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

Solution

Chemistry ma’am · 4 steps

The bead's colour comes from a metaborate, so ask what CuO becomes once it meets B₂O₃.

1. Make the bead. On heating, borax loses its water and then breaks down into sodium metaborate and boric anhydride. The B₂O₃ part is the glassy acidic oxide that reacts with the metal.

   Na₂B₄O₇ →(Δ) 2NaBO₂ + B₂O₃

2. Copper salt reacts. In the flame CuSO₄ first gives CuO. The basic CuO then combines with acidic B₂O₃ to give copper(II) metaborate: CuO + B₂O₃ → Cu(BO₂)₂.
3. Oxidising flame. The oxidising flame keeps copper in the +2 state. The Cu²⁺ ion inside the glassy bead gives the blue green colour.
4. So. The blue green bead is due to Cu(BO₂)₂, option (b).

The idea to keep. In the borax bead test, the colour in the oxidising flame always comes from the metal metaborate, M(BO₂)₂. The reducing flame lowers the metal's oxidation state and changes the colour.

Answer from the NTA final answer key. Solution by PrepWiser.