Q.Differentiate crystalline solids and amorphous solids.

Every solid falls into one of two broad classes, based purely on how its constituent particles (atoms, ions or molecules) are arranged:

• Crystalline solids: long-range, orderly arrangement of constituents extending throughout the solid; sharp, definite melting point; anisotropic (physical properties differ with measurement direction); definite heat of fusion; considered 'true' solids. Examples: NaCl, diamond.
• Amorphous solids: short-range, random arrangement; no sharp melting point -- instead they soften gradually over a range of temperature, which is why they can be moulded; isotropic (properties are the same in every direction, like a liquid); no definite heat of fusion; considered pseudo-solids or supercooled liquids. Examples: glass, rubber, plastics.

Isotropy vs anisotropy: isotropy means a physical property (refractive index, electrical conductance, etc.) has the same value no matter which direction it is measured along; anisotropy means that value changes with direction. Crystalline solids are anisotropic precisely because their constituents are arranged differently along different crystallographic directions, while amorphous solids, lacking any long-range order, look the same statistically in every direction.

The word 'crystal' comes from the Greek 'krystallos' (clear ice), first applied to transparent quartz and then extended to any solid bounded by many flat, symmetrically arranged faces -- the macroscopic signature of the underlying long-range order.