Given below are two statements:
Statement I: $\alpha$ and $\beta$ 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.
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For sulfur allotropes, remember the key temperature: 369 K (96 °C). Below it, rhombic ($\alpha$) is stable. Above it, monoclinic ($\beta$) is stable. The transition between them is a classic example of enantiotropic allotropy.
Let's analyze the two statements about the allotropes of sulfur.
Statement I: Sulfur exists in two common crystalline allotropic forms: rhombic sulfur ($\alpha$-sulfur) and monoclinic sulfur ($\beta$-sulfur). Rhombic sulfur is stable below 369 K (96 °C), while monoclinic sulfur is stable above this temperature. The temperature 369 K is known as the transition temperature. At this temperature, the two forms are in equilibrium and can be interconverted by slow heating or cooling. So, the change is reversible. Statement I is true.
S$_{\text{rhombic}}$ $\rightleftharpoons$ S$_{\text{monoclinic}}$ (at 369 K)
Statement II: Room temperature is typically around 298 K (25 °C), which is well below the transition temperature of 369 K. Therefore, at room temperature, the thermodynamically stable crystalline form of sulfur is rhombic sulfur ($\alpha$-sulfur), not monoclinic sulfur. Statement II is false.
Since Statement I is true and Statement II is false, the correct option is (C).
