CBSE Class 12 Chemistry HOTs The Solid State

Crystalline solids have a sharp melting point and sharp boiling when it reaches a particular temperature.The shape of crystalline solids are definite and having typical arrangements of particles. They show cleavage property i.e. when they are cut with the edge of a sharp tool they split into two pieces and the newly generated surfaces are smooth and plain. They have definite heat of fusion (amount of energy needed to melt a given mass of solid at its melting point).Crystalline solids are anisotropic which means their physical properties like electrical resistance or refractive index show different values when they are measured along with different directions in the same crystal . Crystalline solids are true solids. Crystalline solids are of different types : Ionic solids, Covalent solids , Molecular solids and Metallic Solids.
Amorphous solids are gradually softened over a range of temperature and they can be moulded into different shapes on heating.Amorphous solids are pseudo solids or super cooled liquids which means they have a tendency to flow very slowly. If you observe that the glass panes which is fixed to windows of old buildings they are found to be slightly thicker from the bottom than at the top. Amorphous solids have irregular shape i.e. their constituent particles do not have definite geometry of arrangements. When amorphous solids are cut with a sharp edge tool they form pieces with irregular surfaces. Amorphous solids do not have definite heat of fusion due to its irregular arrangement of the particles. Amorphous solids are isotropic in nature which means the value of any physical property would be same along any direction because of the irregular arrangement of particles.

Question. Why some glass objects from the ancient civilizations are found to become milky
(a) Glass is a crystalline solid, milky appearance is due to its crystalline nature
(b) Glass is amorphous but on heating it become crystalline at some temperature
(c) Glass reacts with the impurities present in air
(d) None of these

Answer: B

Question. Silicon carbide is an example of
(a) Molecular solid
(b) covalent solid
(c) Metallic solid
(d) ionic solid

Answer: B

Question. Which of the following statement is not a characteristic property of amorphous solids
(a) Irregular shape
(b) Gradually soften over a range of temperature
(c) When cut with a sharp edged tool, they cut into two pieces with irregular surfaces
(d) They have a definite and characteristic heat of fusion

Answer: D

Question. Which one of the following is non-crystalline or amorphous?
(a) Diamond
(b) Graphite
(c) Glass
(d) Common Salt

Answer: C

Question. A crystalline solid
(a) has no definite melting point
(b) undergoes deformation of its geometry easily
(c) has irregular three dimensional arrangements
(d) changes abruptly from solid to liquid when heated

Answer: D

The adjective, ‘crystalline’ when applied to solids, implies an ideal crystal in which the structural units, termed as unit cells, are repeated regularly and indefinitely in three dimensions in space. The unit cell, containing at least one molecule has definite orientation and shape defined by the translational vectors, a, b and c. The unit cell therefore has a definite volume, V that contains the atoms and molecules necessary for generating the crystal. Every crystal can be classified as a member of one of the seven possible crystal systems or crystal classes that are defined by the relationships between the individual dimensions, a, b and c of the unit cell and between the individual angles, α, β, and γ of the unit cell. The structure of the given crystal may be assigned to one of the 7 crystal systems, to one of the 14 Bravais lattices, and to one of the 230 space groups. These uniquely define the possible ways of rearranging atoms in a three dimensional solid. Based on these observations, seven crystal systems were identified: triclinic, monoclinic, trigonal or rhombohedral, tetragonal, hexagonal, rhombic or orthorhombic and cubic

Question. The following unit cell with the structure given below represents ............ crystal system    

(a) Tetragonal
(b) Orthorombic
(c) Hexagonal
(d) trigonal

Answer: A

Question. The crystal system of compound with unit cell dimensions a= 10.48, b= 12.92, c= 24.55 Ao and α=β=γ=90ois
(a) Monoclinic
(b) Orthorombic
(c) Hexagonal
(d) Rhombohedral

Answer: B

The discovery of imperfections in an other wise ideally perfect crystal is one of the most fascinating aspects of solid state science. An ideally perfect crystal is one which has the same unit cell and contains the same lattice points throughout the crystal. The term imperfection or defect is generally used to describe any deviation of the ideally perfect crystal from the periodic arrangement of its constituents. If the deviation occurs because of missing atoms, displaced atoms or extra atoms, the imperfection is named as a point defect. Such defects can be the result of imperfect packing during the original crystallisation or they may arise from thermal vibrations of atoms at elevated temperatures because with increase in thermal energy there is increased probability of individual atoms jumping out of their positions of lowest energy. Following diagram show the imperfections :   

Question. Which among the following statement is true about the Schottky defect?
(a) In the defect cation and anion are lacking in stoichiometric proportion
(b) Formation of metal alloy is example of this defect
(c) In this regular cation is replaced by different cation
(d) In this cation or anion moves from regular site to place between lattice site

Answer: A

Question. Which of the following compounds does not show metal deficiency defect?
(a) FeO
(b) FeS
(c) NaCl
(d) NiO

Answer: C

Question. A perfect crystal of silicon is doped with some elements as given in the options.   

Which of these options show n- type semiconductors.

Answer: A

Question. The appearance of colour in solid alkali metal halides is generally due to
(a) Schottky defect
(b) Frenkel defect
(c) Interstitial position
(d) F- centres

Answer: D