Question

During the borax bead test with CuSO₄, a blue-green colour of the bead was observed in oxidizing flame due to the formation of

• A. CuO
• B. \(Cu(BO_2)_2\)
• C. \(Cu_3B_2\)
• D. Cu

Hint:

For borax bead test:
• In the oxidizing flame, metal ions form metaborates, which produce characteristic colours.
• The observed colour is a key indicator of the specific metal ion present.

Answer 1

The Correct Option is B

Borax Bead Test:
1. Definition: The borax bead test is a qualitative test used to identify metal ions based on the colour imparted to the bead in the oxidizing or reducing flame.
In this test, borax (\(\text{Na}_2\text{B}_4\text{O}_7 \cdot 10\text{H}_2\text{O}\)) decomposes to form sodium metaborate (\(\text{NaBO}_2\)) and boric anhydride (\(\text{B}_2\text{O}_3\)) when heated:
\[\text{Na}_2\text{B}_4\text{O}_7 \xrightarrow{\Delta} 2\text{NaBO}_2 + \text{B}_2\text{O}_3.\]
2. Reaction with \(\text{CuSO}_4\): Copper ions react with boric anhydride (\(\text{B}_2\text{O}_3\)) to form copper metaborate (\(\text{Cu(BO}_2)_2\)) in the oxidizing flame:
\[\text{CuSO}_4 + \text{B}_2\text{O}_3 \rightarrow \text{Cu(BO}_2)_2 + \text{SO}_3.\]
3.Observation: \(\text{Cu(BO}_2)_2\) is responsible for the blue-green colour observed in the oxidizing flame.
Final Answer: \((3)\) \(\mathbf{Cu(BO}_2)_2\).