54. Which of the following statements are not true?
(i) Vacancy defect results in a decrease in the density of the substance.
(ii) Interstitial defects results in an increase in the density of the substance.
(iii) Impurity defect has no effect on the density of the substance.
(iv) Frankel defect results in an increase in the density of the substance.
54. Which of the following statements are not true?
(i) Vacancy defect results in a decrease in the density of the substance.
(ii) Interstitial defects results in an increase in the density of the substance.
(iii) Impurity defect has no effect on the density of the substance.
(iv) Frankel defect results in an increase in the density of the substance.
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1 Answer
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54. Options (iii) and (iv) are the statements which are not true since in impurity defect there is a replacement of ions by the ions of another compound. By doing so, even though the electrical neutrality of the solid is maintained, a vacancy defect is caused due to creation of vacant sites in the ionic solid. This results into change in density of the solid. For example, if molten NaCl containing a little amount of SrCl2 is crystallised some of the sites of Na+ are occupied by Sr2+ .
Each Sr2+ replaces two Na+ ions and hence this affects density. Frenkel defect causes vacancy defect at its original site and an interstitial d
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· Density of solid decreases in Schottky defect and vacancy defect.
· Density of solid increases in interstitial defect.
· Density of solid remain unchanged in Frenkel defect.
77. Option (iii) Assertion is correct statement, but reason is wrong statement is the answer since intermediate conductivity in semiconductor is due to the small energy gap between valence band and conduction band and hence the assertion is correct, but the reason is wrong.
76. Option (ii) Assertion and reason both are correct statements, but reason is not correct explanation for assertion is the answer since in fcc structure the number of atoms present = 4 per unit cell which provides a maximum packing efficiency of 74%.
75. Option (iii) Assertion is the correct statement, but the reason is wrong is the answer since besides the body centre there is one octahedral void at the centre of each 12 edges which is shared between four adjacent unit cells. Thus, Octahedral voids present at the body centre of the cube = 1
12 octahedral voids are located at each edge and shared between four-unit cells = 12 * = 3
Total number of octahedral voids = 4.
74. Option (ii) Assertion and reason both are correct statements, but reason is not correct explanation for assertion is the answer since in graphite carbon atoms are arranged in different layers and each atom is covalently bonded to three of its neighbouring atoms in the same layer due to which the fourth valence electron of each atom, present between the different layers is free to move and this makes graphite a good conductor of electricity.
As different layers can slide over each other which makes it soft. while in case of diamond C atom is bonded covalently with their adjacent atoms throughout the crystal. Thus, all the four v
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