Class 12th Chemistry

1. Solids are classified as Crystalline and Amorphous, on the basis of the nature of order
present in the arrangement of their constituent particles. Let us see what they are.
(a) Crystalline solids: A solid is said to be crystalline if the various constituent
particles like atoms, ions, or molecules are arranged in a definite geometric pattern
within the solid, e.g., NaCl, KNO3, LiF, SiO2(Quartz) and CuSO4.5H2O.
(b) Amorphous solids: A solids is said to be amorphous if the various constituent
particles are not arranged in any regular fashion n, e.g., glass and rubber. They
are also called pseudo-solid or super cooled liquid. They are intermediate state
between liquids and solids. Like liquids, amorphous solids have a tendency to flow.
They however do it very slowly and hence are called pseudo solids or super cooled
liquids.
Glass panes fixed to windows or doors of old buildings are found to be slightly thicker
at the bottom than at the top. This is because of the amorphous nature of the glass. It
flows down very slowly and makes the bottom portion slightly thicker.
Difference between crystalline and amorphous solids:
1.) Geometric Shape
Crystalline solids have a definite geometrical shape due to the definite and orderly
arrangement of particles in the three dimensional space while amorphous do not have
an orderly arrangement of particles and therefore, do not possess a definite
geometrical shape.

2. 2.) Melting Points
Crystalline solids have sharp melting point while amorphous do not have sharp
melting point. They melt over a temperature range.
3.) Crystal Symmetry

3. Crystalline solids have crystal symmetry while amorphous do not have crystal
symmetry.
4. Behavior when cut (Cleavage Property)
When Crystalline solids are cut with a sharp edged tool, they split into two pieces
and the newly generated surfaces are plain and smooth while When amorphous are
cut, they split into two pieces and the newly generated surfaces are irregular in shape.
5.) Properties vs. Direction
Crystalline solids possess anisotropic properties, i.e., the magnitude of some of the
physical properties of crystalline solids like refractive index; coefficient of thermal
expansion and electrical and thermal conductivities is different in different directions
within the crystal while amorphous have isotropic properties, i.e., the magnitude of
physical properties is same in all directions.

4. 6.) Heat of Fusion
Crystalline solids have a definite heat of fusion however amorphous solids don’t.
Example 1:- Which of the following is pseudo-solids.
(a) KCl
(b) Rubber
(c) NaCl
(d) Barium chloride dehydrate
Answer: - (b)
Example 2: - Which of the following statement is correct?
(a) Amorphous solids show anisotropic properties.
(b) Crystalline solids show anisotropic properties.

5. (c) Amorphous solids can be crystalline on slow heating.
(d) Crystalline solids have definite shape and volume.
Answer: - (b,d)