Definition check: Heat of transition in phase/allotrope changes:\nHeat of transition is the heat evolved or absorbed when a substance is converted from which state to which?

Introduction / Context:
Thermochemical quantities distinguish different types of transformations: fusion (melting), vaporisation, sublimation, and transition. Heat of transition specifically involves rearrangements between allotropic forms—distinct structural forms of the same element (e.g., graphite ⇄ diamond, rhombic ⇄ monoclinic sulfur).

Given Data / Assumptions:

• Allotropy refers to structural variations at the same aggregate state (usually solid).
• Process occurs at constant pressure with measurable heat effect.

Concept / Approach:
Identify the transformation type: phase changes between solid, liquid, and vapor have their own heats (fusion, vaporisation, sublimation). A transition between two allotropic forms is neither simple melting nor boiling; the associated enthalpy change is the heat of transition.

Step-by-Step Reasoning:

Verification / Alternative check:
Thermochemistry tables list separate ΔH values for transition distinct from fusion or vaporisation, confirming the definition.

Why Other Options Are Wrong:

• Vapor ↔ liquid/solid and solid → liquid are standard phase-change heats (vaporisation, condensation, sublimation, fusion) not “transition.”
• Liquid to supercritical fluid involves approaching the critical point, not an allotropic change.

Common Pitfalls:
Equating allotropic transitions with polymorphic phase changes between different aggregate states; mixing up heat-of-transition with latent heat of fusion.

Final Answer:
One allotropic form to another allotropic form