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JEE Main 2023 · 29 Jan · Shift 1

JEE Main 2023 Chemistry question on Practical Chemistry

During the borax bead test with CuSO4\text{CuSO}_4, a blue green colour of the bead was observed in oxidising flame due to the formation of
  1. A.
    CuO\text{CuO}
  2. B.
    Cu\text{Cu}
  3. C.
    Cu(BO2)2\text{Cu}(\text{BO}_2)_2
  4. D.
    Cu3B2\text{Cu}_3\text{B}_2
Show answer and solution

Correct option: C

Solution

In the borax bead test, borax (Na₂B₄O₇·10H₂O) melts and loses water to form sodium metaborate (NaBO₂) and B₂O₃, which acts as a glassy flux. In the oxidising flame, CuSO₄ reacts with this melt to form copper(II) metaborate, Cu(BO₂)₂, which gives the characteristic blue-green colour to the bead. The oxidising conditions keep copper in its +2 state, stabilised as the metaborate complex rather than being reduced to Cu⁺ or Cu⁰.

[JEE Main 2023 · 29 Jan · Shift 1]

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