Chemistry 12: Chapter –1 Exemplar Solutions-State Solid State Part 5

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Question 47:

A perfect crystal of silicon (Fig. 1.1) is doped with some elements as given in the options. Which of these options show n-type semiconductors?

Pure Crystal

Pure Crystal

(i)

Crystal

Crystal

(ii)

Crystal

Crystal

(iii)

Crystal

Crystal

(iv)

Crystal

Crystal

Answer: (i), (iii)

Question 48:

Which of the following statements are correct?

(i) Ferrimagnetic substances lose ferrimagnetism on heating and become paramagnetic.

(ii) Ferrimagnetic substances do not lose ferrimagnetism on heating and remain Ferrimagnetic.

(iii) Antiferromagnetic substances have domain structures similar to ferromagnetic substances and their magnetic moments are not cancelled by each other.

(iv) In ferromagnetic substances all the domains get oriented in the direction of magnetic field and remain as such even after removing magnetic field.

Answer: (i), (iv)

Question 49:

Which of the following features are not shown by quartz glass?

(i) This is a crystalline solid.

(ii) Refractive index is same in all the directions.

(iii) This has definite heat of fusion.

(iv) This is also called super cooled liquid.

Answer: (i), (iii)

Question 50:

Which of the following cannot be regarded as molecular solid?

(i) (Silicon carbide)

(ii)

(iii) Diamond

(iv)

Answer: (i), (ii), (iii)

Question 51:

In which of the following arrangements octahedral voids are formed?

(i)

(ii)

(iii) Simple cubic

(iv)

Answer: (i). (iv)

Question 52:

Frenkel defect is also known as ________.

(i) Stoichiometric defect

(ii) Dislocation defect

(iii) Impurity defect

(iv) Non-stoichiometric defect

Answer: (i), (ii)

Question 53:

Which of the following defects decrease the density?

(i) Interstitial defect

(ii) Vacancy defect

(iii) Frankel defect

(iv) Schottky defect

Answer: (ii), (iv)

III. Short Answer Type

Question 54:

Why are liquids and gases categorised as fluids?

Answer:

The liquids and gases have a property to flow i.e. the molecules can move past and tumble over one another freely. Hence, they have been categorised as fluids.

Question 55:

Why are solids incompressible?

Answer:

The distance between the constituent particles (atoms, ions, molecules etc.) is very less in solids. On bringing them still closer repulsion will start between electron clouds of these particles. Hence, they cannot be brought further close to each other

Question 56:

Inspite of long range order in the arrangement of particles why are the crystals usually not perfect?

Answer:

Crystals have long range repeated pattern of arrangement of constituent particles but in the process of crystallization some deviations from the ideal arrangement (i.e. defects) may be introduced, therefore, crystals are usually not perfect.

Question 57:

Why does table salt, , sometimes appear yellow in colour?

Answer:

Yellow colour in sodium chloride is due to metal excess defect due to which unpaired electrons occupy anionic sites. These sites are called F-centres. These electrons absorb energy from the visible region for the excitation which makes crystal appear yellow.

Question 58:

Why is not formed in stoichiometric composition?

Answer:

In the crystals of , some of the cations are replaced by ions. Three ions are replaced by two ions to make up for the loss of positive charge. Eventually there would be less amount of metal as compared to stoichiometric proportion.

Question 59:

Why does white becomes yellow upon heating?

Answer:

On heating loses oxygen according to the following reaction.

ions and electrons move to interstitial sites and F-centres are created which impart yellow colour to .

Question 60:

Why does the electrical conductivity of semiconductors increase with rise in temperature?

Answer:

The gap between conduction band and valence band is small in semiconductors (Fig. 1.1), therefore, electrons from the valence can jump to the conduction band on increasing temperature. Thus they become more conducting as the temperature increases.

Semi-Conductor

Semi-Conductor

Fig 1.1: Semi-Conductor

Question 61:

Explain why does conductivity of germanium crystals increase on doping with gallium.

Answer:

On doping germanium with gallium some of the positions of lattice of germanium are occupied by gallium. Gallium atom has only three valence electrons. Therefore, fourth valency of nearby germanium atom is not satisfied. The place remains vacant. This place is deficient of electrons and is therefore called electron hole or electron vacancy. Electron from neighbouring atom comes and fills the gap, thereby creating a hole in its original position. Under the influence of electric field electrons move towards positively charged plates through these holes and conduct electricity. The holes appear to move towards negatively charged plates.

Question 62:

In a compound, nitrogen atoms (N) make cubic close packed lattice and metal atoms (M) occupy one-third of the tetrahedral voids present. Determine the formula of the compound formed by M and N?

Answer:

Question 63:

Under which situations can an amorphous substance change to crystalline form?

Answer:

See page no. 3 of NCERT textbook for Class XII.

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