Flame Test & Borax Bead: Preliminary Test Colours

The flame test and the borax bead test are preliminary tests in qualitative salt analysis: quick, simple checks that hint at which metal ion a salt contains before confirmatory tests are done. A tiny sample is heated in a flame, and the colour the flame or bead takes on points to specific cations.

These tests matter because they save time and chemicals. Instead of running a full analysis blindly, a chemist reads the flame or bead colour and narrows the suspect list to one or two ions. They are taught in school laboratories because they need almost no equipment: a burner, a platinum wire, and borax.

Key Concepts

• Flame test: a clean platinum wire dipped in the salt is held in a non-luminous Bunsen flame; volatile metal chlorides colour the flame.
• Standard flame colours: lithium crimson red, sodium intense golden yellow, potassium lilac (best seen through blue glass), calcium brick red, strontium crimson, barium apple green, copper bluish-green.
• Borax, Na2B4O7.10H2O: on heating it loses water and forms a clear glassy bead of sodium metaborate and boric anhydride, which dissolves metal oxides to give coloured metaborates.
• Oxidising vs reducing flame: the bead is heated first in the outer oxidising flame and then in the inner reducing flame; many metals give different colours in the two.
• Characteristic bead colours: cobalt deep blue in both flames; copper blue in oxidising and red opaque in reducing; iron yellow in oxidising and bottle green in reducing; chromium green in both; manganese violet in oxidising and colourless in reducing.

Formulas

• Bead formation: Na2B4O7.10H2O --heat--> 2NaBO2 + B2O3 + 10H2O. The bead is a glassy mixture of sodium metaborate and boric anhydride.
• Colour reaction: CuO + B2O3 --> Cu(BO2)2, copper metaborate, which is blue in the oxidising flame.
• Reduction: 2Cu(BO2)2 + C --> 2Cu + 2B2O3 + CO2, where carbon comes from the inner flame; the bead turns red and opaque from metallic copper.
• These equations assume strong heating; a weakly heated bead stays cloudy and gives poor colours.

Worked Example

A student heats 1.91 g of borax, Na2B4O7.10H2O (molar mass 381.4 g/mol), to make beads. Moles of borax = 1.91 / 381.4 = 0.00501 mol, about 0.005 mol. From the equation, 1 mol of borax gives 2 mol of NaBO2, so 0.005 mol gives 0.010 mol of sodium metaborate - far more bead material than a whole class needs.

Common Misconceptions

• The flame colour alone confirms the ion. Sodium's intense yellow masks weaker colours such as potassium's lilac, so potassium must be viewed through cobalt-blue glass, which absorbs the yellow.
• The bead colour is the same in both flames. Many metals change colour: copper is blue in the oxidising flame but red and opaque in the reducing flame, and that difference is diagnostically useful.
• The borax bead test works for every metal. It gives useful colours only for certain transition metals (Cu, Co, Fe, Cr, Mn, Ni); alkali and alkaline-earth metals give colourless beads.

Real-World Applications

• School and college qualitative analysis schemes for salt identification.
• Testing ceramic glazes and glass batches for transition-metal colourants.
• Pyrotechnics: the same flame colours make coloured fireworks (strontium for red, barium for green).
• Field geology: a quick bead test can hint at cobalt or copper minerals in an ore sample.

The flame test reads volatile chlorides by their flame colours, and the borax bead test reads metal oxides by the coloured metaborates they form in oxidising and reducing flames. Together they give a fast first answer in salt analysis, but each has blind spots, so confirmatory tests always follow.