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Class 10 Class 12
Ferric oxide crystallizes in a hexagonal close packed array of oxide ions with two out of every three tetrahedral holes occupied by ferric ions. Derive the formula of the ferric oxide.

Let us Suppose the number of oxide (O2–) ions = N.
Number of octahedral void = number of anions
So that the number of octahedral voids = N

We have Given that

Two out of every three octahedral holes are occupied by ferric ions.

So that the number of ferric (Fe3+) ions = 2N/3
The ratio of the number of Fe3+ ions to the number of O2− ions,

Fe3+ : O2− = 2N/3 : N

Multiply 3 and divide by N we get

Fe3+ : O2− = 2 : 3

Hence, the formula of the ferric oxide is Fe2O3.

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 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 .

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.

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