Red phosphorus PYQs MCQs

Red Phosphorous MCQs
NEET, IIT-JEE, CSIR-NET, GATE, IIT-JAM

JEE Main 2020

Question: Red phosphorus is chemically less reactive than white phosphorus because

1. it is amorphous in nature
2. it has polymeric structure
3. it is red in colour
4. its density is low

Correct Answer: B

Explanation:
Red phosphorus has a polymeric chain/network structure with strong P–P covalent bonds → high lattice energy and activation energy → much less reactive than discrete P₄ molecules of white phosphorus.

NEET 2019, 2021, 2023 Repeated

Question: Red phosphorus is used in safety matches because

1. it has low ignition temperature
2. it is highly poisonous
3. it has high ignition temperature and is non-poisonous
4. it shows phosphorescence

Correct Answer: C

Explanation:
Ignition temperature ≈ 260°C (high) and non-poisonous nature make it safe for striking surface.

JEE Main 2022

Question: Red phosphorus is obtained by heating white phosphorus at 573 K in an inert atmosphere of

1. air
2. CO₂ only
3. N₂ only
4. CO₂ or N₂

Correct Answer: D

Explanation:
Inert atmosphere (CO₂ or N₂) prevents oxidation of white phosphorus during conversion.

JEE Advanced / CSIR-NET

Question: The catalyst used to convert white phosphorus to red phosphorus at lower temperature is

1. sulphur
2. iodine
3. selenium
4. iron

Correct Answer: B

Explanation:
Trace iodine catalyses the polymerisation and reduces temperature from ~300°C to ~200°C.

NEET & JEE Main

Question: Which allotrope of phosphorus is insoluble in CS₂?

1. White phosphorus
2. Red phosphorus
3. Black phosphorus
4. All are soluble

Correct Answer: B

Explanation:
Most important identification test — polymeric red P is insoluble in CS₂.

NEET

Question: Red phosphorus does NOT exhibit

1. polymeric structure
2. chemiluminescence
3. reaction with hot conc. NaOH
4. oxidation to phosphoric acid

Correct Answer: B

Explanation:
Chemiluminescence (glowing in dark) is shown only by white phosphorus due to slow oxidation.

JEE Advanced / CSIR-NET

Question: The reaction of red phosphorus with hot concentrated NaOH gives

1. PH₃ only
2. Na₃PO₄ only
3. PH₃ and NaH₂PO₂
4. PH₃ and Na₃PO₄

Correct Answer: C

Explanation:
4P + 3NaOH + 3H₂O → PH₃ + 3NaH₂PO₂ (classic disproportionation reaction)

NEET Repeated

Question: Ignition temperature of red phosphorus is approximately

1. 30°C
2. 260°C
3. 44°C
4. 573 K

Correct Answer: B

JEE Main

Question: The most stable allotrope of phosphorus is

1. white
2. red
3. black
4. violet

Correct Answer: C

Explanation:
Stability order: Black > Red > White phosphorus

NEET 2020, 2022

Question: Red phosphorus is present on the

1. tip of the matchstick
2. striking surface of matchbox
3. both
4. none

Correct Answer: B

NEET

Question: Which phosphorus glows in the dark?

1. Red
2. White
3. Black
4. Violet

Correct Answer: B

JEE Main Assertion-Reason

Question: Assertion: Red phosphorus is less poisonous than white phosphorus.
Reason: It has polymeric structure.

1. Both A and R true, R explains A
2. Both true, R does not explain A
3. A true, R false
4. A false, R true

Correct Answer: A

JEE Advanced

Question: In red phosphorus, each phosphorus atom is linked to how many other P atoms?

1. 2
2. 3
3. 4
4. 1

Correct Answer: B

JEE Main

Question: Red phosphorus on strong heating with conc. HNO₃ gives

1. H₃PO₃
2. H₃PO₄
3. HPO₃
4. (HPO₃)ₙ

Correct Answer: B

GATE / CSIR-NET

Question: Hittorf’s violet phosphorus is a

1. amorphous form
2. crystalline form of red phosphorus
3. separate allotrope
4. mixture

Correct Answer: B

NEET

Question: Red phosphorus is used in smoke screens because it produces

1. PH₃
2. dense P₄O₁₀ smoke
3. H₃PO₄ mist
4. NO₂

Correct Answer: B

JEE Main

Question: Which of the following has garlic odour?

1. White phosphorus
2. Red phosphorus
3. Phosphine
4. Both A and C

Correct Answer: D

JEE Advanced

Question: Red phosphorus acts as a flame retardant because it forms a coating of

1. phosphine
2. phosphoric acid
3. P₄O₁₀
4. both B and C

Correct Answer: D

NEET

Question: White phosphorus is stored under

1. water
2. CS₂
3. kerosene
4. alcohol

Correct Answer: A

JEE Main

Question: Which phosphorus is used on the head of matchstick?

1. Red P
2. White P
3. Black P
4. None (only KClO₃ + S)

Correct Answer: D

CSIR-NET

Question: The structure of amorphous red phosphorus is best described as

1. discrete P₄ tetrahedra
2. random network of P₄ units linked by P–P bonds
3. layered structure like black P
4. chain of P₈ units

Correct Answer: B

JEE Main

Question: Red phosphorus sublimes at

1. 416°C
2. 280°C
3. 44°C
4. 590°C

Correct Answer: A

NEET

Question: Which is non-poisonous allotrope of phosphorus?

1. White
2. Red
3. Both
4. None

Correct Answer: B

JEE Advanced

Question: The conversion of white P → red P is

1. exothermic and irreversible
2. endothermic and reversible
3. exothermic but reversible at high T
4. endothermic and irreversible

Correct Answer: A

GATE

Question: Black phosphorus is

1. best conductor of electricity
2. best conductor of heat
3. thermodynamically most stable
4. all of the above

Correct Answer: D

NEET

Question: Which statement is incorrect for red phosphorus?

1. Non-poisonous
2. Insoluble in CS₂
3. High ignition temperature
4. Glows in dark

Correct Answer: D

JEE Main

Question: Red phosphorus is used as

1. flame retardant
2. smoke screen
3. safety matches
4. all of these

Correct Answer: D

CSIR-NET

Question: Scarlet phosphorus is

1. another name for red P
2. a separate allotrope
3. mixture of red and white P
4. none

Correct Answer: B

JEE Advanced

Question: The number of P–P bonds broken when white P₄ converts to red P network is

1. 0
2. 6 per P₄
3. 3 per P₄
4. infinite

Correct Answer: B

Explanation:
P₄ has 6 P–P bonds; in polymeric red P, bonds are shared → effectively 3 bonds per P atom.

NEET / JEE Main

Question: Red phosphorus is thermodynamically more stable than white phosphorus because

1. it has higher density
2. it has polymeric structure
3. it is red in colour
4. it is non-poisonous

Correct Answer: B

Must Read Red phosphorus: Preparation, Properties, Structure and Uses

Related Topics
White Phosphorus
Red Phosphorus
Black Phosphorus
White Phosphorus PYQs MCQs
Red Phosphorus PYQs MCQs
Black Phosphorus PYQs MCQs