The most important chemistry chapter 5 short questions for class 11. 1st Year Chapter 5 of Chemistry is related to atomic structure. These questions are for the Punjab Textbook Board and can be used within all of Punjab where this syllabus is taught.
Students are advised to prepare these questions in order to perform the best in the board examination.
Chemistry Chapter 5 Short Questions for Class 11
Q.1: What is atmospheric pressure? Give its units.
Ans: Atmospheric pressure:
“The pressure of air that can support 760mm Hg column at sea level is called as one atmosphere or atmospheric pressure.”
(OR)
“It is the force exerted by 760mm or 76cm long column of mercury on an area of 1cm² at 0 °C.”
1atm = 760mm of Hg
1atm = 760 torr
∴ 1mm of Hg = 1 torr
In S.I. units,
1atm = 101325 Nm⁻²
1atm = 101325 p.a.
∴ 1Nm⁻² = 1 Pascal
Q.2: What is meant by diffusion of gases?
Ans: DIFFUSION:
“The spontaneous intermixing of molecules of one gas with another gas at a given temperature and pressure is called diffusion.”
In diffusion the molecules of both gases move into each other until a homogenous mixture is obtained.
Example:
The spreading of fragrance of a rose or a scent is due to diffusion.
Q.3: What is Joule Thomson effect?
Ans: Joule-Thomson effect:
“When a compressed gas is allowed to expand suddenly into a region of low pressure it causes cooling. This phenomenon is called as Joule-Thomson effect.”
Explanation:
The molecules of the compressed gas are very close to each other and appreciable attractive forces are present among them. When a gas is allowed to undergo sudden expansion, the molecules move apart. The energy required to overcome these inter-molecular forces of attractions is taken from the gas itself and hence it is cooled.
Q.4: How can we determine the Molar mass of a gas from general gas equation?
Ans: Molar mass of gas:
According to general gas equation:
PV = nRT
∴ n = m/M
PV = (m/M) RT
Molar mass of the gas can be calculated from above equation, as.
M = mRT / PV
Q.5: Calculate the value of “R” in general units of pressure and volume.
Ans: Value of ‘R’:
Consider 1 mole of an ideal gas at STP. According to Avogadro’s principle:
T = 0°C = 273K
P = 1 atm
n = 1 mole
V = 22.414 dm³
According to the General gas equation,
PV = nRT
R = PV / nT
Putting Values in above equation,
R = (1 atm × 22.414 dm³) / (1 mole × 273K)
= 0.0821 atm. dm³. mole⁻¹. K⁻¹
Q.6: What is physical meaning of value of “R”?
Ans: Physical meaning of “R”:
For 1 mole of an ideal gas at STP (i.e., 1 atm pressure and 273.16K), The value of gas constant “R” is 0.0821atm.dm³.K⁻¹ mol⁻¹.
The physical meaning of this value is that, if we have one mole of an ideal gas at STP and its temperature is increased by 1 kelvin, then it will absorb 0.0821 atm.dm³ of energy. Hence, the value of R is a universal parameter for all the gases. It tells us that the Avogadro’s number of molecules of all ideal gases has same demand of energy.
Q.7: What are intermolecular forces? Give two examples?
Ans: Intermolecular forces:
“The force of attraction between the atoms of two different molecules are called as Inter molecular forces of attraction.”
These types of attractive forces are called as intermolecular forces and these are very weak as compared to the strength of a covalent bond. But these forces sufficiently affect the physical state of matter.
Example:
I⁶⁺ —— I⁶⁻ ……… I⁶⁺ —— I⁶⁻ ……… I⁶⁺ —— I⁶⁻
Q.8: What are dipole-dipole forces?
Ans: Dipole-dipole forces:
“The electrostatic forces of attraction between the positive end of one polar molecule and the negative end of other polar molecule are called as dipole-dipole forces”.
Examples:
H⁶⁺ — Cl⁶⁻ ……… H⁶⁺ — Cl⁶⁻ ……… H⁶⁺ — Cl⁶⁻
(Dipole Forces in Chloroform Molecule)
They are approximately one percent as effective as a covalent bond. The strength of these forces depend upon the electronegativity difference between the bonded atoms.
Q.9: Discuss the factors affecting the strength of dipole-dipole forces?
Ans: Factors affecting Dipole-Dipole Forces:
The strength of dipole-dipole forces depends upon the following factors:
(i) Electronegativity difference between the bonded atoms:
The strength of dipole-dipole forces depends upon the difference in the electronegativity of the bonded atoms. Greater the difference, greater will be the charge and hence stronger will be the attractive forces and vice versa.
(ii) Distance between the molecules:
The distance between the molecules in gaseous phase is greater, so the attractive forces are very weak in gases. While as the molecule are come close to each other these forces become more significant, and hence these forces are reasonably strong in liquids.
Q.10: What are London dispersion forces or id-id forces?
Ans: Instantaneous dipole- induced dipole or London forces:
“The momentary forces of attraction that exist between non-polar molecules due to formation of an instantaneous dipole which induced-dipoles in other molecules are called as instantaneous dipole-induced dipole forces or London dispersion forces”.
Explanation:
When the atoms of He come close to each other, the electrons of one atoms repels the electrons of the other atom. In this way a temporary dipole is created in the He atom. This instantaneous dipole disturbs the electronic cloud of the nearby atoms, hence it induce new dipoles in the atoms around it.
Q.11: What is Polarizability?
Ans: Polarizability:
“Polarizability is the quantitative measurement of the extent to which the electronic Cloud can be polarized or disturbed.”
When the electronic cloud is distorted, temporary poles are created in the molecule. With the increase in the atomic number of the elements down the group, the outermost electrons move away from the nuclei and the distortion of electronic cloud become more and more easy. So, the polarizability of these atoms go on increasing. This increased distortion of electronic cloud creates stronger London forces and hence the boiling points are increased down the group.
Q.12: How the size of the electronic cloud affect the strength of London forces?
Ans: Size of the electronic cloud:
The size of the electronic cloud of the atoms or molecules determines the strength of London forces. When the size of electron cloud increases, the dispersion becomes easier and hence forces become more prominent.
This distortion in the electron cloud of a molecule is called polarizability.
Example:
Due to greater size of iodine molecules, they are easily polarized and develop stronger attractive forces.
Q.13: Why the boiling points of Halogens increases down the group?
Ans: Boiling points of halogens:
All the halogens exist as non-polar diatomic molecules. In the group (from fluorine to iodine) there is an increase in the size of electronic cloud of the molecules. Therefore, the polarizability of the molecules increases. This increased distortion of electronic cloud creates stronger London forces and hence the boiling points of the halogens increased down the group.
| VII A | B.P °C |
|---|---|
| F₂ | – 188.1 |
| Cl₂ | – 34.6 |
| Br₂ | + 58.8 |
| I₂ | + 184.4 |
| At | + 337 |
Q.14: Why F₂ is a gas while I₂ is a solid at room temperature?
Ans: F₂ is a gas while I₂ is a solid:
The physical state of a substance depends on the strength of attractive forces. All the halogens exist as non-polar diatomic molecules. Moving down the group from fluorine to iodine the size of the molecules increases. Therefore, the attractive forces become stronger.
F₂ has the smallest size so it is gas at room temperature while I₂ has the greatest size hence it is solid at room temperature.
Examples:
F₂(g) = -188 °C ⇒ Gas
I₂(s) = +184 °C ⇒ Solid
Q.15: Why the boiling points of noble gases increases down the group?
Ans: Boiling points of noble gases:
The noble gases like He, Ne, Ar, Kr, Xe and Rn exist as monatomic molecules. The atomic number of the elements increases down the group and hence the size of the electron cloud of the atoms also increases. The outer most electrons are at a greater distance from the nucleus, hence the distortion of electronic cloud become more and more easy. So as we move down the group the polarizability of these atoms increases and so as their boiling points.
| VIIIA | B.P °C |
|---|---|
| He | -268.6 |
| Ne | -245.9 |
| Ar | -185.7 |
| Kr | -152.3 |
| Xe | -107.1 |
| Rn | -61.7 |
Q.16: Why the boiling points of alkanes increases with increase in the number of carbon atoms?
Ans: Boiling points of alkanes:
Alkanes exist as non polar molecules having different number of carbon atoms. The boiling points of these hydrocarbons increase as the number of carbon atom increases in the carbon chain. The reason is that longer molecules have more places along its length where they can be attracted to other molecules. Hence the boiling points of alkanes increases with increase in the number of atoms in the chain.
| Alkanes | B.P °C |
|---|---|
| CH₄ | – 161.5 |
| C₂H₆ | – 88.6 |
| C₃H₈ | – 42.1 |
| C₄H₁₀ | – 0.5 |
| C₆H₁₄ | + 68.7 |
Q.17: Why methane is a gas while hexane is a liquid at room temperature.?
Ans: CH₄ is a gas and hexane is a liquid:
The physical state of a substance depends upon the strength of intermolecular forces. The strength of London dispersion forces increases with increase in the number of atoms in molecules. Methane (CH₄) has only one carbon atom in the chain, while Hexane (C₆H₁₄) has 6 carbon atoms. Therefore, due to longer chain hexane have more places along its length, where they can be attracted by other molecules.
Hence, methane (B.p = -161.5°C) is a gas while hexane (B.p = 68.7°C) is a liquid at room temperature.
Q.18: Why ethane is a gas while hexane is a liquid at room temperature?
Ans: C₂H₆ is a gas and hexane is a liquid:
The physical state of a substance depends upon the strength of intermolecular forces. The strength of London dispersion forces increases with increase in the number of atoms in molecules. Ethane (C₂H₆) has only two carbon atom in the chain, while Hexane (C₆H₁₄) has 6 carbon atoms. Therefore, due to longer chain hexane have more places along its length, where they can be attracted by other molecules.
Hence, ethane (B.p = -88.6°C) is a gas while hexane (B.p = 68.7°C) is a liquid at room temperature.
Q.19: What is hydrogen bonding? Give an example. / Explain the formation of hydrogen bonding in water? / What is the origin of inter molecular forces in water?
Ans: Hydrogen bonding:
“The strong electrostatic forces of attraction between the lone pair of an electronegative atom and partially positive hydrogen which is already covalently bonded with other electronegative atom are called as hydrogen bonding.”
Example:
In water molecule the hydrogen atoms create strong electrical field and hence attract the nearby oxygen atom with great extent. As the oxygen atom contains two lone pairs, it donates one of its pair to hydrogen atom and form a loose co-ordinate covalent bond. The new bond which is formed is called as hydrogen bond.
Q.20: Why H₂O is a liquid while H₂S is a gas at room temperature?
Ans: H₂O is liquid & H₂S is gas:
The physical state of a substance depends upon the strength of intermolecular forces. Water is the best example of hydrogen bonded system. The H₂O molecules arrange themselves in such a way that each oxygen atom can make two hydrogen bonds with the neighbouring water molecules. So due to strong attractive forces H₂O is liquid at room temperature.
While, H₂S is a weakly polar substance having weak dipole-dipole forces. Due to weaker forces it is a gas at room temperature.
Q.21: In hydrogen bonded structure of HF, which is the strong bond the shorter covalent bond or the longer hydrogen bond between different molecules?
Ans: In HF, which is stronger bond:
In hydrogen bonded structure of HF, the molecules of HF join with each other in a zig-zag manner. There are two types of bonds in the structure of HF.
The solid line represents the covalent bond while the dotted line between H and F atoms represents the longer hydrogen bond. As the strength of hydrogen bond is generally twenty times less than that of covalent bond, so the shorter covalent bond is stronger than the longer hydrogen bond between different molecules.
Q.22: HF is a weaker acid than HCl, HBr and HI. Justify.
Ans: HF is a weaker acid:
In HF, the molecules are join with each other in a zig-zag manner due to hydrogen bonding. This arrangement can be shown as.
The exceptional, low acidic strength of HF molecules as compared to HCl, HBr and HI is due to strong hydrogen bonding. In zigzag structure of HF, the partially positive hydrogen is entrapped between two highly electronegative fluorine atoms. So it is very difficult to ionize the hydrogen from HF molecules. Hence HF is a weaker acid that other halogen acids in which no hydrogen bonding is present.
Q.23: Hydrogen bond in HF is strongest, but why the boiling point of water is greater than that of HF?
Ans: Boiling point of H₂O is greater than HF:
HF is a highly polar molecule. In liquid state, the molecule of HF arrange themselves in a zigzag manner due to hydrogen bonding. This structure allows the flourine atom to make only one hydrogen bond with the partial positive hydrogen of the neighboring molecule. (B.P = 19.9°C)
While the H₂O molecules although less polar than HF but they have tetrahedral structure. The H₂O molecules arrange themselves in such a way that each oxygen atom can make two hydrogen bonds with the neighbouring water molecules. As water molecules can make more hydrogen bonds, so boiling point of H₂O is greater than HF. (B.P = 100°C)
Q.24: Why water and ethanol can mix easily with each other?
Ans: Water and ethanol can mix easily:
Water is the best example of hydrogen bonded system. H₂O molecule have two partially positive hydrogen atoms and two lone pairs. Similarly ethyl alcohol also have one partially positive hydrogen and two lone pairs. So ethyl alcohol can dissolve in water in all proportions, because both can form hydrogen bonds with each other.
Q.25: Justify that ice occupy 9% more volume than liquid water? / Why the density of ice is less than water? / Why ice floats on the surface of the water?
Ans: Density of ice is less than water:
The molecules of water have tetrahedral structure, having two lone pairs and two hydrogen atoms. In liquid state, water molecules are extensively associated with each other. When the temperature of water is decreased and ice is formed then the molecules become more regular and this regularity extends throughout the whole structure.
Due to regular arrangement of H₂O molecules, empty spaces are created among the molecules. When water freezes it occupy 9% more space and hence its density decreases. The result is that ice floats on the surface of water.
Q.26: In very cold winter the fish in the garden pond owe their lives to hydrogen bonding?
Ans: Fish in the pond owe their lives to hydrogen bonding:
The low density of ice than liquid water at 0°C causes water in ponds and lakes to freeze from surface to the downward direction. During winter as the outer atmosphere becomes cold, the water at the surface becomes less dense. This less dense water stays on the top of slightly warm water underneath. A stage reaches when it freezes. This layer of ice insulates the water underneath for further heat loss.
Fish and plants survive under this blanket of ice for months. So we can say that fish and plants in garden pond owe their lives to hydrogen bonding present in water molecules.
Q.27: Define evaporation. Write the names of the factors which affect the rate of evaporation.
Ans: Evaporation:
“The spontaneous change of a liquid into its vapours (Gas phase) at any temperature is called as evaporation.”
The rate of evaporation of the liquids depends upon three factors.
(i) Strength of the intermolecular forces.
(ii) Surface area.
(iii) Temperature.
Q.28: Evaporation takes place at all temperatures justify.
Ans: Evaporation takes place at all temperature:
“The spontaneous change of a liquid into its vapour is called as evaporation.”
The molecules of a liquid are constantly moving in the container. The energy of molecules is not equally distributed among the molecules. The molecules with low kinetic energy move slowly, while others with high kinetic energy move faster. If one of the high speed molecules reaches the surface it may escape from the liquid by breaking the attractions of its neighbouring molecules and leaves the bulk of the liquid. So we can say that evaporation takes place at all temperatures.
Q.29: Evaporation causes cooling. Give reason.
Ans: Evaporation causes cooling:
Evaporation causes cooling, the reason is that the high energy molecules leave the surface of the liquid during evaporation, thus leaving behind the low energy molecules. As a result the average kinetic energy of the liquid molecules decreases and hence the temperature of the liquid falls. Now heat moves from the surrounding to the liquid and therefore the temperature of the surrounding also falls. Therefore, we can say, evaporation cause cooling.
Q.30: Earthenware vessels keep the water cool. Justify.
Ans: Earthenware vessels keep the water cool:
Earthenware vessels have very small pores in them. The water molecules evaporate from these pores. During evaporation, high energy molecules of the water escapes from these tiny pores, leaving behind low energy molecules of water, hence the temperature decrease.
Moreover the surface of the vessel acts as an insulator to keep the water inside the vessel cool.
Q.31: One feels sense of cooling under the fan after bath?
Ans: One feels sense of cooling:
After taking bath a thin layer of water is present on the body. When a person sits under the fan after bath, the water molecules starts evaporating from the surface of body.
The rate of evaporation is also increased due to wind pressure. These evaporating water molecules absorb energy from the surface of body. Hence, the temperature of the body falls and a person feel sense of cooling.
Q.32: How rate of evaporation is affected by temperature?
Ans: Rate of evaporation is affected by temperature:
Temperature of the liquid strongly affects the rate of evaporation. At high temperature, the kinetic energy of the liquid increases. It increases the velocity of the molecules, these fast moving molecules have greater tendency to the leave the liquid surface. Hence rate of evaporation of a liquid is increases by increase of temperature.
Q.33: How rate of evaporation is affected by surface area?
Ans: Rate of evaporation is affected by surface area:
Evaporation of a liquid is a surface phenomenon. When the surface area of a liquid is increased, more and more molecules have chance to escape from the surface of the liquid into vapour phase. Hence increase in surface area increases the rate of evaporation.
Q.34: How strength of intermolecular forces affect the rate of evaporation?
Ans: Rate of evaporation and Intermolecular forces:
Intermolecular forces among molecules also affect the rate of evaporation. Liquids having strong molecular forces have slow evaporation rate. Similarly in liquids having weak inter molecular forces, the rate of evaporation is high e.g., gasoline having weak London forces evaporates much rapidly than water which has strong hydrogen bonding.
Q.35: Define vapour pressure? Write the names of the factors which affect vapour pressure.
Ans: Vapour pressure:
“The pressure exerted by the vapours on the surface of a liquid at equilibrium with the liquid at a constant temperature is called as vapour pressure.”
The magnitude of vapour pressure depends upon three factors.
(i) Strength of the intermolecular forces.
(ii) Size of the molecules.
(iii) Temperature.
Q.36: Dynamic equilibrium is established during evaporation of a liquid in a closed vessel at constant temperature?
Ans: Dynamic equilibrium is established:
If some liquid is placed in a closed container and left it for some time. The molecules of the liquid having high energy evaporates and starts gathering above the surface of the liquid.
Whenever the gaseous molecules collide with the surface of the liquid it is recaptured and again converted into liquid. This is called as condensation.
Liquid ⇌ Vapours
The two processes i.e., evaporation and condensation continue till a stage reaches when the rate of evaporation becomes equal to the rate of condensation. This is called the state of dynamic equilibrium.
Q.37: How vapour pressure of a liquid is affected by temperature?
Ans: Effect of temperature:
Temperature is the most important parameter which controls the vapour pressure of a liquid. At higher temperature, the kinetic energy of the molecules is increased. Hence the molecules move with high velocities and have greater chances to escape the surface of the liquid. As a result the vapour pressure of the liquid is increased.
Example:
Vapour pressure of water at various temperatures is given as.
At 0°C, vapour pressure of water is 4.579 torr.
At 100°C, vapour pressure of water is 760 torr.
Q.38: What is the effect of intermolecular forces on vapour pressure?
Ans: Effect of Intermolecular Forces:
The strength of intermolecular forces in different liquids is directly related to their vapour pressures. Stronger the intermolecular forces lower is the vapour pressure and vice versa.
Iso-pentane is a non-polar liquid having very high vapour pressure due to weak intermolecular forces, while glycerol is a highly polar molecule with very strong hydrogen bonding has very low vapour pressure.
| Name of compound | Formula | Vapour pressure at 20°C [mm Hg] |
|---|---|---|
| Iso-pentane | CH₃-CH₂-CH(CH₃)-CH₃ | 580 |
| Glycerol | C₃H₈O₃ | 0.00016 |
Q.39: The vapour pressures of solids are far less than liquids. Give reason
Ans: Vapour pressures of solids and liquids:
The vapour pressure of different substances are directly related with the strength of attractive forces among their particles. Stronger the attractive forces lower is the vapour pressure and vice versa.
Solids have very strong attractive forces among their particles therefore there vapour pressures are far less as compared to liquids which have weaker attractive forces.
Example:
The vapour pressure of diethyl ether (liquid) is about 200 torr at 0°C, While, that of ice (solid) is 4.579 torr at the same temperature.
Q.40: What is Boiling point?
Ans: Boiling point:
“It is defined as the temperature at which the vapour pressure of a liquid becomes equal to the external atmospheric pressure is called boiling point”.
Explanation:
When a liquid is heated, the vapour pressure of the liquid also increases with increase in temperature. A stage comes when the vapour pressure of a liquid becomes equal to the external pressure. This temperature is referred as the boiling point of the liquid.
Q.41: Boiling needs a constant supply of heat. Explain.
Ans: Boiling needs a constant supply of energy:
When a liquid is heated, the kinetic energy of its molecules increases and hence the temperature of the liquid increases. At the boiling point, the kinetic energy of the molecules become maximum and further heating at this stage will not increase the temperature rather this heat will only be utilized to break the intermolecular forces and convert the liquid into its vapours. Hence a constant supply of energy is required to boil a liquid.
Q.42: Why bubbles are produced during boiling of a liquid?
Ans: Bubbles are produced during boiling of liquid:
When a liquid is heated from the bottom, the temperature of lower layer is slightly higher than the upper layer. Therefore the liquid in interior have greater internal pressure than the surface of the liquid.
Due to that, the liquid in the interior boils and converted into gas. This gas (bubbles) comes out of the liquid and burst upon the surface. Thus a continuous stream of bubbles comes out at the boiling point of a liquid.
Q.43: Why temperature remains constant during boiling of a liquid?
Ans: Temperature remains constant:
When a liquid is heated, the kinetic energy of its molecules also increases. Since kinetic energy is directly proportional to absolute temperature, the temperature of the liquid also increases.
At the boiling point of a liquid, the kinetic energy of the liquid molecules become maximum and hence further heating at this stage will not increase the temperature, rather this heat will be utilized to break the intermolecular forces and convert the liquid into vapours.
Q.44: What is molar heat of vapourization?
Ans: Molar heat of vapourization (ΔHv):
“The amount of heat required to vapourize one mole of a liquid at its boiling point is called its molar heat of vapourization.”
Example: The molar heat of vapourization of water is +40.6 kJ mol⁻¹.
Significance:
Molar heat of vapourization is an index for the strength of intermolecular forces between the molecules of a liquid.
Q.45: Why steam causes more severe burns than boiling water?
Ans: Steam causes more severe burns than boiling water:
When a liquid is heated, the kinetic energy of its molecules increases and hence the temperature of the liquid increases. At the boiling point, the kinetic energy of the molecules become maximum and further heating at this stage will not increase the temperature rather this heat will only be utilized to break the intermolecular forces and to increase the potential energy of molecules. Therefore the total energy (i.e. K.E & P.E) of the steam is greater than the boiling water. So steam causes more severe burn than boiling water.
Q.46: How the boiling point of a liquid is affected by the external pressure?
Ans: Boiling point and external pressure:
The boiling point of a liquid depends on the external atmospheric pressure. If external pressure on a liquid is high, it requires more heat to equalize its vapour pressure to the external pressure and hence boiling point is increased. e.g., at 1489 torr the water boils at 120°C.
Similarly at lower external pressure the liquid requires less amount of heat and hence its boiling point is decreased e.g., at 23.7 torr the water boils at 25°C.
Q.47: What is molar heat of fusion?
Ans: Molar heat of fusion(ΔHf):
“It is the amount of heat absorbed by one mole of a solid when it melts into liquid form, at its melting point.”
It is denoted by ΔHf. The pressure during the change is kept one atmosphere or 760 torr. The value of ΔHf indicates the energy required to separate molecules from each other. So these values give the idea about the strength of intermolecular forces in different compounds.
Q.48: Why heat of vapourization of water is greater than CH₄?
Ans: Molar heat of vapourization of CH₄:
“It is defined as the amount of heat absorbed by one mole of a liquid, when it is changed into vapours at its boiling point.”
The molar heat of vapourization of a substance depends on the strength of attractive forces of a substance.
Water molecules are highly polar, having very strong attractive forces, so its ΔHv is very high i.e. +40.7 kJ mol⁻¹.
While CH₄ is non polar with weak attractive forces, so its ΔHv is low i.e. +8.6 kJ mol⁻¹.
Q.49: What are amorphous solids?
Ans: Amorphous solids:
The word amorphous means ‘shapeless’.
“Amorphous substances are those whose constituent atoms, ions, or molecules do not possess a regular orderly arrangement”.
Examples:
The best examples are glass, plastics, rubber, glue, etc. these substances have solid state properties and virtually complete maintenance of shape and volume. But they do not have ordered crystalline state.
Q.50: Differentiate between crystalline and amorphous solids.
Ans:
| Crystalline Solids | Amorphous Solids |
|---|---|
| 1. Those solids in which atoms, ions or molecules are arranged in a definite three dimensional pattern are called crystalline solids. Examples: NaCl, Sucrose, KBr etc. | 1. Amorphous substances are those whose constituent atoms, ions or molecules do not possess a regular orderly arrangement. Examples: Glass, plastics, rubber etc. |
| 2. They have sharp melting points. | 2. They do not have sharp melting points. |
| 3. In crystalline solids, a definite geometrical shape repeats three dimensionally in the structure. | 3. A long-range regularity does not exist in the amorphous solids. |
Q.51: What are crystallites?
Ans: Crystallites:
“A long regularity does not exist in amorphous solids but they can possess small regions of orderly arrangements. These crystalline parts of otherwise amorphous solids are known as crystallites.”
Q.52: Amorphous solid like glass is also called super cooled liquid.
Ans: Amorphous solids are called super cooled liquid:
“Amorphous solids are those substances in which the particles do not possess a regular orderly arrangement.”
Sometimes, the temperature of a liquid may be lowered below its freezing point without crystallization, such liquids are called super cooled or under cooled liquids. When the temperature of the fused glass is lowered, the large and irregular units of the material become less and less mobile and finally become rigid without any definite order. So, glass is a super cooled liquid.
Q.53: Crystals can only broken along definite planes. Justify. / Define cleavage plane? Give one example.
Ans: Cleavage planes:
Whenever the crystalline solids are broken they do so along definite planes. These planes are called the cleavage planes and they are inclined to one another at a particular angle for a given crystalline solid. The value of this angle varies from one solid to another solid.
Example:
Graphite can be cleaved only parallel to length of sheet and not easily in the other direction.
Q.54: Why most of solids cannot be compressed easily?
Ans: Compressibility of solids:
Solids are those substances which are rigid, hard, have definite shape and definite volume. The atoms, ions and molecules that makes up a solid are closely packed due to very strong attractive forces. Since particles of solids have not enough spaces among their particles, they cannot be compressed by applying pressure.
Q.55: What are liquid crystals?
Ans: Liquid crystals:
Some of the crystalline solids when heated, they change into a turbid liquid before finally melting to a clear liquid.
Example:
Crystalline solid ⇌ (Melting Temperature / Clearing Temperature) ⇌ Turbid liquid ⇌ Liquid
Such turbid liquid have properties like liquids as surface tension, viscosity and they can also flow as liquids. But on other hand these liquids possess some degree of order, due to which they resemble in optical properties with solids. These turbid liquids are called as liquid crystals.
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