CHAPTER ONE The Solid State TOPIC 1 General Characteristics of Solid State As we know, matter can exist in three states namely solid, liquid and gas. For different applications, we need solids with widely different properties, which depend upon the nature of constituent particles and the binding forces operating among them. Solids differ from liquids, and gases in the fact that gases and liquids possess fluidity, i.e. they can flow and hence called fluids, whereas solids do not possess fluidity, instead they possess rigidity. The fluidity in gases and liquids is due to the fact that the molecules are free to move about, while rigidity in solids is due to the fixed positions of their constituent particles which can only oscillate about their mean positions. Therefore, under a given set of conditions of temperature and pressure, which of these would be the most stable state of a given substance depends upon the net effect of two opposing factors: (i) Intermolecular forces which tend to keep the constituent particles closer. (ii) Thermal energy which tends to keep them apart by making them move faster. At sufficiently low temperature, thermal energy is low and intermolecular forces bring molecules so close that they cling to one another and occupy fixed position and the substance exists in solid state. The solids possess following characteristic properties: (i) Solids have definite mass, volume and shape. (ii) They have strong intermolecular forces. (iii) They are incompressible and rigid. (iv) They have short intermolecular distances. Chapter Checklist General Characteristics of Solid State Structure of Solids Crystal Defects and Properties of Solids 1
2 ll ne Chemistry Class 12th (v) The constituent particles (atoms, molecules or ions) have fixed position and can only oscillate about their mean position. Classification of Solids Solids are classified into two categories on the basis of the nature of order present in arrangement of their constituent particles: 1. Crystalline Solids A crystalline solid usually consists of a large number of small crystals, each of them having a definite characteristic geometrical shape. In a crystal, the arrangement of constituent particles is ordered. It has long range order which means that there is a regular pattern of arrangement of particles which repeats itself periodically over the entire crystal. Sodium chloride and quartz are typical examples of crystalline solids. Crystalline solids have a sharp melting point that means they are true solids. Nature Crystalline solids are anisotropic in nature. This arises from different arrangement of particles in different directions. Since, the arrangement of particles is different along different directions, the value of same physical property is found to be different along each direction. B D 2. Amorphous Solids The term amorphous is derived from the Greek word amorphos which means no form. An amorphous solid consists of particles of irregular shape. The arrangement of constituent particles in such a solid has only short range order. A regular and periodically repeating pattern is observed over short distances only. Such portions are scattered and in between the arrangement is disordered. Quartz glass, rubber and plastic are typical examples of amorphous solids. Amorphous solids soften over a range of temperature and can be moulded and blown into various shapes. On heating, they become crystalline at some temperature. That s why a few glass objects from ancient civilisations are found to become milky in appearance due to crystallisation. Like liquids, amorphous solids have a tendency to flow but very slowly. Therefore, they are called pseudo solids or supercooled liquids. The tendency to flow in amorphous solids is illustrated by the fact that the glass pans fixed to windows or doors of old buildings are found slightly thicker at the bottom than at the top. This is because the glass flows down very slowly and makes the bottom portion slightly thicker. Nature Amorphous solids are isotropic in nature. It is because there is no long range order in them and arrangement is irregular along all the directions. Isotropy It is the property due to which amorphous substances show identical electrical and optical properties in all directions. The structures of quartz (a crystalline) (Fig. a) and quartz glass (Fig. b) (amorphous) are shown in fig. below, C A Anisotropy in crystals is due to different arrangement of particles along different directions (a) (b) Anisotropy It is the property due to which crystals show different electrical and optical properties in different planes of the same crystal. The structure of amorphous solids is similar to that of liquids. Due to difference in the arrangement of constituent particles, the two types of solids differ in their properties. 2
ll ne The Solid State 3 Differences between crystalline and amorphous solids Basis of difference Shape Melting point Cleavage property Nature Order in arrangement of constituent particles Uses Crystalline solids They have definite characteristic geometrical shape. They melt at a sharp and characteristic temperature. On cutting with sharp edged tool, they split into two pieces and the newly generated surfaces are plain and smooth. They are anisotropic in nature and called true solids. They have long range order. Calcite crystal is used in making optical instrument such as prism. Amorphous solids They have irregular shape. They melt over a range of temperature. They give irregular surfaces. They are isotropic in nature and called pseudo solids or supercooled liquids. They have short range order. Amorphous silicon is used as photovoltaic material for conversion of sunlight into electricity. Examples Cu, Ag, Fe, S, etc. Glass, rubber, plastic, etc. Classification of Crystalline Solids Crystalline solids are classified (on the basis of nature of intermolecular forces) into four categories: 1. Molecular Solids In molecular solids, constituent particles are molecules. These are again classified on the basis of type of forces existing between the molecules. (a) Non-polar Molecular Solids Constituent particles are either atoms (noble gas Ar, He) or the molecules formed by non-polar covalent bonds (H 2, Cl 2 and I2). In these solids, atoms or molecules are held together by weak dispersion forces or London forces. These solids are soft, non-conductors of electricity and have low melting point and exist in gaseous or liquid state at room temperature and pressure. (b) Polar Molecular Solids They consist of molecules having polar covalent bonds. In these solids, molecules are held together by relatively stronger dipole-dipole interactions. These solids are soft, non-conductors of electricity and have comparatively higher melting point than that of non- polar molecules. They exist in gaseous or liquid state at room temperature, e.g. solid SO 2 and solid NH 3. (c) Hydrogen Bonded Molecular Solids They consist of molecules containing polar covalent bonds between H and F, O or N atoms. Strong H-bonding binds molecules of such solids (e.g. H2O ice). These solids are volatile liquids or soft solids at room temperature and pressure. They are also non-conductors of electricity. Generally, they are volatile liquids or soft solids at room temperature. 2. Ionic Solids In these solids, constituent particles are ions. Cations and anions are arranged in three-dimensional space. These ions are held together by strong coulombic (electrostatic) forces. These solids are hard, brittle and have high melting and boiling points. In solid state, ionic solids are electrical insulators because ions are not free to move. But in molten state or in aqueous solution, they conduct electricity because ions become free to move. 3. Metallic Solids In these solids, constituent particles are positively charged metal ions called Kernels and free electrons. These electrons can easily flow throughout the metal crystal like water in the sea. Hence, we call it a sea of free electrons. Each metal atom contributes one or more electrons toward this sea of mobile electrons. These free and mobile electrons are responsible for high electrical and thermal conductivity of metals. Metal possesses lustre and colour in some cases. due to the presence of free electrons in them. Metals are highly malleable and ductile because of unlike ionic crystals, the position of the positive ions can be altered without destroying the crystal. Metals possess high melting point due to the strong metallic bonds. Metallic bond The force that holds the metal ions together in the crystal, is called metallic bond. 3
4 ll ne Chemistry Class 12th 4. Covalent or Network Solids A wide variety of crystalline solids of non-metals result from the formation of covalent bonds between adjacent atoms throughout the crystal. They are also called giant molecules. Covalent bonds are strong and directional in nature, therefore atoms are held very strongly at their positions. These solids are very hard, brittle and have extremely high melting points. They are insulators and do not conduct electricity, e.g. diamond and silicon carbide. Special Case of Covalent Solids : Graphite Graphite is also a covalent solid but it is a soft solid and a good conductor of electricity. In graphite, carbon atoms are arranged in different layers and each atom is covalently bonded to other three of its neighbouring atoms in the same layer. The fourth valence electron is free in each carbon atom. These free electrons make graphite a good conductor of electricity. In graphite, different layers can slide one over the other. For this reason, graphite is soft and acts as a good solid lubricant. The structure of diamond and graphite is shown below: 141.5 pm 340 pm Network structure of diamond Network structure of graphite Types of solids Molecular solids Constituent particles Binding/attractive forces (a) Non-polar Molecules Weak dispersion or London forces (b) Polar Dipole-dipole interactions (c) Hydrogen bonded Hydrogen bonding lonic solids Ions Coulombic (electrostatic) forces of attraction Metallic solids Covalent or network solids Positive ions in a sea of mobile electrons Metallic bonding Different types of solids Physical nature Melting and boiling point Electrical conductivity Examples Soft Very low Insulator He, Ar, Kr, H 2, Cl 2, CH4, I2, CO2, CCl4 Soft Low Insulator HCl, SO 2 Hard Low Insulator H O 2 (ice) Hard but brittle Hard but malleable and ductile Atoms Covalent bonding Very hard except graphite (soft) High Fairly high Very high Insulators in solid state but conductors in molten state and in aqueous solution Conductors in solid state as well as in molten state Insulators except graphite KCl, NaCl, CuSO 4, CaF 2, CsCl, ZnS, MgO Fe, Cu, Mg, Co, Al, Au, Ag SiO 2 (quartz), SiC, C (diamond), AlN, C (graphite) 4
Very Short Answer Type Questions [1 Mark] 1. Why are solids rigid? Sol. Because of strong attractive forces, particles have fixed positions and they can only oscillate about their mean position. So, solids are rigid. 2. Why do solids have a definite volume? Sol. Strong attractive forces bind the solid particles and thus they have fixed positions, so they cannot show motion and possess definite volume. 3. Why is glass considered a supercooled liquid? Sol. Glass is an amorphous solid. When heated it softens and then starts flowing without changing state, thus it is considered as a supercooled liquid. 4. 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? Sol. SiO 2 (quartz), covalent (network) solid. 5. Ionic solids conduct electricity in molten state but not in solid state. Explain. Sol. In ionic solids, the ions are not free to conduct electricity. However, in molten state the ions become free to move, so they conduct electricity. 6. What types of solids are electric conductors, malleable and ductile? Sol. Metallic solids 7. Which types of solids have definite enthalpy of fusion? Sol. Crystalline solids have definite enthalpy of fusion. 8. Which types of solids have sharp melting points? Sol. Crystalline solids have sharp melting points. 9. Which type of intermolecular force is present in ice? Sol. H-bonding is present in ice. 10. Why are amorphous solids called pseudo solids or supercooled liquids? Sol. Amorphous solids have tendency to flow like liquids. Hence, these are called pseudo solids or supercooled liquids. 11. What is the number of carbon atoms per unit cell of diamond unit cell? Sol. The diamond lattice contains an fcc Bravais point lattice which have two identical atoms per lattice point. The diamond lattice contains 4 lattice points per unit cell but contains 8 atoms per unit cell. 12. Why are liquids and gases categorised as fluids? NCERT Exemplar Sol. Liquids and gases have the tendency to flow that means their molecules can move freely from one place to another. Therefore, they are known as fluids. 13. Inspite of long range order in the arrangement of particles, why are the crystals usually not perfect? NCERT Exemplar Sol. Crystals have long range repeated pattern of arrangement of constituent particles but in the process of crystallisation, some deviations from the ideal arrangement (i.e. defects) may be introduced. Therefore, crystals are usually not perfect. 5
6 ll ne Chemistry Class 12th Short Answer Type Questions [2 Marks] 1. Classify the following as amorphous or crystalline solids, Polyurethane, naphthalene, benzoic acid, teflon, potassium nitrate, cellophane, polyvinyl chloride, fibre glass, copper. Sol. Crystalline solids Naphthalene, benzoic acid, potassium nitrate, copper. (1) Amorphous solids Polyurethane, teflon, polyvinyl chloride, fibre glass, cellophane. (1) 2. Metallic solids are conductors of heat and electricity. Why? Sol. Metallic solids are collections of positive ions embedded in a sea of electrons around it. These free electrons are responsible for electric conduction in metallic solids. (2) 3. Graphite is a conductor of electricity. Give reasons. Sol. In graphite, three electrons of each carbon atom are bonded to three other C atoms in covalent bonding. The fourth valence electron of each atom forms a delocalised layer of free electrons. These free electrons are responsible for the conduction of electricity. That s why, graphite is a conductor of electricity. (2) 4. Why glass panes fixed to windows or doors of old buildings are found to be thicker at the bottom? Sol. Glass panes fixed to windows or doors of buildings are found to become thicker at the bottom because the glass flows down slowly and makes the bottom portion thicker. (2) 5. Some glass objects from ancient civilisations are found to become milky in appearance. Explain. Sol. Some glass objects from ancient civilisations are found to become milky in appearance because of crystallisation of glass. (2) Short Answer Type Questions [3 Marks] 1. 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. Sol. Covalent solids Graphite, silicon carbide. Molecular solids Benzene, ammonia, argon, urea, water. Ionic solids Potassium sulphate, zinc sulphide. Metallic solids Rubidium, tin. (3) 6. Define the term amorphous. Give few examples of amorphous solids. NCERT Sol. The term amorphous is derived from Greek word amorphos which means no form. In amorphous solids, the constituent particles have a random disordered arrangement. These are not true solids, and are also called pseudo solids, e.g. glass, rubber, plastics, etc. (2) 7. Under which situation can an amorphous substance change to crystalline form? NCERT Exemplar Sol. As a result of slow heating and cooling, over a long period, an amorphous solid acquires some crystalline character. (2) 8. Why are solids incompressible? NCERT Exemplar Sol. The distance between the constituent particles 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 together and are incompressible. (2) 9. What makes a glass different from a solid such as quartz? Under what conditions could quartz be converted into glass? NCERT Sol. Glass is an example of amorphous solid in which the constituent particles (SiO 4, tetrahedral) have only a short range order and there is no long range order whereas in quartz, the constituent particles (SiO 4, tetrahedral) have short as well as long range order. On melting and then cooling rapidly, it is converted into glass. (2) 2. 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? Sol. As solid has the same value of refractive index, along all directions it is isotropic and thus amorphous. Therefore, it would not show a clean cleavage when cut with a knife, it would break into pieces with irregular surfaces. (3) 6
ll ne The Solid State 7 3. Classify each of the following solids as ionic, metallic, molecular, network (covalent) or amorphous. (i) Tetraphosphorus decaoxide (P4O 10 ) (ii) Ammonium phosphate (NH 4) 3PO4 (iii) SiC (iv) I 2 (v) P 4 (vi) Graphite (vii) Brass (viii) Rb (ix) LiBr (x) Si (xi) Plastics NCERT Sol. Ionic solids Metallic solids Molecular solids Network (covalent) solids ( NH ) PO, LiBr Brass, Rb P4O 10,P 4, I 2 Graphite, SiC, Si Plastics 4 3 4 Amorphous solids (3) 4. Explain (i) the basis of similarities and differences between metallic and ionic crystals. (ii) ionic solids are hard and brittle. NCERT Sol. (i) Similarities between ionic and metallic crystals (a) Both ionic and metallic crystals have electrostatic forces of attraction. In ionic crystals, these are between the oppositely charged ions. In metals, these are between kernels and valence electrons. That is why both have high melting points. (1) (b) In both cases, the bond is non-directional. Difference between ionic and metallic crystals Basis of difference Ionic crystals Metallic crystals Electrical Conductivity Binding Forces They conduct electricity in the molten state or in aqueous solution but not in the solid state. This is because in the solid state the ions are not free to conduct electricity as their positions are fixed. In molten state or in aqueous solution, their ions become mobile and hence, conduct electricity. It is strong due to electrostatic forces of attraction. They conduct electricity in the solid state because in metals, the valence electrons are free to flow. It may be weak or strong depending upon the number of valence electrons and the size of kernels. Physical Nature Ionic crystals are hard but brittle. Metallic crystals are usually hard and malleable. (1) (ii) Strong electrostatic forces of attraction among oppositely charged ions make ionic crystals hard and since, the ionic bond is non-directional, they are brittle. (1) 5. Stability of a crystal is reflected in the magnitude of its melting point, comment. Collect melting points of solid water, ethyl alcohol, diethyl ether and methane from data book. What can you say about the intermolecular forces between these molecules? NCERT Sol. Higher the melting point, greater are the forces holding the constituent particles together and hence greater is its stability. (1) The intermolecular forces in water and ethyl alcohol are mainly the hydrogen bonding but the intermolecular H-bonding in water is stronger than in ethyl alcohol. Therefore, water has a higher melting point than ethyl alcohol. Diethyl ether is a polar molecule. The intermolecular forces present in them are dipole-dipole attraction. Methane is a non-polar molecule. Therefore, the intermolecular forces operating in them are the weak van der Waals forces (London dispersion forces). (2) 7
8 ll ne Chemistry Class 12th Value Based Questions [4 Marks] 1. Manu went to the market to purchase a glass for window pane. The shopkeeper had shown two types of glasses one is transparent, and other is milky coloured glass. A student of chemistry standing nearby advised Manu to take the transparent glass. (i) Why did the student advice Manu to buy the transparent glass? (ii) What phenomenon is associated with milkiness of glass? (iii) What values would you associate with the gesture of the chemistry student? Sol. (i) The transparent glass is stronger and more tempered than milky glass. (2) (ii) The phenomenon associated with milkiness of glass is the crystallisation of glass molecules. (1) (iii) The values that can be associated with the gesture of the chemistry student is kindness and the will to share knowledge with the underprivileged. (1) 2. Quartz is a crystalline solild whereas glass is amorphous. It can be converted into glass and used in photovoltaic cell. (i) Why can quartz be used in a photovoltaic cell? (ii) Give, any one difference between quartz and glass. (iii) What values can be inferred from the nature of the quartz depicted in the statement above? Sol. (i) Quartz has the properties to convert sunlight into electric energy thus, it is used in photovoltaic cell. 1 1 2 (ii) Quartz is a crystalline solid whereas glass is amorphous. (1) (iii) The values that we can infer from the nature of quartz depicted by the given statement is that it is versatile in nature, helpful in energy conservation and eco-friendly. 1 1 2 Long Answer Type Question [5 Marks] 1. Give a differentiation between crystalline and amorphous solids with examples. Sol. Refer to text on page 3. (5) Q.1 Why is glass of window panes of very old buildings found to be thicker at the bottom than at the top and why is it milky? Q.2 Why does urea have a sharp melting point but glass does not? Q.3 Solid A is very hard, electrical insulator in solid state as well as in molten state and melts at extremely high temperature. What type of solid is it? Q.4 How do the structures of quartz and quartz glass differ from each other? Q.5 Diamond and solid rhombic sulphur both are covalent solids but the latter has very low melting point than the former. Explain, why? 8