\(\alpha \)- sulfur converts to \(\beta \)-sulfur above 369K. This temperature is known as:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Transition temperature
Rhombic sulphur (α-sulphur) is stable below 369 K and monoclinic sulphur (β-sulphur) above it; at 369 K the two forms are in equilibrium, and the temperature at which one allotrope changes into the other is the transition temperature:
\( \displaystyle \ce{S(rhombic)} \overset{369\ \text{K}}{\rightleftharpoons} \ce{S(monoclinic)} \)
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Monoclinic Sulphur (β-sulphur) – “Its m.p. is 393 K and specific gravity 1.98. It is soluble in CS2. This form of sulphur is prepared by melting rhombic sulphur in a dish and cooling, till crust is formed. Two holes are made in the crust and the remaining liquid poured out. On removing the crust, colourless needle shaped crystals of β-sulphur are formed. It is stable above 369 K and transforms into α-sulphur below it. Conversely, α-sulphur is stable below 369 K and transforms into β-sulphur above this. At 369 K both the forms are stable. This temperature is called transition temperature.”