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The Solid State

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Chemistry I

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Class 10 Class 12
The density of chromium is 7.2 g cm–3. If the unit cell is cubic with edge length of 289 pm, determine the type of the unit cell (Atomic mass of Cr = 52 amu).

We have given that 

Gram atomic mass of Cr(M) = 52.0 g mol-1
   Edge length of unit cell (a) = 289 pm
   Density of unit cell (ρ) = 7.2 g cm-3
 Avogadro's Number (N0) = 6.022×1023 mol-1       

     ρ = Z×Ma3×NA×10-30

or              Z=ρ×a3×NA×10-30M
   Z=(7.2 g cm-3) × (289)3× (6.022×1023 mol-3) × (10-30 cm3)(52.0 g mol-1)=2

Since the unit cell has 2 atoms, it is body centre in nature.


Why are solids rigid?

Solids are rigid because the internuclear distance between the  molecules of a solid are very less and closely packed and their positions are fixed due to the strong forces of attraction between them.

Classify the following as amorphous or crystalline solids: Polyurethane, naphthalene, benzoic acid, teflon, potassium nitrate, cellophane, polyvinyl chloride, fibre glass, copper.

Amorphous solids: Polyurethane, teflon, cellophane, polyvinyal chloride, fibre glass.
Crystalline solids: naphthlene, benzoic acid, potassium nitrate, copper.

Why do solids have a definite volume?

A solid have a definite volume because the inter molecular distance  between its molecules are fixed.

Refractive index of a solid is observed to have the same value along all directions. Comment on the nature of this solid. Would it show clevage property?

Refractive index of a solid which have the same value along all directions are isotropic in nature. It would not show cleavage property.

Why is glass considred a super-cooled liquid?

Glass is amorphous solids and  have a tendency to flow, though very slowly. Therefore, these are called pseudo solids or super cooled liquids .