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The Solid State

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Chemistry I

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CBSE Gujarat Board Haryana Board

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Class 10 Class 12
An element ‘A’ of atomic mass 100 having bcc structure has unit cell edge of 400 pm. Calculate the density of ‘A’ and the number of unit cells for 10 g of ‘A’.

length of the unit cell edge = 400Pm =400x10-10 cm
 volume of the unit cell  = ( 400 x10-10cm)3
                                 = 6.4 x 10-23 cm-3

as the element A forms a body centred  cubic lattice so no of atom per units cell is 2

z= 2 atoms unit cell

atomic mass of the element  =100 g/mol
density of element is given by

md= M xZa3 xNA d= 100 x26.4 x 10-23 x 6.023 x 10 23 = 51.88 g/cm3volume of 10 g of A = massDensity                             = 10g5.188g cm-3                           = 1.9275 cm3number of unit cell in 1.9275 cm3 volume  =volume of the substance unit cell volume                                                           = 3.0 x1022 unit cell


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.

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 .

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.