Question

A pure crystalline substance, on being heated gradually, first forms a turbid looking liquid and then the turbidity completely disappears. This behaviour is the characteristic of substances forming.
(a) Isometric crystals
(b) Liquid crystals
(c) Isomorphous crystals
(d) Allotropic crystals

Answer 1

Hint: Liquid crystals can flow like the liquid, but their molecules can be arranged like crystalline solids. The liquid crystals also exist in nature. For example, Lyotropic liquid-crystalline phases.

Complete Step by Step Solution:
The crystals of some compounds tend to exist in a crystalline solid but can run like a liquid substance by heating them above their melting points. Such a substance is known as liquid crystals.

The liquid crystal has a special property, when white light falls on them, they reflect only a single colour. Whereas on changing the temperature they have different various lights. Therefore, the liquid crystals can be used to determine the change in temperature.
The liquid crystals have the tendency to form a transparent phase by changing the temperature range.

Types of liquid crystals
Nematic liquid crystals
These are also known as needle-like crystals.
These are transparent or translucent liquid crystals.
These crystals can change the speed of passing polarised light.
The magnitude to change the speed of polarised light depends upon the intensity of an applied electric field.

The cyanobiphenyl is an example of nematic liquid crystal.
In nematic liquid crystals, the constituent (molecules) particles do not have layer structure but all the molecules are arranged in one direction.
Smectic liquid crystals.

They are also known as soap-like crystals.
The smectic liquid crystals are found at low temperatures.
The constituent particles are arranged in one direction.
Smectic liquid crystals have flat layers of cigar-shaped constituent particles (molecules) and are perpendicular to the flat layer.

The above points indicate that the ionic crystal on heating can be converted into a transparent form . Therefore, option (b) will be the correct answer.

Note: The display of LCD consists of twisted nematic liquid crystals. The para-azoxyanisole (PPA) was the first synthetic liquid crystal, prepared by Ludwing Gattermann in Heidelberg.