Q.A compound is prepared from the mineral colemanite by boiling it with a solution of sodium carbonate. It is white crystalline solid and used for inorganic qualitative analysis.
Write the reaction that explains its formation.

Fusing borax with a metal salt on a wire loop produces a coloured glassy bead whose colour (in oxidising vs reducing flame) is characteristic of the metal ion present — a classic qualitative test for coloured/transition-metal salts.

Principle: When borax (Na₂B₄O₇·10H₂O) is strongly heated, it loses water and decomposes to a mixture of sodium metaborate and boric anhydride:

Na₂B₄O₇ → 2NaBO₂ + B₂O₃

B₂O₃ is an acidic oxide that combines with coloured transition-metal oxides to form characteristically coloured metaborates (glasses), e.g. B₂O₃ + CoO → Co(BO₂)₂ (blue).

Procedure:

1. A platinum (or nichrome) wire with a small loop at the end is heated in a Bunsen flame until red hot, then dipped into powdered borax.
2. The adhering borax is heated in the flame; it swells, froths, and finally fuses to a clear, colourless, transparent glassy bead (of NaBO₂ and B₂O₃).
3. The hot bead is touched to a trace of the powdered metal salt/oxide to be tested, so a little of it sticks to the bead.
4. This bead is then reheated first in the oxidising flame (outer, hotter zone) and its colour noted after cooling; then reheated in the reducing flame (inner, luminous zone) and its colour again noted after cooling.

Result: The metal oxide dissolves in the B₂O₃–NaBO₂ melt to form a metaborate whose colour depends on the metal and its oxidation state, which can differ between the oxidising and reducing flames (since the flame type can change the metal's oxidation state). For example: …

The borax bead test identifies coloured metal ions by the characteristic colour their metaborate forms when fused with borax glass in a flame.

Borax is sodium tetraborate decahydrate, Na2B4O7.10H2O. When a small amount of borax is taken on a heated platinum/nichrome wire loop and strongly heated in a Bunsen flame, it first loses its water of crystallisation and swells up, then fuses into a clear, colourless, transparent glassy bead consisting of sodium metaborate and boric anhydride:

Na2B4O7.10H2O --(heat)--> Na2B4O7 --(heat)--> 2NaBO2 + B2O3 (colourless transparent bead)

This hot bead is then touched to a trace amount of the coloured metal salt to be tested, so a little of the salt sticks to it, and the bead is reheated in the Bunsen flame — first in the oxidising flame (outer, blue) and then in the reducing flame (inner, luminous).

In the oxidising flame, the metal salt is converted to its oxide, which combines with the B2O3 of the bead to form a metaborate of the metal, giving the bead a characteristic colour depending on the metal ion present, e.g.:

CuO + B2O3 -> Cu(BO2)2

…

The borax bead test identifies coloured metal ions by the characteristic colour their metaborate gives when fused into a hot borax-glass bead in the Bunsen flame.

Formation of the bead

A small amount of borax (sodium tetraborate, Na2B4O7.10H2O) is taken on a loop at the end of a platinum or nichrome wire and heated strongly in a Bunsen flame. It first loses its water of crystallisation, swells up, and then fuses to a clear, colourless, glassy bead consisting of sodium metaborate and boric anhydride:

Na2B4O7 -> 2NaBO2 + B2O3

Performing the test

The hot bead is touched to a trace of the powdered metal salt to be tested, and the loaded bead is then heated again — first in the oxidising (outer) flame, then in the reducing (inner) flame. The metal oxide formed reacts with B2O3 in the bead to give a metal metaborate (or borate), which is often a distinctive colour, allowing identification of the metal present.

Example — cobalt salt

With a cobalt(II) salt (e.g. CoSO4 or CoCl2), the bead turns a deep blue colour in both the oxidising and the reducing flame, because cobalt metaborate, Co(BO2)2, is blue regardless of flame type.

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