Chemistry Solids BUMHS 2024
PMDC Verified Question 15 of 54
The term 'Transition temperature' is use for the temperature:
A
At which one crystalline form of a substance changes into another
B
At which liquid crystals are formed in a liquid
C
Within which the habit of a crystal remains the same
D
Showing a sharp change in the anisotropic properties of a crystal
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option A: At which one crystalline form of a substance changes into another
Concept:
Many substances can exist in more than one crystalline structural form (polymorphism or allotropy). These different forms are stable at different temperatures.

Solution:
  • The specific equilibrium temperature at which one distinct crystalline form (polymorph/allotrope) of a substance reversibly transforms into a different crystalline form is defined as its transition temperature.
  • For example, Rhombic Sulphur transforms into Monoclinic Sulphur at exactly \( 95.5^\circ\text{C} \). At this specific temperature, both solid crystalline forms stably co-exist in equilibrium.


Why other options are incorrect:
  • B: This describes the formation of mesophases, not standard transition temperature between solid states.
  • C: Crystal habit refers to the macroscopic external shape, which is related but not the fundamental thermodynamic definition of transition temperature.
  • D: While properties may change during the transition, the definition is strictly based on the structural change from one crystalline phase to another, not just the properties themselves.

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