When borax is heated with CoO on a platinum loop, blue coloured bead formed is largely due to :

Solution

Chemistry ma’am · 4 steps

Heat strips borax down to B₂O₃, and that acidic oxide grabs CoO to make blue cobalt metaborate.

1. Borax loses water. On heating, borax first loses all its water of crystallisation. It swells up and then melts into anhydrous Na₂B₄O₇.
2. Glassy bead forms. The anhydrous salt breaks down into sodium metaborate and boric anhydride. Together they form a clear, colourless glassy bead.

   Na₂B₄O₇ →(Δ) 2NaBO₂ + B₂O₃

3. Metal oxide reacts. B₂O₃ is an acidic oxide, so it combines with the basic oxide CoO to give the metaborate: CoO + B₂O₃ → Co(BO₂)₂. This cobalt metaborate is what gives the deep blue colour.
4. So. The blue bead is largely due to Co(BO₂)₂, option (b).

The idea to keep. In any borax bead test the coloured species is the metal metaborate, M(BO₂)₂, made from B₂O₃ and the metal oxide. Blue in both flames means cobalt.

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