Question

Why does the window glass of old buildings look milky?

Answer 1

Hint: Glass is an amorphous solid and thus it is not a true solid and has irregular arrangement of molecules. It is considered as a super cooled solid due to its property to flow which is a very slow process. Thus, due to a long exposure of these window panes to heat differences in the day and night fades them.

Complete Answer:
Solid particles have a definite shape and a definite volume and high densities. There are two types of solids-
Amorphous solids: They have irregular shapes and short range order. They are isotropic in nature. Silicon and glass are some examples of amorphous solids.
Crystalline solids: They have definite and regular arrangement. They are anisotropic in nature. This type of solid has a flat face and sharp edge. They are true solids. Sodium chloride is an example of crystalline solids.
As we know that glass is an example of amorphous solids. Thus they are not true solids and like liquids, they have the tendency to flow through very slowly. The glass panes in the windows of old buildings are found to be a little thicker at the bottom as compared to the top of the window panes and this because of the tendency of glass to flow very slowly and thus, glass is known as super cooled liquid.
The window panes of buildings are exposed to continuous heat and change in temperature heat from day to night which converts the amorphous glass to crystalline form. This process is very slow just like the flow in glass. Hence, with time the window panes tend to fade due to continuous exposure to sun’s heat and cooling nights. Therefore, the window panes of old buildings appear milky.

Note: The three dimensional arrangement of particles of a solid in a crystal is known as a space lattice and unit cell is the smallest portion in a space lattice which is repeated to make the arrangement of particles in a crystal. These crystal systems are divided on the basis of difference in axial distances and axial angles of the particles.