Question

Which one of the following is a pseudo solid?
       (A) Calcium fluoride
       (B) Glass
       (C) Sodium chloride
       (D) All of the above

Answer 1

Hint: Pseudo solids are those which possess liquid like properties and hence they have the ability to flow. This material will flow like a liquid but at a very slower rate than the liquids. While the compounds which are made of ions and have proper arrangements are true solids.

Complete step by step answer:
-First of all let us see what pseudo solids are.
Pseudo solids are also known as amorphous solids. Amorphous solids are those solids in which the particles are not arranged in some specific already designed order or sequence. These solids lack the overall order of the crystal lattice. They will thus possess liquid-like properties.
These pseudo or amorphous solids are not the same as an ideal crystal because an ideal crystal has a definite atomic arrangement with infinite translational symmetry while pseudo solids have no definite arrangement of atoms.
-Some examples of such pseudo or amorphous solids are: glass, gels, nanostructured materials, some polymers, etc.
Glass, being a pseudo solid possesses liquid like properties. Hence it flows like a liquid in a very slow process due to which the base of the glass is thicker will always be thicker than the top.

-Now we will talk about some key features of these pseudo or amorphous solids:
(1) By nature they can be known to be supercooled liquids.
(2) Melting point: Their melting points are not sharp. The pseudo solids soften gradually over a particular range of temperature.
(3) Geometry: In pseudo solids the molecules are randomly dispersed and there is no specific ordered arrangement. Hence they have irregular shapes.
(4) Rigidity: They are not rigid at all and hence even small changes or deflections from their ideal conditions may change their shapes.
(5) Heat of fusion: The pseudo solids do not have a definite heat of fusion.
(6) Cleavage property: Since they are randomly arranged the two resulting halves after cutting with a sharp edge have irregular surfaces.
(7) Isotropism: Due to the random arrangement they are isotropic in nature, which means that the physical properties have the same magnitude along all the directions of the solids.
So, the correct answer is “Option B”.

Note: The solids are classified into crystalline and amorphous solids based on their crystal structures. The crystalline solids are also known as true solids and are exactly opposite of the amorphous or pseudo solids. Calcium fluoride ($Ca{F_2}$) and sodium chloride (NaCl) are made up of positive and negative ions (cations and anions) having proper arrangements, hence they are crystalline or true solids.