White Phosphorus – Preparation, Properties, Structure, Uses and Previous Year Questions
Highly Important for NEET, JEE Main/Advanced, GATE & CSIR-NET
1. Preparation of White Phosphorus
| Method | Reaction / Process | Key Exam Points |
|---|---|---|
| From Rock Phosphate (Industrial) | Ca3(PO4)2 + 3SiO2 → 3CaSiO3 + P4O10 P3O10 + 10C → P4↑ + 10CO (Electric furnace, 1400–1600°C) |
Most asked in JEE Main & GATE NEET favourite one-liner: White P is obtained by heating phosphate rock with sand and coke in electric furnace |
| From Bone Ash (Old Method) | Ca3(PO4)2 + 3H2SO4 → 3CaSO4 + 2H3PO4 2H3PO4 + 12C → P4 + 6H2O + 12CO |
JEE Advanced & CSIR-NET level |
JEE Trick: Electric furnace method → always gives white phosphorus (not red).
2. Physical Properties of White Phosphorus
| Property | White Phosphorus | Comparison with Red P |
|---|---|---|
| Appearance | Waxy, translucent, pale yellow solid | Reddish-brown powder |
| Odour | Garlic-like smell | Odourless |
| Density | 1.82 g/cm³ | 2.1–2.34 g/cm³ |
| Melting Point | 44.2°C | 590°C (sublimes) |
| Solubility | Soluble in CS2, benzene, ether Insoluble in water | Insoluble in CS2 & water |
| Chemiluminescence | Glows in dark (greenish glow) | No glow |
| Ignition Temperature | Very low (~30°C) | High (~260°C) |
| Poisonous Nature | Highly poisonous (causes phossy jaw) | Non-poisonous |
| Storage | Under water (to prevent contact with air) | In air (stable) |
NEET Most Repeated:
• Glows in dark → Chemiluminescence
• Garlic odour
• Stored under water
• Highly poisonous
• Soluble in CS2
• Glows in dark → Chemiluminescence
• Garlic odour
• Stored under water
• Highly poisonous
• Soluble in CS2
3. Structure of White Phosphorus
Tetrahedral P4 molecule (Regular tetrahedron)
- Molecular formula: P4
- Shape: Regular tetrahedron
- Each P atom is sp³ hybridized
- Bond angle: 60° (highly strained → very reactive)
- Contains 6 P–P single bonds
- Discrete covalent molecules (not polymeric)
JEE Advanced Favourite: “Angle strain in P4 (60°) is responsible for its high reactivity.”
4. Chemical Properties of White Phosphorus
- Highly reactive – most reactive allotrope
- Catches fire spontaneously in air at ~30°C → P4O10
- Glows in dark due to slow oxidation (chemiluminescence)
- With halogens: Direct combination (even in dark)
- With hot conc. NaOH: Disproportionation
P4 + 3NaOH + 3H2O → PH3↑ + 3NaH2PO2
(Produces phosphine + sodium hypophosphite) - With conc. HNO3: Oxidised to H3PO4
- With metals: Forms phosphides (e.g., Ca3P2)
CSIR-NET & JEE Advanced Classic:
P4 + 3NaOH(hot conc.) + 3H2O → PH3 + 3NaH2PO2
Oxidation state: P(0) → P(-3) in PH3 & P(+1) in H2PO2⁻
P4 + 3NaOH(hot conc.) + 3H2O → PH3 + 3NaH2PO2
Oxidation state: P(0) → P(-3) in PH3 & P(+1) in H2PO2⁻
5. Uses of White Phosphorus
| Use | Reason |
|---|---|
| Manufacture of rat poison | Highly poisonous |
| In incendiary bombs & smoke screens | Burns spontaneously, produces dense P4O10 smoke |
| Tracer bullets & fireworks | Chemiluminescence & spontaneous ignition |
| Synthesis of PCl3, PCl5, H3PO4, etc. | Starting material |
| Formerly used in matchsticks (strike-anywhere matches) | Banned now due to toxicity |
Important for NEET:
White P was used in old strike-anywhere matches → banned due to phossy jaw disease.
White P was used in old strike-anywhere matches → banned due to phossy jaw disease.
Quick Comparison: White vs Red Phosphorus (Exam Favourite)
| Property | White P | Red P |
|---|---|---|
| Structure | P4 molecules | Polymeric |
| Reactivity | Highly reactive | Less reactive |
| Ignition Temp | 30°C | 260°C |
| CS2 Solubility | Soluble | Insoluble |
| Glow in dark | Yes | No |
| Poisonous | Yes | No |
| Use in matches | Old strike-anywhere | Safety matches (striking surface) |
High-Yield One-Liners for Last Minute Revision
- White P has P4 tetrahedral structure with 60° bond angle → highly strained → very reactive
- Stored under water → prevents contact with oxygen
- Glows in dark → chemiluminescence due to slow oxidation
- Garlic odour + highly poisonous
- Spontaneously inflames in air at 30°C
- With hot conc. NaOH → PH3 + NaH2PO2 (disproportionation)
- Used in smoke bombs and incendiary devices
- Converted to red P by heating at 573 K in inert atmosphere
Must Read White Phosphorus MCQs asked in NEET, IIT-JEE, GATE, IIT-JAM & CSIR-NET
Related Topics
White Phosphorus
Red Phosphorus
Black Phosphorus
White Phosphorus PYQs MCQs
Red Phosphorus PYQs MCQs
Black Phosphorus PYQs MCQs