Law of Constancy Of Interfacial Angles

The crystals of a substance are obtained by cooling the liquid (or the melt) or the solution of that substance. The size of the crystals depends upon

  Shapes of crystals (crystal habits) of a particular substance.

The of cooling. If cooling is carried out slowly, crystals of large size are obtained because the particles (viz. ions, atoms or molecules) get sufficient time to arrange themselves in proper positions. Similarly, it is found that the shape of the crystals of a substance also depends upon the conditions under which the crystallization takes place i.e. in the absence or presence of any other substance etc. For example, shapes of the crystals of a particular substance are shown in Fig.

However inspire of the differ sizes and shapes of the crystal of the same substance, the following regularity is observed:

The angles between the corresponding faces called the interfacial angles of the crystals of a particular substance are always the same. This is called the law of constancy of interfacial angles. The instrument used for the measurement of interfacial angles is called a goniometer.

Further it is found that the form or shape (or habit ) of the crystals of a substance also depends upon the conditions under which the crystallization takes place, i.e., in the presence or absence of any other substance, etc. For example, sodium chloride grows as cubes form water solution but as octahedral form 15% aqueous urea solution.

A large variety of shapes of the crystalline substances have been observed. One is, therefore, tempted to classify the variety of shapes on some basis. Such classification is based upon the symmetries exhibited by the crystals and their internal structure. In the next section, we shall , therefore, discuss the elements of symmetry possessed by crystals and molecules.

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