What makes the crystal of KCl appear sometimes violet?

This is due to F centre, electron become exicted and when they come to ground state they radiate energy in visible reigon.
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Give reasons for the following:
Covalent crystals like diamond and silicon carbide are quite hard and difficult to break.

In covalent crystals, the shared pair of electrons give rise to very strong forces between them, therefore covalent solids like diamond and silicon carbide are quite hard and difficult to break and show very high melting points.
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Name the crystal defect which lowers the density of an ionic crystal

Density of crystal is lowered by shottky defect.

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Give reasons for the following:
Ionic solids, inspite of being made up of ions, do not conduct electricity.

Solid ionic compounds do not conduct electricity because the ions (charged particles) are locked into a rigid lattice or array. The ions cannot move out of the lattice, so the solid cannot conduct electricity.

When molten, the ions are free to move out of the lattice structure.

 

  • Cations (positive ions) move towards the negative electrode (cathode) 
    M+ + e -----> M

     

  • Anions (negative ions) move towards the positive electrode (anode) 
    X- -----> X + e

When an ionic solid is dissolved in water to form an aqueous solution, the ions are released from the lattice structure and are free to move so the solution conducts electricity just like the molten (liquid) ionic compound.

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What is the effect of Schottky and Frenkel defects on the density of crystalline solids?


Schottky defect decrease the density solid where as no such defect arise due to frenkel defect.

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