Borax bead, charcoal cavity tests — Hinglish notes

3.5.4 · Chemistry › Inorganic Qualitative Analysis

1. The Borax Bead Test

Clear bead kyun banta hai? (chemistry derive karo)

Step 1 — paani hatao. Heating se 10 water molecules nikal jaate hain: $Na_{2} B_{4} O_{7} \cdot 10 H_{2} O Δ Na_{2} B_{4} O_{7} + 10 H_{2} O ↑$ Yeh step kyun? Salt pehle phool jaata hai ("intumescence") jaise steam nikalti hai — isliye fresh bead bubble karta hai.

Step 2 — do active species mein decompose karo. Aur heating se borax sodium metaborate + boric anhydride mein split ho jaata hai: $Na_{2} B_{4} O_{7} Δ metaborate 2 NaBO_{2} + boric anhydride (the glass) B_{2} O_{3}$ Yeh step kyun? $B_{2} O_{3}$ ek transparent glassy solvent hai. Yahi reactive part hai jo metal ions ko pakad ke coloured glasses banata hai.

Yeh coloured kyun ho jaata hai?

$B_{2} O_{3}$ ek metal oxide ke saath react karke us metal ka ek coloured metaborate banata hai: $B_{2} O_{3} + CuO ⟶ Cu(BO_{2})_{2} (copper metaborate)$

Figure — Borax bead, charcoal cavity tests

Colour table (woh 20% jo tumhe ZAROOR pata hona chahiye)

Metal ion	Oxidising flame (OF)	Reducing flame (RF)
$C u^{2 +}$	Blue / green	Red / colourless
$F e^{3 +}$	Yellow-brown	Green / colourless
$C r^{3 +}$	Green	Green
$M n^{2 +}$	Violet / amethyst	Colourless
$C o^{2 +}$	Blue	Blue
$N i^{2 +}$	Brown	Grey (Ni metal)

2. The Charcoal Cavity Test

Charcoal kyun? (reduction ki chemistry)

Garam charcoal ek reducing agent hai: yeh carbon aur CO supply karta hai jo metal oxide se oxygen kheench lete hain. $MO + C Δ M + CO ↑ MO + CO Δ M + CO_{2} ↑$

$Na_{2} CO_{3}$ kyun add karte hain? Yeh ek flux + reducing aid hai: yeh salt ko metal carbonate/oxide mein convert karta hai (jo oxide mein decompose hota hai) taaki charcoal use cleanly reduce kar sake: $PbSO_{4} + Na_{2} CO_{3} \to PbCO_{3} + Na_{2} SO_{4}, PbCO_{3} Δ PbO + CO_{2}$ $PbO + C \to Pb + CO ↑$

Kya observe karte ho — aur kyun

Do cheezein matter karti hain: bead/incrustation (metal) aur koi bhi coloured residue (oxide).

Metal	Charcoal par	Incrustation colour	Kyun
$P b$	Grey malleable bead	Yellow (PbO)	Pb melt ho jaata hai (mp 327 °C), oxide yellow hai
$B i$	Brittle bead	Yellow	Pb jaisa hi
$C u$	Red bead	—	Cu metal mein reduce ho jaata hai
$Z n$	Koi bead nahi	Garam par yellow, thanda par white (ZnO)	"yellow-hot, white-cold" rule
$S b$	Brittle bead	White	volatile oxide
$C d$	—	Brown (CdO)
$M g, A l, C a$	White residue, glows	White infusible oxide	high mp, koi metal bead nahi

Flashcards

Borax ka chemical formula kya hai?

Na_{2} B_{4} O_{7} \cdot 10 H_{2} O

(sodium tetraborate decahydrate)

Borax bead mein kaun si species transparent glass hai jo metal oxides dissolve karti hai?

Boric anhydride

B_{2} O_{3}

(

Na_{2} B_{4} O_{7} \to 2 NaBO_{2} + B_{2} O_{3}

se)

Borax bead colour

C u^{2 +}

ke liye oxidising vs reducing flame mein?

Blue/green (OF) → red/colourless (RF)

Co aur Cu beads ko kaise distinguish karoge jo dono OF mein blue hain?

Reducing flame use karo: Co blue rehta hai, Cu red/colourless ho jaata hai

Borax bead: chromium ka colour dono flames mein?

Dono OF aur RF mein green

Reducing flame fuel-rich kyun hoti hai?

Excess unburnt carbon/CO electrons supply karta hai, metal ki oxidation state ko lower karta hai

Charcoal cavity test mein charcoal kyun use karte hain?

Yeh reducing agent ki tarah kaam karta hai (C aur CO metal oxide se oxygen hata dete hain)

Charcoal cavity test mein

Na_{2} CO_{3}

kyun add karte hain?

Flux + reducing aid ki tarah kaam karta hai, salt ko oxide mein convert karta hai jise charcoal reduce kar sake

Zinc ki diagnostic incrustation behaviour?

Garam par yellow, thanda hone par white (ZnO)

Charcoal cavity mein lead kya deta hai?

Soft grey malleable bead + yellow incrustation (PbO)

Charcoal cavity test ke liye kaun si flame use karte hain?

Reducing (luminous) flame

White glowing infusible residue kaun se metals suggest karta hai?

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

Borax bead ke liye salt ki sirf tiny speck kyun use karni chahiye?

Bahut zyada dene par opaque dark bead banta hai jo diagnostic colour chhupa leta hai

Recall Feynman: ek 12-saal ke bachche ko samjhao

Socho tum clear glass ko ek chhote se blob mein pighla lete ho. Agar tum us mein thodi si metal "dust" chirakate ho, toh glass ek colour mein badal jaati hai — copper use blue banata hai, chromium use green banata hai. Toh glass bead ke colour ko dekh kar tum andaza laga sakte ho ki andar kaun sa metal chhupa hua hai! Doosre test mein, tum metal ko ek charcoal (jali hui lakdi) ke tukde par rakhte ho aur us par flame blow karte ho. Charcoal oxygen ka bhooka hai aur metal se oxygen chura leta hai, peeche shiny pure metal chodke — jaise rust saaf karo aur asli metal paao. Lead ek soft grey ball chhod jaata hai yellow daag ke saath; zinc ek daag chhod jaata hai jo garam hone par yellow aur thanda hone par white hota hai.

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 $B_{2} O_{3}$ as slag-former)