Chapter 1 The Solid State Solutions
Question - 21 : - What type of defect can arise when a solid isheated? Which physical property is affected by it and in what way?
Answer - 21 : -
When a solid is heated, vacancy defect canarise. A solid crystal is said to have vacancy defect when some of the latticesites are vacant.
Vacancy defect leads to a decrease in thedensity of the solid.
Question - 22 : - What type of stoichiometric defect is shown by:
(i)┬аZnS┬а(ii)┬аAgBr
Answer - 22 : -
(i)┬аZnS shows Frenkel defect.
(ii)┬аAgBr shows Frenkel defect as well as Schottky defect.
Question - 23 : - Explain how vacancies are introduced in an ionicsolid when a cation of higher valence is added as an impurity in it.
Answer - 23 : -
When a cation of higher valence is added to anionic solid as an impurity to it, the cation of higher valence replaces morethan one cation of lower valence so as to keep the crystal electricallyneutral. As a result, some sites become vacant. For example, when Sr2+┬аisadded to NaCl, each Sr2+┬аion replaces two Na+┬аions.However, one Sr2+┬аion occupies the site of one Na+┬аionand the other site remains vacant. Hence, vacancies are introduced.
Question - 24 : - A group 14 element is to be converted inton-type semiconductor by doping it with a suitable impurity. To which groupshould this impurity belong?
Answer - 24 : -
An┬аn-type semiconductor conductsbecause of the presence of extra electrons. Therefore, a group 14 element canbe converted to┬аn-type semiconductor by doping it with a group 15element.
Question - 25 : - What type of substances would make betterpermanent magnets, ferromagnetic or ferrimagnetic. Justify your answer.
Answer - 25 : -
Ferromagnetic substances would make betterpermanent magnets.
In solid state, the metal ions of ferromagneticsubstances are grouped together into small regions. These regions are calleddomains and each domain acts as a tiny magnet. In an unmagnetised piece of aferromagnetic substance, the domains are randomly oriented. As a result, themagnetic moments of the domains get cancelled. However, when the substance isplaced in a magnetic field, all the domains get oriented in the direction ofthe magnetic field and a strong magnetic effect is produced.
The ordering of the domainspersists even after the removal of the magnetic field. Thus, the ferromagneticsubstance becomes a permanent magnet.
Question - 26 : - Define the term 'amorphous'. Give a few examplesof amorphous solids.
Answer - 26 : -
Amorphous solids are the solids whoseconstituent particles are of irregular shapes and have short range order. Thesesolids are isotropic in nature and melt over a range of temperature. Therefore,amorphous solids are sometimes called pseudo solids or super cooled liquids.They do not have definite heat of fusion. When cut with a sharp-edged tool,they cut into two pieces with irregular surfaces. Examples of amorphous solidsinclude glass, rubber, and plastic.
Question - 27 : - What makes a glass different from a solid suchas quartz? Under what conditions could quartz be converted into glass?
Answer - 27 : -
The arrangement of the constituent particlesmakes glass different from quartz. In glass, the constituent particles haveshort range order, but in quartz, the constituent particles have both longrange and short range orders.
Question - 28 : - (i)┬аWhat is meant by the term 'coordination number'?
Answer - 28 : -
(ii)┬аWhat is the coordination number of atoms:
(a)┬аin a cubic close-packed structure?
(b)┬аin a body-centred cubic structure?
Answer
(i)┬аThe number of nearest neighbours of any constituent particlepresent in the crystal lattice is called its coordination number.
(ii)┬аThe coordination number of atoms
(a)┬аin a cubic close-packed structure is 12, and
(b)┬аin a body-centred cubic structure is 8
Question - 29 : - How can you determine the atomic mass of anunknown metal if you know its density and the dimension of its unit cell?Explain.
Answer - 29 : -
By knowing the density of an unknown metal andthe dimension of its unit cell, the atomic mass of the metal can be determined.
Let┬атАШaтАЩ be the edge length of a unitcell of a crystal, тАШdтАЩ be the density of the metal,┬атАШmтАЩ┬аbethe mass of one atom of the metal and┬атАШzтАЩ┬аbe the number ofatoms in the unit cell.
Now, density of the unitcell
[Since┬аmass of the unit cell = Number ofatoms in the unit cell ├Ч mass of one atom]
[Volume of the unit cell = (Edge length of thecubic unit cell)3]
From equation (i), we have:
Now, mass of one atom of metal (m)┬а
Therefore,
If┬аthe edge lengths are different(say┬аa,┬аb┬аand┬аc), then equation (ii)becomes:
M┬а=┬аd(abc)NAz┬а┬а┬а┬а┬а(iv)M┬а=┬аd(abc)NAz┬а┬а┬а┬а┬а(iv)
From equations (iii) and (iv), wecan determine the atomic mass of the unknown metal.
'Stability of a crystal is reflected inthe┬аmagnitude of its melting point'. Comment.┬а
Answer - 30 : - Collect melting points of solid water, ethyl alcohol, diethyl ether and methane from a data book. What can you say about the intermolecular forces between these molecules?
Answer
Higher the melting point, greater is theintermolecular force of attraction and greater is the stability. A substancewith higher melting point is more stable than a substance with lower meltingpoint.
The melting points of the given substances are:
Solid water┬атЖТ┬а273 K
Ethyl alcohol┬атЖТ┬а158.8 K
Diethyl ether┬атЖТ┬а156.85 K
Methane┬атЖТ┬а89.34 K
Now, on observing the values of the meltingpoints, it can be said that among the given substances, the intermolecularforce in solid water is the strongest and that in methane is the weakest.