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Class 10 Class 12
Give the significance of a 'lattice point'.

(i) Each point in a lattice is called lattice point or lattice side.
(ii) Each point in a crystal lattice represents one constituent particle which may be an atom, a molecule or an ion.
(iii) Every lattice point has identical surroundings except those which are on the surface or corner of a crystal.
(iv) If a line is drawn by joining two lattice points, it would pass through a series of similar points at regular intervals.
(v) Lattice points are joined by straight lines to bring out the geometry of the lattice. These lines do not represent chemical bond.

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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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Why do solids have a definite volume?

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