The Solid State

Question 177
CBSEENCH12005525

Give reasons for the following:
Covalent crystals like diamond and silicon carbide are quite hard and difficult to break.

Solution
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.
Question 178
CBSEENCH12005526

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

Solution

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.

Question 179
CBSEENCH12005527

A liquid which is permanently supercooled is frequently called a _____

Solution
glass
Question 180
CBSEENCH12005528

The smallest portion of a crystal which has the structure characteristic of space lattice is called the__________.

Solution
unit cell

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