Borax bead, charcoal cavity tests

1. The Borax Bead Test

WHY does a clear bead form? (derive the chemistry)

Step 1 — lose water. Heating drives off the 10 water molecules: $\text{Na}_2\text{B}_4\text{O}_7 \cdot 10H_2O \xrightarrow{\Delta} \text{Na}_2\text{B}_4\text{O}_7 + 10H_2O\uparrow$ Why this step? The salt first puffs up ("intumescence") as steam escapes — that's why a fresh bead bubbles.

Step 2 — decompose into the two active species. Further heating splits borax into sodium metaborate + boric anhydride: $\text{Na}_2\text{B}_4\text{O}_7 \xrightarrow{\Delta} \underbrace{2\,\text{NaBO}_2}_{\text{metaborate}} + \underbrace{\text{B}_2\text{O}_3}_{\text{boric anhydride (the glass)}}$ Why this step? $\text{B}_2\text{O}_3$ is the transparent glassy solvent. It is the reactive part that grabs metal ions to make coloured glasses.

WHY does it become coloured?

$\text{B}_2\text{O}_3$ reacts with a metal oxide to form a coloured metaborate of that metal: $\text{B}_2\text{O}_3 + \text{CuO} \longrightarrow \text{Cu(BO}_2)_2 \quad (\text{copper metaborate})$

Figure — Borax bead, charcoal cavity tests

The colour table (the 20% you MUST know)

Metal ion	Oxidising flame (OF)	Reducing flame (RF)
$Cu^{2+}$	Blue / green	Red / colourless
$Fe^{3+}$	Yellow-brown	Green / colourless
$Cr^{3+}$	Green	Green
$Mn^{2+}$	Violet / amethyst	Colourless
$Co^{2+}$	Blue	Blue
$Ni^{2+}$	Brown	Grey (Ni metal)

2. The Charcoal Cavity Test

WHY charcoal? (the chemistry of reduction)

Hot charcoal is a reducing agent: it supplies carbon and CO that pull oxygen out of the metal oxide. $\text{MO} + \text{C} \xrightarrow{\Delta} \text{M} + \text{CO}\uparrow \qquad \text{MO} + \text{CO} \xrightarrow{\Delta} \text{M} + \text{CO}_2\uparrow$

Why add $\text{Na}_2\text{CO}_3$ ? It is a flux + reducing aid: it converts the salt to the metal carbonate/oxide (which decomposes to oxide) so charcoal can reduce it cleanly: $\text{PbSO}_4 + \text{Na}_2\text{CO}_3 \to \text{PbCO}_3 + \text{Na}_2\text{SO}_4, \quad \text{PbCO}_3 \xrightarrow{\Delta} \text{PbO} + \text{CO}_2$ $\text{PbO} + \text{C} \to \text{Pb} + \text{CO}\uparrow$

What you observe — and WHY

Two things matter: the bead/incrustation (metal) and any coloured residue (oxide).

Metal	On charcoal	Incrustation colour	Why
$Pb$	Grey malleable bead	Yellow (PbO)	Pb melts (mp 327 °C), oxide is yellow
$Bi$	Brittle bead	Yellow	similar to Pb
$Cu$	Red bead	—	Cu reduced to metal
$Zn$	No bead	Yellow hot, white cold (ZnO)	"yellow-hot, white-cold" rule
$Sb$	Brittle bead	White	volatile oxide
$Cd$	—	Brown (CdO)
$Mg, Al, Ca$	White residue, glows	White infusible oxide	high mp, no metal bead

Flashcards

What is the chemical formula of borax?

\text{Na}_2\text{B}_4\text{O}_7\cdot 10H_2O

(sodium tetraborate decahydrate)

Which species in the borax bead is the transparent glass that dissolves metal oxides?

Boric anhydride

\text{B}_2\text{O}_3

(from

\text{Na}_2\text{B}_4\text{O}_7 \to 2\text{NaBO}_2 + \text{B}_2\text{O}_3

)

Borax bead colour for

Cu^{2+}

in oxidising vs reducing flame?

Blue/green (OF) → red/colourless (RF)

How do you distinguish Co and Cu beads which are both blue in OF?

Use reducing flame: Co stays blue, Cu turns red/colourless

Borax bead: chromium colour in both flames?

Green in both OF and RF

Why is the reducing flame fuel-rich?

Excess unburnt carbon/CO supplies electrons, lowering the metal's oxidation state

In charcoal cavity test, why is charcoal used?

It acts as a reducing agent (C and CO remove oxygen from the metal oxide)

Why is

\text{Na}_2\text{CO}_3

added in charcoal cavity test?

Acts as flux + reducing aid, converts salt to oxide that charcoal can reduce

Zinc's diagnostic incrustation behaviour?

Yellow when hot, white when cold (ZnO)

Lead in charcoal cavity gives what?

Soft grey malleable bead + yellow incrustation (PbO)

Which flame is used for the charcoal cavity test?

Reducing (luminous) flame

A white glowing infusible residue suggests which metals?

Mg, Al, Ca, Ba (confirm with cobalt nitrate: Al→blue, Mg→pink, Zn→green)

Why should you use only a tiny speck of salt for the borax bead?

Too much gives an opaque dark bead, masking the diagnostic colour

Recall Feynman: explain to a 12-year-old

Imagine you melt clear glass into a little blob. If you sprinkle a tiny bit of metal "dust" into it, the glass turns a colour — copper makes it blue, chromium makes it green. So by looking at the colour of the glass bead, you can guess which metal is hiding inside! In the second test, you put the metal on a piece of charcoal (burnt wood) and blow a flame on it. The charcoal is hungry for oxygen and steals it from the metal, leaving behind the shiny pure metal — like cleaning rust off to find the real metal underneath. Lead leaves a soft grey ball with a yellow stain; zinc leaves a stain that's yellow when hot and white when cool.

Connections

Dry Tests in Qualitative Analysis
Flame Test and Atomic Emission
Oxidising vs Reducing Flame
Cobalt Nitrate Test
Transition Metal d-d Colour
Cation Analysis - Group Salts
Fluxes in Metallurgy (same $\text{B}_2\text{O}_3$ as slag-former)

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, ye dono dry tests hain — matlab salt ko paani me ghol-ne ke bajaye hum usko garam karte hain. Borax bead test me hum borax ( $\text{Na}_2\text{B}_4\text{O}_7\cdot 10H_2O$ ) ko garam karte hain. Pehle paani udta hai, phir borax tut-ke $\text{NaBO}_2$ aur $\text{B}_2\text{O}_3$ banta hai. Ye $\text{B}_2\text{O}_3$ ek transparent glass hota hai jo metal oxide ke saath mil-ke coloured bead banata hai. Copper neela, chromium green, manganese violet — bas colour dekho aur metal pehchaano!

Sabse important baat: do flame hote hain. Oxidising flame (bahar wala, oxygen zyada) metal ko higher oxidation state me rakhta hai — true bright colour. Reducing flame (andar wala, carbon zyada) electron deta hai, oxidation state niche karta hai — colour badal jaata hai. Isliye Co aur Cu dono OF me blue dikhte hain, par RF me Cu red/colourless ho jaata hai jabki Co blue hi rehta hai — yahi se confusion door hoti hai.

Charcoal cavity test me charcoal block me ek chhota gaddha banate hain, salt + $\text{Na}_2\text{CO}_3$ daal-ke reducing flame maarte hain. Charcoal oxygen kha jaata hai (reducing agent), isliye pure metal nikal aata hai — Pb ka soft grey bead + yellow incrustation, Zn ka "yellow-hot white-cold" daag. Agar koi bead na bane, sirf white glowing residue ho, to woh Mg/Al/Ca ho sakta hai — confirm karne ke liye cobalt nitrate test karo (Al→blue, Mg→pink, Zn→green).

Ye tests fast aur powerful hain kyunki bina solution banaye seedha cation identify ho jaata hai — exam aur lab dono me bahut kaam aata hai.

1. The Borax Bead Test

WHY does a clear bead form? (derive the chemistry)

WHY does it become coloured?

The colour table (the 20% you MUST know)

2. The Charcoal Cavity Test

WHY charcoal? (the chemistry of reduction)

What you observe — and WHY

Flashcards

Connections

Concept Map

Hinglish (regional understanding)

Go deeper — visual, from zero

Test yourself — Inorganic Qualitative Analysis