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The Solid State

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Chemistry I

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Class 10 Class 12
An element crystallises in fcc structure with an edge of 200 pm. Calculate density if 200 g of this element contains 24 x 1023 atoms. 

we have 
edge length =200Pm
volume of the unit = ( 200 x10-10 cm)3
                         = 8 x10-24 cm-3

In a FCC unit cell there are four atoms per unit cell.therefore  mass of unit cell  =200 x 424 x1023 =33.3 x10-23gDensity  =mass of unit cell volume of unit cell =33.3 x10-23g8 x10-24cm3  =41.6 g cm-3

density = 41.6 g cm-3.


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 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 .