During the borax bead test with CuSO4, a blue-green colour of the… — Solution | JEE Chemistry

MCQEasyJEE 2023Salt Analysis (Cations & Anions)

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

Answer

Correct answer:C

Standard Method

Given: In the borax bead test with $\mathrm{CuSO_4}$ , a blue-green colour is observed in the oxidizing flame.

Find: The species responsible for this colour.

In the borax bead test, borax decomposes on heating to give sodium metaborate and boric anhydride:

\mathrm{Na_2B_4O_7 \xrightarrow{\Delta} 2NaBO_2 + B_2O_3}

In the oxidizing flame, copper ions react with boric anhydride to form copper metaborate:

\mathrm{CuSO_4 + B_2O_3 \rightarrow Cu(BO_2)_2 + SO_3}

The compound $\mathrm{Cu(BO_2)_2}$ is responsible for the blue-green colour of the bead in the oxidizing flame.

Therefore, the correct option is C.

Assuming that the metal forms its simple oxide $\mathrm{CuO}$ in the oxidizing flame. In the borax bead test, the characteristic colour is due to formation of a metaborate, not the simple oxide. Check the specific borax bead reaction before choosing the product.
Choosing metallic $\mathrm{Cu}$ by confusing oxidizing and reducing flames. Metallic copper is associated with reduction, whereas the oxidizing flame favors formation of the coloured metaborate species.
Focusing only on the blue-green colour and guessing without recalling the borax decomposition step. First identify that borax gives $\mathrm{B_2O_3}$ , then determine which borate compound of copper is formed.

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