Chemistry: Solid State: Properties of Crystalline Solids: Melting Point of a Solids (For CBSE, ICSE, IAS, NET, NRA 2022)

Doorsteptutor material for CBSE/Class-6 is prepared by world's top subject experts: get questions, notes, tests, video lectures and more- for all subjects of CBSE/Class-6.

Properties of Crystalline Solids

  • Crystalline solids have smooth surfaces. These are called ‘faces’ of the crystal.
  • These faces are developed in the process of crystal formation by ordered arrangements of the constituent particles.
  • It is generally observed that the faces of crystals are developed unequally.
  • The internal angle between a pair of faces is called interfacial angle and is defined as the angle between the normal to the intersecting faces.
  • In crystalline solids the interfacial angle between a pair of faces is always the same.
  • This was stated by Steno as the law of constancy of interfacial angles
Crystalline Solids

Melting Point of a Solids

  • When a solid is heated it becomes hot and eventually gets converted into a liquid. This process is called as melting.
  • The temperature at which a solid melts to give a liquid is called as melting point.
  • Every solid is characterized by a definite melting point and it is a test of the purity of the solid and it gives an idea about the nature of binding forces between constituent particles.
  • The solids like sodium chloride have very high melting points due to strong columbic forces between the ions.
  • The molecular solids like naphthalene have low melting points.
  • The effect of heat on a solid can be understood in terms of energy and motion of the constituent particles.
  • In a solid the constituent particles vibrate in their mean position.
  • When solid is heated, the constituent particles gain energy and vibrate vigorously in their equilibrium positions.
  • When heat is increases, the energy also increases, and it becomes greater than the binding forces between them and it results in the solid converted into a liquid.

Crystal Lattice and Unit Cells

  • The constituent particles in crystalline solids have long-range order, they are closely packed and are arranged in three-dimensional pattern.
  • The points in 3-D pattern represents the location of a constituent particle and is known as lattice point and such an arrangement is called a crystal lattice or space lattice or simply a lattice.

Two-Dimensional Lattice and Unit Cells

  • A two-dimensional lattice has a set of points arranged in a regular pattern on a plane or a surface.
  • The arrangement of lattice points is as shown in Figure below:
Two Dimensional Lattice

When these lattice points are joined, the geometry of the lattice becomes clear as shown in figure below:

Unit Cells
  • The entire pattern can be generated by repeating a set of four points. When these points are joined, we get a unit cell of the lattice.
  • The unit cell of a two-dimensional lattice is a parallelogram which is defined in terms of two sides a and b and the angle between them are represented as .
  • Only five types of unit cells are possible in two-dimensional lattice. These are shown in figure below:
Lattice and Unit Cells

Developed by: