Unit – I B

Classification of Solids

On the basis of the nature of order present in the arrangement of their constituent particles, solids can be classified as crystalline or amorphous Due to the differences in the arrangement of the constituent particles, the two types of solids differ in their properties.

Crystalline Solids:

  • A crystalline solid usually consists of a large number of small crystals, each of them having a definite characteristic geometrical shape.
  • In a crystal,the arrangement of constituent particles (atoms, molecules or ions) is ordered. It has long range order which means that there is a regular pattern of arrangement of particles which repeats itself periodically over the entire crystal.
  • Sodium chloride and quartz are typical examples of crystalline solids.

Characteristics of Crystalline Solids:

  • A crystalline solid usually consists of a large number of small crystals, each of them having a definite characteristic geometrical shape.
  • These are true solids.
  • We get clean cleavage in case of crystalline solids.
  • Cooling curves of crystalline solids are not smooth, there are break points in the curve.

  • Crystalline solids may or may not be isotropic (a physical property which has the same value when measured in different directions).
  • Heat of fusion is definite and depends upon arrangement of the particles in the crystalline solids.
  • Crystalline solids have a sharp melting point.
  • Crystalline solids are anisotropic in nature, that is, some of their physical properties like electrical resistance or refractive index show different values when measured along different directions in the same crystals. This arises from different arrangement of particles in different directions. This is illustrated in Fig. Since the arrangement of particles is different along different directions, the value of same physical property is found to be different along each direction.



Amorphous Solids:

Crystalline                                                Amorphous

  • An amorphous solid (Greek Omorphe = no form or shapeless) consists of particles of irregular shape.
  • The arrangement of constituent particles (atoms, molecules or ions) in such a solid has only short range order. In such an arrangement, a regular and periodically repeating pattern is observed over short distances only. Such portions are scattered and in between the arrangement is top. This is because the glass flows down very slowly and makes the bottom portion disordered. Amorphous solids in many respects resemble liquids which flow very slowly at room temperature and can be considered as super cooled liquids in which cohesive forces are almost as strong as solids.
  • We get irregular cleavage in case of amorphous solids.
  • The cooling curves of amorphous solids are smooth, without any break

  • The structures of quartz (crystalline) and quartz glass (amorphous) are shown in Fig. (a) and (b) respectively.
  • While the two structures are almost identical, yet in the case of amorphous quartz glass there is no long range order. The structure of amorphous solids is similar to that of liquids.
  • Glass, rubber and plastics are typical examples of amorphous solids.

Characteristics of Amorphous Solids:



  1. The arrangement of constituent particles (atoms, molecules or ions) in such a solid has only short range order. In such an arrangement, a regular and periodically repeating pattern is observed over short distances only. Such portions are scattered and in between the arrangement is disordered.
  2. These are pseudo solids or super cooled liquids. Like liquids, amorphous solids have a tendency to flow, though very slowly. Therefore, sometimes these are called pseudo solids or super cooled liquids.
  3. Amorphoussolidsareisotropiclikeliquids
  4. Heat of fusion is not definite.
  5. Meltingpointisnotsharp.Theymeltover a range of temperature and can be moulded and blown into various shapes.
  6. On heating they become crystalline at some temperature.
  7. Glass panes fixed to windows or doors of old buildings are invariably found to be slightly thicker at the bottom than at the top. This is because the glass flows down very slowly and makes the bottom portion slightly thicker.
  8. Amorphous solids are isotropic in nature. It is because there is no long range order in them and arrangement is irregular along all the directions. Therefore, value of any physical property would be same along any direction

Uses of Amorphous Solids:

  1. Amorphous silica is used in photovoltaic cell which has capacity to convert quantum energy from light into electrical energy.
  2. Glass is amorphous solid, which has wide applications in everyday life and also has industrial uses.
  3. Manufacturing concept of plastic, rubber is based on chemistry of amorphous substance.
  4. Unbreakable glass is an amorphous substance.

Glass:

  • Glass is optically transparent material produced by fusing together silicon oxide with sodium oxide, boron oxide. Trace of transition metal oxide is added to impart colour to the glass.
  • By changing composition 800 types of glasses are produced. Quartz is obtained from silicon oxide only.
  • Composition of different types of glasses is as given below

Type of Glass Composition

Pyrex glass :  60 to 80 % SiO2, 10 to 20 % B2O3, remaining Al2O3.
Soda lime glass :  75% SiO2, 15 % Na2O, 10 % CaO

Trace Colour of glass

Gold, copper :   Red
UO2                        Yellow
CaO, CuO            Blue
Fe2O3, CuO       Green

Interconversion Between Crystalline and Amorphous Substance

  • Crystalline solids can be converted into amorphous form or glassy form by rapidly cooling its melt or rapidly cooling its vapours.
  • Amorphous solids can be converted into crystalline form by heating and cooling it very slowly (annealing).
  • Old glasses becomes translucent due to a small extent of crystallization.

Polycrystalline Solids:

  • Some solids like aluminum and steel have a structure which is intermediate between crystalline and amorphous.
  • They contain crystal consisting of small units with a definite and regular arrangement but the crystals themselves are randomly arranged.

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