JEE Main 2021 · 25 February · evening · Single correct · +4 / −1
Given below are two statements :
Statement I : α and β forms of sulphur can change reversibly between themselves with slow heating or slow cooling.
Statement II : At room temperature the stable crystalline form of sulphur is monoclinic sulphur.
In the light of the above statements, choose the correct answer from the options given below :
- (a) Statement I is true but Statement II is false.
- (b) Statement I is false but Statement II is true.
- (c) Both Statement I and Statement II are true.
- (d) Both Statement I and Statement II are false.
Answer and solution
Answer (a) Statement I is true but Statement II is false.
Solution
Chemistry ma’am · 5 steps
Remember 369 K: rhombic below it, monoclinic above it, and slow heating or cooling switches them.
- Two allotropes. Rhombic sulphur is the α form and monoclinic sulphur is the β form. Both are built from the same puckered S₈ rings, packed differently in the crystal.
- Transition temperature. Rhombic sulphur is stable below 369 K and monoclinic sulphur is stable above it. At 369 K both forms are equally stable.
- Check Statement I. Heat α sulphur slowly past 369 K and it turns into β. Cool β slowly below 369 K and it goes back to α. The change is reversible, so Statement I is true.
- Check Statement II. Room temperature (about 298 K) is well below 369 K. So the stable form there is rhombic, not monoclinic, and Statement II is false.
- So. Statement I is true and Statement II is false, which is option (a).
The idea to keep. Every stability question about sulphur allotropes comes back to 369 K. Below it, rhombic (α) is stable; above it, monoclinic (β). Slow heating or cooling across it interconverts them.
Answer from the NTA final answer key. Solution by PrepWiser.