text{O}}_{\text{2}}}{\text{ + }}{{\text{B}}_{\text{2}}}{{\text{O}}_{\text{3}}}\]
Thus, metaborate is formed in borax bead test.

Hence, the correct option is the option A.

Note:
You can use the borax bead test to identify the presence of various transition metal cations in the sample. Transition metals form metaborates that have characteristic colour. From this colour, you can identify the transition metal cation present in the sample. You can analyse copper, manganese, nickel and iron through borax bead test. For example, copper forms blue green coloured cupric metaborate \[{\text{Cu}}{\left( {{\text{B}}{{\text{O}}_{\text{2}}}} \right)_2}\].