Chemistry Solids DUHS 2024
PMDC Verified Question 13 of 54
It is an example of a crystalline solid:
A
Plastics
B
Charcoal
C
Glass
D
Alum
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option D: Alum
Concept:
Crystalline solids possess a sharp melting point and a perfectly ordered, repeating 3D internal arrangement of particles. Amorphous solids do not.

Solution:
  • Alum (such as potash alum, \( \text{KAl(SO}_4\text{)}_2 \cdot 12\text{H}_2\text{O} \)) is an ionic double salt.
  • When it solidifies, its constituent ions arrange themselves into a highly regular, repeating geometric lattice, yielding distinct, faceted crystals with sharp physical properties.
  • Therefore, alum is a true crystalline solid.


Why other options are incorrect:
  • A, B, C: Plastics, charcoal (a form of carbon), and glass are classic examples of amorphous solids. They have disordered, random internal structures and melt over broad temperature ranges.

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