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Q1 Why are solids rigid? Ans: Solids have a definite volume & shape. They are rigid & lack the ability to flow.In solids, the atoms,ions & molecules are held together by relatively strong chemicalmforces-ionic bond, covalent bond, or by intermolecular Van der waals forces. They do not translate although they vibrate to some extent in their fixed positions.The interparticle spaces or voids are so small that their movement is almost negligible.
Q2 Why do solids have a definite volume? Ans: Gases & liquids can flow & take up the shape of their container because they have weak intermolecular forces between them & the ion are not arranged in a regular method. As a result, when certain pressure or temperature is applied to them,their ions or molecules rearranges themselves & show flow properties. But in case of solids, the ions or molecules are arranged in a regular & repeated three dimensional pattern. So, whenever pressure or heat is applied to solids ,their ions or molecules do not rearranges themselves, and hence do not show flow properties & have a definite volume.
Q3 Classify the following as amorphous or crystalline solids: Polyurethane, naphthalene, benzoic acid, teflon, potassium nitrate, cellophane, polyvinyl chloride, fibre glass, copper. Ans: Solids are classified into 2 types:
Crystalline solids
Amorphous solids
Crystalline solids-exists as small crystals,each crystal having a characterstic geometrical shape.The atoms,ions or molecules are arranged in a regular, repeating three dimensional pattern called the crystal lattice.
Amorphous solids( Greek term amorphous-no form)- has atoms, molecules or ions arranged at random & lacks the ordered crystalline lattice.
Naphthalene, benzoic acid, potassium nitrate, copper
Polyurethane, teflon, cellophane, polyvinyl chloride, fibre glass
Q4 Why is glass considered a super cooled liquid? Ans: Amorphous solids( Greek term amorphous-no form)- has atoms, molecules or ions arranged at random & lacks the ordered crystalline lattice. For eg glass, plastics. In their disordered structure,amorphous solids resemble liquids.Being a amorphous solid, glass has atoms arranged in random order with weak intermolecular forces, when ever heat is applied in gradual pattern the glass behave as a super cooled liquid or highly viscous liquids. The liquid nature of glass is sometimes apparent in very old window panes that have become slightly thicker at the bottom due to gradual downward flow.
Q5 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 cleavage property? Ans: The solids are classified into two types:
a) Crystalline /true solids
b) Amorphous solids
To distinguish between crystalline and amorphous solids, isotropy and anisotropy plays a vital role.
Isotropy – The substances which have the same physical properties such as electrical conductivity, thermal conductivity, mechanical strength, refractive index etc. in all the directions in the space are called isotropic substances and this property is called as isotropy.
Anisotropy – The substances which have physical properties such as electrical conductivity, thermal conductivity, mechanical strength, refractive index etc. different in different directions in the space are called anisotropic substances and the property is called as anisotropy.
So a isotropic solid will have same refractive index in all the directions. Hence, amorphous solids are isotropic in nature. The reason for this is that in amorphous solids the arrangement of particles is random and disordered, therefore all directions are equivalent and properties are independent of direction .On the other hand, the particles in a crystalline solids are arranged & well ordered. Thus the arrangement of particles may be different in different directions.
CLEAVAGE-A crystalline solid when cut with a sharp edged tool such as knife gives a clean cleavage but an amorphous solid undergoes an irregular cleavage i.e. when an amorphous solid is cut with a sharp edged tool, it cuts into two pieces with irregular surfaces.
Q6 Classify the following solids in different categories based on the nature of intermolecular forces operating in them: Potassium sulphate, tin, benzene, urea, ammonia, water, zinc sulphide, graphite, rubidium, argon, silicon carbide. Ans: Potassium sulphate → Ionic bond is formed, wherein potassium loses electrons & sulphate gains electrons.
Tin → Tin can form two ions Sn 2+ or Sn 4+ now depending on what form it is in and what other element it is bonding with then only the type of bonding will be known.
Benzene → It comprises of sp2 hybridized carbon atoms which makes covalent bond with each other by mutual sharing and pairing of electrons.
Urea → Polar molecular solid and they mostly forms hydrogen bond with other atoms.
Ammonia → Covalently bonded polar compound formed by mutual sharing and pairing of electrons between nitrogen and three hydrogen atoms.
Water → Covalently bonded polar compound formed by mutual sharing of electrons between oxygen and hydrogen atoms.
Zinc sulphide → It exist in the form of crystals and makes coordination bond.
Graphite → It consist of carbon atoms and have covalent bond between them forming a 2-D (flat, planar etc.) network, which in itself is strong. These planes are then held together by London Forces, which are very weak.
Rubidium → Metallic bond is formed by rubidium because it is a metal. Metallic bond is hydride of covalent bond and ionic bond.
Argon → Non-polar molecular solid
Silicon carbide → Covalent or network solid
Q7 Solid A is a very hard electrical insulator in solid as well as in molten state and melts at extremely high temperature. What type of solid is it? Ans: The solids which are very hard have their atoms bind to each other by covalent forces which are very strong, as a result they have high melting and boiling points and do not conduct heat are called as covalent crystals (crystalline solids). Hence, solid A shows the properties of an covalent crystalline solids.
For example diamond (C), quartz (SiO2), & silicon carbide SiC)
Q8 Ionic solids conduct electricity in molten state but not in solid state. Explain. Ans: In an ionic solids the lattice is made of positive and negative ions. These are held together by ionic bonds-the strong electrostatic attractions between oppositely charged ions. Consequently, the cations and anions attract each other and pack together in an arrangement so that the attractive forces maximize. The ionic solids are insulators in the solid state because the ions are entrapped in fixed places in the crystal lattice and cannot move when electric field is applied. However, in molten state, they become good conductors of electricity. This is due to the fact that in molten state, the well- ordered arrangement of ions in the solids is destroyed and the ions are in a position to move about in the liquid medium when an electric field is applied. For example NaCl.
Q9 What type of solids are electrical conductors, malleable and ductile? Ans: Metallic solids are electrical conductors, malleable, and ductile. They consists of an assemblage of positive ions (kernels) immersed in a ‘sea’ of mobile electrons. Thus, each electron belongs to a number of positive ions and each positive ion belongs to a number of electrons. The force that binds a metal ion to a number of electrons within its sphere of influence is known as metallic bond. This force of attraction is sufficiently strong and is thus responsible for compact solid structure of metals and therefore these solids are good conductors, malleable and ductile in nature. For example metals like copper, nickel etc.
Q10 Give the significance of a lattice point. Ans: In a crystalline solid the constituent particles (atoms, ions) are arranged in a definite and regular order in space and the relative positions of such particles in a crystal are shown by points (.) The arrangement of these points in a crystal is called space lattice. The location of the points in the space lattice are called lattice points or lattice sites. These points are linked by lines to depict the picture of a space lattice. These lattice points arranges in repeated pattern in different directions to form the complete lattice known as the unit cell. These unit cells are of different types and constitutes of different solids.
Q11 Name the parameters that characterize a unit cell. Ans: A unit cell is the smallest repeating unit in space lattice which when repeated over and over again produces the complete space lattice. It represents the shape of entire crystal. The parameters which characterizes a unit cell are as follows:
1 Relative lengths (distance) of the edges along the three axis (a, b, c). These edges may or may not be equal.
2 Angle between the edges (α, β, γ).The angle α is between the edges b & c, angle β is between the edges c & a, and angle γ is between the edges a & b.
Q12 Distinguish between (i)Hexagonal and monoclinic unit cells (ii) Face-centred and end-centred unit cells. Ans: (i) Hexagonal unit cell
For a hexagonal unit cell,
Monoclinic unit cell
For a monoclinic cell,
(ii) Face-centred unit cell
In a face-centred unit cell, the constituent particles are present at the corners and one at the centre of each face.
End-centred unit cell
Anend-centred unit cell contains particles at the corners and one at the centre of any two opposite faces.
Q13 Explain how much portion of an atom located at (i) corner and (ii) body-centre of a cubic unit cell is part of its neighbouring unit cell. Ans: (i) An atom located at the corner of a cubic unit cell is shared by eight adjacent unit cells.
Therefore, 1/8 portion of the atom is shared by one unit cell.
(ii) An atom located at the body centre of a cubic unit cell is not shared by its neighbouring unit cell. Therefore, the atom belongs only to the unit cell in which it is present i.e., its contribution to the unit cell is 1.
Q14 What is the two dimensional coordination number of a molecule in square close packed layer? Ans: Square Close Packing: In this type, the spheres are packed in such a way that the centres of all of them are in straight line. This means that the different rows have horizontal as well as vertical alignment. Since each sphere is in contact with four other spheres in the same plane, this packing is called square close packing.
Coordination Number: The number of spheres which are touching a given sphere is called the coordination number. In square close-packed layer, a molecule is in contact with four of its neighbours. Therefore, the two dimensional coordination number of a molecule in square close-packed layer is 4.
Q15 A compound forms hexagonal close-packed structure. What is the total number of voids in 0.5 mol of it? How many of these are tetrahedral voids? Ans: Number of close-packed particles = 0.5 x 6.022 x 1023 = 3.011 x 1023
Therefore, number of octahedral voids = 3.011 x 1023
And, number of tetrahedral voids = 2 x 3.011 x 1023 = 6.022 x 1023
Therefore, total number of voids = 3.011 x 1023 + 6.022 x 1023 = 9.033 x 1023
Q16 A compound is formed by two elements M and N. The element N forms ccp and atoms of M occupy 1/3rd of tetrahedral voids. What is the formula of the compound? Ans: The ccp lattice is formed by the atoms of the element N.
Here, the number of tetrahedral voids generated is equal to twice the number of atoms of the element N.
According to the question, the atoms of element M occupy 1/3rd of the tetrahedral voids.
Therefore, the number of atoms of M is equal to 2x1/3 = 2/3rd of the number of atoms of N.
Therefore, ratio of the number of atoms of M to that of N is M: N =2/3 :1 = 2 : 3
Thus, the formula of the compound is M2N3.
Q17 Which of the following lattices has the highest packing efficiency (i) simple cubic (ii) body-centred cubic and (iii) hexagonal close-packed lattice? Ans: Hexagonal close-packed lattice has the highest packing efficiency of 74%. The packing efficiencies of simple cubic and body-centred cubic lattices are 52.4% and 68% respectively.
Q18 An element with molar mass 2.7 x 10-2kg mol-1 forms a cubic unit cell with edge length 405 pm. If its density is 2.7 x 103 kg m-3, what is the nature of the cubic unit cell? Ans: It is given that density of the element, d = 2.7 ×103 kg m-3
Molar mass, M = 2.7 ×10-2 kg mol-1
Edge length, a= 405 pm = 405 ×10-12 m = 4.05 ×10-10 m
It is known that, Avogadro's number, NA= 6.022 ×1023 mol-1
Applying the relation, This implies that four atoms of the element are present per unit cell. Hence, the unit cell is face-centred cubic (fcc) or cubic close-packed (ccp).
Q19 What type of defect can arise when a solid is heated? Which physical property is affected by it and in what way? Ans: When a solid is heated, vacancy defect can arise. A solid crystal is said to have vacancy defect when some of the lattice sites are vacant.
Vacancy defect leads to a decrease in the density of the solid.
Q20 What type of stoichiometric defect is shown by: (i) ZnS (ii) AgBr Ans: (i) ZnS shows Frenkel defect.
(ii) AgBr shows Frenkel defect as well as Schottky defect.
Q21 Explain how vacancies are introduced in an ionic solid when a cation of higher valence is added as an impurity in it. Ans: When a cation of higher valence is added to an ionic solid as an impurity to it, the cation of higher valence replaces more than one cation of lower valence so as to keep the crystal electrically neutral. As a result, some sites become vacant. For example, when Sr2+ is added to NaCl, each Sr2+ ion replaces two Na+ ions. However, one Sr2+ ion occupies the site of one Na+ ion and the other site remains vacant. Hence, vacancies are introduced.
Q22 Ionic solids, which have anionic vacancies due to metal excess defect, develop colour. Explain with the help of a suitable example. Ans: The colour develops because of the presence of electrons in the anionic sites. These electrons absorb energy from the visible part of radiation and gets excited.
For example, when crystals of NaCl are heated in an atmosphere of sodium vapours, the sodium atoms get deposited on the surface of the crystal and the chloride ions from the crystal diffuse to the surface to form NaCl with the deposited Na atoms. During this process, the Na atoms on the surface lose electrons to form Na+ ions and the released electrons diffuse into the crystal to occupy the vacant anionic sites. These electrons get excited by absorbing energy from the visible light and impart yellow colour to the crystals.
Q23 A group 14 element is to be converted into n-type semiconductor by doping it with a suitable impurity. To which group should this impurity belong? Ans: An n-type semiconductor conducts because of the presence of extra electrons. Therefore, a group 14 element can be converted to n-type semiconductor by doping it with a group 15 element.
Q24 What type of substances would make better permanent magnets, ferromagnetic or ferrimagnetic. Justify your answer. Ans: Ferromagnetic substances would make better permanent magnets.
In solid state, the metal ions of ferromagnetic substances are grouped together into small regions. These regions are called domains and each domain acts as a tiny magnet. In an un-magnetised piece of a ferromagnetic substance, the domains are randomly oriented. As a result, the magnetic moments of the domains get cancelled. However, when the substance is placed in a magnetic field, all the domains get oriented in the direction of the magnetic field and a strong magnetic effect is produced.
The ordering of the domain persists even after the removal of the magnetic field. Thus, the ferromagnetic substance becomes a permanent magnet.