# Crystalline and Amorphous Solids

Differences between Crystalline and Amorphous Solids

(i)  Characteristic Geometry: In crystalline solids the particles (atoms, ions, or molecules) are definitely and orderly arranged thus these have characteristic geometry while amorphous solids do not have characteristic geometry.

(ii) Melting Point: A crystalline solid has a sharp melting point, i.e., it changes into liquid state at a definite temperature. On the contrary an amorphous solid does not have a sharp melting point. For example, when glass is heated, it softens and then starts flowing without undergoing any abrupt or sharp change from solid to liquid state. Therefore, amorphous solids are regarded as “liquids at all temperatures”.

(iii)  Cooling curve: Amorphous solids show smooth cooling curve while crystalline solids show two breaks in cooling curve. In the case of crystalline solids two break points ‘a’ and ‘b’ appear. These points indicate the

beginning and the end of the process of crystallization. In this time interval temperature remains constant. This is due to the fact that during crystallization process energy is liberated which compensates for the loss of heat, thus the temperature remains constant.

(iv) Isotropy and Anisotropy: Amorphous solids differ from crystalline solids and resemble liquids in many respects. The properties of amorphous solids, such as, electrical conductivity, thermal conductivity, mechanical strength, refractive index, coefficient of thermal expansion etc. are same in all directions. Such solids are known as isotropic. Gases and liquids are also isotropic.

On the other hand, crystalline solids show these physical properties different in different directions. Therefore crystalline solids are called anisotropic. The anisotropy itself is a strong evidence for the existence of orderly molecular arrangement in crystals. For example, the velocity of light passing through a crystal is different in different directions. A ray of light entering in a crystal may split up

into two components each following a different path and “aveling with a different velocity. This phenomenon is called double refraction. In the figure two different kinds of atoms are shown in two dimensional arrangement. If the properties are measured along the direction CD, they will be different from those measured along the direction AB. This is due to the fact that in the direction AB each row is made up of one type of atoms while in the direction CD each row is made up of two types of atoms. It is important to note that in the case of amorphous solids, liquids and gases atoms or molecules are arranged disorderly therefore all directions are identical and all properties are same in all directions.

(v) Cutting: Crystalline solids give clean cleavage while amorphous solids give irregular cut, due to conchoidal fracture on cutting with a sharp edged tool.

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