Chapter 05: States and Phases of Matter

Matter exists in four states i.e., solid, liquid, gas and plasma. The simplest form of matter is the gaseous state and most of matter around us in the solid state. Liquids are less common than solids, gases and plasmas. The reason is that the liquid state of any substance can exist only within a relatively narrow range of temperature and pressure. Properties of gases (i) Shape and volume: Gases do not have a definite shape and volume. The volume of a gas is the volume of the container. (i) Ideal gas: Ideal gases have zero particle volume. (ill) Intermolecular Spaces: The molecules of gases are widely separated from one another and most of the volume of the gas is empty space (nearly 99.9%). So, gases can be compressed easily. When sudden expansion of gases occurs cooling takes place. It is called Joule-Thomson effect (iv) Collision: Pressure of a gas is due to the collisions of gas molecules with the walls of the container are (V) Intermolecular forces: There negligible intermolecular forces in ideal gases. Properties of liquids (i) Diffusion: Liquids also diffuse like gases, however the rate of diffusion in liquids is much lower than that in gases. (ii) Spaces: There are little spaces among liquid molecules because of relatively stronger intermolecular forces. (wii) Compressible: Liquids are 105 times less compressible than gases, but about 10 times more compressible than solids. (iv) Motion: The molecules of a liquid are in constant random motion, but their speed of movement is lower than gases. Diffusion among the miscible liquids is an evidence for the free motion of liquid molecules. Similarly, liquids can flow because of the free movement of their molecules. 1 atm 1 atm 2 atm 2 atm All Air Water Water (Gas) (Gas) (a) (b) (b) (a) (a) Volume of gas is (b) Volume of a liquid changes by about 0.0045% when pressure is doubled Fig: Effect of pressure on the volumes of gases and liquids (v) Expansion: Liquids expand when their temperature is increased. However, this expansion is negligible compared to that in gases. (vi) Kinetic molecular theory: According to this theory, molecules of a liquid are in constant motion, They possess a certain amount of kinetic energy at any temperature. The kinetic energy of the molecules may increase or decrease by the rise or fall in temperature respectively. A liquid is converted into a solid by decreasing its kinetic energy (on cooling) and converted into vapor by increasing kinetic energy (on heating).

Ideal gas It is a matter of common observation that when external conditions of temperature and pressure are changed, the volume of a given quantity of a gas is affected. Gas Laws: The gas laws describe the relationships between volume of a given amount of a gas and the prevailing conditions of temperature and pressure. In Boyle's and Charles's laws, some of the variables are held constant during the changes produced in the gases. According to Boyle's law. V oc - (when 'n' and "T' are held constant) According to Charles's law Vo T (where 'n' and 'P' are held constant) According to Avogadro's law "the volume of the given gas at constant temperature and pressure is directly proportional to the number of moles" Von (when 'P' and "T' are held constant) Three relationships can be joined together if no variable is to be kept constant. By combining gas laws: Voc V = constant - The constant suggested is 'R' which is called ideal gas constant. Its value is 0:0821 atm dm? K' mol-1. Its value depends upon units of pressure and volume. The value for 'R' in SI unit is 8.3143 Nm K-'mol-1 Hence V = R PV = nRT This equation is called the ideal gas equation. Calculation of Relative Molecular Mass (Mr) of a Gas General gas equation can be employed to calculate the relative molecular mass of a gas whose P, T, V and mass in grams are known. This is achieved by rearranging equation. Putting n = m/M in equation. Where m = mass in g and M = molar mass of the gas P nT P nT P PV = - Now rearranging equation M= At molecular level, the molar mass becomes relative molecular mass of a compound, therefore, we can write M, = - Sample Problem 5.1 The volume of 134g of a gas at -73°C under 10 atm pressure is 5 dm. Determine the relative molecular mass of the gas. Solution: The molecular mass of the gas can be calculated by using the formula: M = M= (134g)(0.0821 atm dm' mol K)(200K) (10 atm) (5 dm") M = 44 g mol Therefore, the relative molecular mass of the gas is 44 amu. Quick Check 5.1 (a) Explain why gases can be compressed easily! Ans. Gases can be compressed easily because they have enough empty spaces among their molecules (about 99%). That's why they are compressible. (b) What is Joule-Thomson effect? Ans. When a compressed gas is allowed to expand suddenly, then it creates cooling effect. It is called "Joule-Thomson effect" (c) The volume of 21g a gas is 2 dm' at -90°C under 7 atm pressure. Calculate the relative molecular mass of the gas. Ans. mass = 21g Volume = 8 dm Temperature = -90°C= 183K Pressure = 7 atm R = 0.0821 atm dm mol-1 K-1 M. mass =? Solution m - RT PV = M mRI M=- PV M= 218) (0.0821 atm dm mol K) 183K) (7 atm) (8 dm) M = 5.63 g/mol-1 Therefore, the relative molecular mass of the gas is 5.63 amu. mRT M mRT PV mRT PV mRT PV

Intermolecular Forces Definition: The forces of attraction among the molecules are called inter molecular forces. The intermolecular forces among the molecules of a liquid are reasonably stronger. Three types of such forces are (i) Instantaneous dipole-induced dipole forces (id-id) (London dispersion forces). (ii) Permanent dipole-permanent dipole forces. (wii) Hydrogen bonding. Permanent dipole-permanent dipole forces (pd-pd) Definition: The force of attraction between the positive end of a polar molecule and the negative end of a nearby polar molecule is called permanent dipole-permanent dipole force. Examples: Whenever the HC! molecules are close to each other (as in liquid state), they tend to line up as in figure. Similar type of forces is present among the molecules of chloroform (CHCI) The molecules of chloroform have a positive center at the H atom and the negative center on the end with Cl atoms.

Instantaneous dipole-induced dipole forces (id-id) Definition: The momentary force of attraction between an instantaneous dipole and an induced dipole is called instantaneous dipole-induced dipole force. Examples: They are present in all molecules but significant in non-polar molecules like F2, Cl (Halogens) and He, Ne (noble gases) etc. These are only forces that exist among the molecules of non-polar compounds. 0- This molecule has an instantaneous dipole 8- Fig: Creation of instantaneous dipole-induced dipole force 0+ 0+ 0+ -ci - H H Cl S+ 8+ C Cl -H Fig: Permanent dipole-permanent dipole forces in HCl and chloroform This molecule does not yet have a dipole, but its electrons are being repelled by the dipole next to it.... &+ ...making it become a dipole which is attracted to the dipole next to it. Factors affecting Instantaneous dipole-induced dipole forces (id-id) The strength of these forces depend upon the following factors. (i) Molecular mass and size When the molecular mass (Mr) of a compound is greater, the id-id forces are stronger and vice verse. Reason: A larger atom or molecule the dispersion of electronic cloud and polarization is relatively easier. Example: This factor operates in the halogen and noble gas families. Physical state: The physical state changes in the halogen family from top to bottom due to the increase in size and polarizability. The first two members in the halogen family (F2 and Cl2) are gases, but bromine is a liquid and iodine a solid (B.P. - 458K). The boiling points of noble gases in group 18 also increase from helium (He) to xenon (Xe) as in table 5.1. Table 5.1 Molar masses and bonding points of group 17 and 18 of periodic table. Molecular 17 Boiling (VII-A) Mass points (K) 85 38 F2 71 239 Cl2 333 159 Br2 458 253.8 12 (ii) Surface area (shape of molecule) A molecule with larger surface area has more number of contacts for a nearby molecule. The forces among the molecules of a compound having larger surface area are stronger. Saturated Hydrocarbons • These have chains of carbon atoms linked with hydrogen atoms. • Compare the length of the chains in C2H6 and C6H14. They have the boiling points -88.6°C and 68.7°C, respectively. • Molecules with larger chain length experiences stronger attractive forces. Reason: The longer molecules have more places along the length where they can be attracted to other molecules. Boiling points of some hydrocarbons are given in the following table Table: Boiling points and physical states of some hydrocarbons B.P.°C Physical state Name at S.T.P (1 atm) Gas -164 Methane (CH4): Gas -88.6 Ethane (CzH6) -42.1 Gas Propane Gas Butane -0.5 (C4H10) Isomers • Three isomers of pentane have the same relative molecular mass (Mr), however, they differ in their surface areas. 18 Molecular Boiling mass VIII points (K) 4.22 4.003 He 27.1 Ne 20.18 87.3 Ar 39.95 120 Kr 83.8 165 Xe 131.3 B.P.°C Physical state Name at S.T.P (1 atm) 36.1 Pentane (CsH12) Liquid Hexane (C.H14) 68.7 Liquid Decane (C1oH22) Liquid 174.1 327 Isodecane (C2oH42) Solid • Straight chain pentane has largest surface area so its boiling point is the highest among the three isomers of pentane. 2-Methylbutane (Isopentane) has smaller surface area than n- pentane but larger than 2,2-Dimethylpropane (Neopentane). • The forces among 2-Methylbutane molecules are intermediate and the least strong forces occur in 2,2-Dimethylpropane. The isomers of pentane alone with their boiling points are given in figure. pentane 2-Methylbutane (n-pentane) (iso-pentane) 36.2°C Fig: The three isomers of pentane have different structures, surface areas and intermolecular Quick Check 5.2 (a) To improve the quality of gasoline (petrol), straight chain hydrocarbons in the gasoline fraction of petroleum are converted to branched chain ones? What could be the possible reason? Ans. To improve the quality of gasoline straight hydrocarbons are converted into branched chain hydrocarbons because straight chain molecules have strong intermolecular forces due to larger surface area, need more energy to break the bonds as compared to branched chain molecules. So they can be broken easily and the quality is enhanced. (b) Which forces are present among the molecules of the following substances? CCl4, SiF 4. Ans. CCl4 & SiF4 are non-polar molecules and the forces of attraction among non-polar molecules are instantaneous dipole-induced dipole (London dispersion forces). (c) Differentiate id-id and pd-pd forces with examples. Ans. The difference is in the table. id-id forces forces among • The momentary instantaneous dipole and induced dipole instantaneous molecules are called dipole-induce dipole forces. • These forces are present among non- polar molecules i.e. CCl4, SiF4 Xe, I2 etc.

Hydrogen Bonding Definition: The forces of attraction between partially positively charged hydrogen atom and highly electronegative elements (F, O, N) are called hydrogen bonding. Conditions of hydrogen bond: A hydrogen bond is formed when the following conditions are fulfilled: (i) The hydrogen atom is connected to a highly electronegative atom, such as F, O, or N (ii) The electronegative atom must have a lone pair of electrons on it. CH3 CH3 2,2-Dimethylpropane (neo-pentane) 28°C 9.5°C forces pd-pd forces the • The forces of attraction among positive end of one molecule and negative end of other nearly polar molecule called permanent dipole-permanent dipole forces. • These forces are present among polar molecules i.e. Hcl, HBr etc. Explanation • Hydrogen bond is a special type of dipole-dipole force. • It is the strongest force among intermolecular forces, but is weaker than all the major types of bond; i.e. ionic, metallic, and covalent bonds. • The covalent bond between a highly electronegative atom and hydrogen is highly polarized. It means the hydrogen atom carries a sufficient &t charge and the electronegative atom S- charge. • The presence of lone pair on the electronegative atom allows the partial positively charged hydrogen atom to make a bond with F, O, or N called a hydrogen bond. • This bond is represented by a dotted line (...) and its strength is one tenth of an ordinary covalent bond Examples of hydrogen bonding Liquid water (H2O) • A water molecule contains two hydrogen atoms and two lone pairs on oxygen. • It can form two bonds on average. • Water molecules are extensively hydrogen bonded with one another because of two hydrogen bond per molecules. • This unique feature of water is responsible for its characteristics properties, like high boiling point of water (100°C). • The water molecules in liquid form are arranged three dimensionally as shown in figure. • Hydrogen bonding acts as a bridge between two electronegative oxygen atoms. Ammonia (NH3) • Ammonia can form only one hydrogen bond per molecule despite having three hydrogen atoms. Because of availability of only one lone pair on the N atom as shown in figure. • The hydrogen bonding in liquid ammonia is much weaker than water. So, its boiling point is much lower (-33°C). Hydrogen fluoride (HF) HF is even more distinct than water and ammonia. • It has three lone pairs on the F atom which allows it to form three H-bonds. • However, it has only one H atom and due to this reason, it is restricted to form only one bond again. • Its hydrogen bond is (B.P =19.9°C) is much higher than ammonia (B.P = -33.5°C). б / 5t/ H / б б- - H°* б+ H Fig: Hydrogen bonding in water • The molecules of HF join with one other in a zig-bag manner in solid form due to the presence of the hydrogen bonds. Hydrogen halides N St • The exceptional low acidic H g*\ 8+ strength of HF molecule as H compared to Hcl, HBr and Hl is attributed to this strong hydrogen bonding, because the partial positively charged is entrapped hydrogen two between highly electronegative atoms.

Influence of Hydrogen bonding on the properties of water (i) Structure and low density of ice • The molecules of water have tetrahedral structure. • Two lone pairs of electrons on oxygen atom occupy two corners of the tetrahedron. • When the temperature of water is decreased and ice is formed, the molecules become more regular and this regularity extends throughout the whole structure. • Empty spaces are created in the extensively hydrogen bonded structure as shown in the following diagram figure. • When water freezes, it occupies 9% more space and its density decreases. Due to decrease in density ice floats on water. • The fish and other living creatures survive under the frozen lakes and oceans, due to this so called anomalous behavior of ice. • The ice blanket covers the water surface due to its low density and the water beneath is insulated from the environment. 2.8 A 1.0 A 1.8 A 28° 28° Hydrogen bond Fig: Extensive hydrogen bonding in ice and spaces among molecules 8t COLL S+ 8- F 8* / H 8- St St H H Fig: Hydrogen bonding in NH3 and HF This part of a glacier is floating on the surface due to low density of ice (i) High heat capacity Water has a high specific heat capacity. This is due to its unique molecular structure which allows strong hydrogen bonds. (ill) Anomalous heat of vaporization and boiling point The general trend of enthalpy change of vaporization of group 16 hydrides is a regular increase from H¿S to HaPo figure. This is because the molecules of these compounds are bound by weak London dispersion forces. 40 H,Po H, Te H.Se H2S Enthalpy of vaporization kJ mol-! 10 At. No. of Element Fig: A graphical expression of variation in enthalpy of vaporization and boiling points of group 16 hydrides • The van der Waals forces become stronger, with the increasing atomic size down the group, from S to Po. • If water had followed this trend, the heat of vaporization of water would be the lowest among the group members. • Water has the highest enthalpy change of vaporization (41 kJmol) in the group. Water is exceptionally hydrogen bonded, which makes it difficult to break forces among water molecules and vaporize it. • The boiling point of water is also remarkably higher than the rest of group 16 hydrides. • The energy required to break the water molecules apart is very high due to extensive hydrogen bonding in it. It needs high temperatures to provide so much energy as to boil water into its vapor. (iv) Surface tension and viscosity Surface tension: The surface tension and viscosity of water are also very high. • Surface tension is a downward pull of water molecules at the surface, making the water surface stretched and strained. • The downward pull is due to the attraction of inner molecules through hydrogen bonds. Viscosity: Viscosity is the resistance by a liquid to its flow. • Water has higher viscosity than many of liquids including lower hydrocarbons and alcohols (hexane and ethanol). • The high viscosity of water is also due to strong hydrogen bonds among the molecules. Alcohols can make hydrogen bonds, but the strength and extent of hydrogen bonding is far less than that in water • Hydrocarbons lack the ability to make the hydrogen bond. 35 - н,0 60- O 35- H,PO A -10 - -15 - H,Te -40 - HaSe -65 - -90 At. No. of Element Quick Check 5.3 (a) Can the CHF3 molecule make a hydrogen bond? Explain why or why not? Ans. CHF: (fluoroform) cannot form hydrogen bond because hydrogen atom is bonded to carbon atom and carbon atom bonded to three fluorine atoms. The overall molecule is polar but the individual C-H bond does not have the necessary polarization to participate in hydrogen bonding (b) Show a hydrogen bond between two molecules of ethanol. Ans. The hydrogen bend between two ethanol molecule is given below: H -C- -C H H Hydrogen bond (c) Describe which forces are present in the following and arrange them in increasing order of boiling point. Ans. (i) CH3CH2CH3 (Propane, a non-polar molecule) It has London dispersion forces (id-id) It has hydrogen bonding (ii1)CH3CH2C1 (Ethyl chloride, a weakly polar molecule) It has permanent dipole - permanent dipole (pd-pd) forces Increasing order of boiling point. Propane < Ethyl chloride < Ethanol (d) The boiling point difference in each of the following pairs is given. (i) CH3CH3 (-89°C) and CH:OH (65°C), Difference = 154°C (ii) CH3CH2CH2CH: (0°C) and CH:CH2CH2OH (97°C), Difference = 97°C. Explain why the difference decreases as the size of the molecules increases. Ans. As we notice the molecular sizes increase than the difference in boiling point decreases due to strong intermolecular forces because of large surface area, and high polarizability. (e) Molecules of ethanoic acid (acetic acid) exist in the form of dimers in pure form but not in aqueous solution. How hydrogen bond can explain this? Ans. Molecules of ethanoic acid (acetic acid) exist as dimer form in the pure form, while in aqueous solution ethanoic acid can form hydrogen bond with water and does not exist as dimer. H CH3 -H (A dimer in pure form) H H H H H (Hydrogen bond with water H in aqueous solution)

Surface tension of liquids Definition: Surface tension is property of liquids that describes the force acting along the surface of a liquid, causing it to behave like an elastic sheet. Explanation (iv) Force arises due to the intermolecular forces between the molecules at the surface. (v) Molecules at the surface experience a net force inward due to intermolecular forces, leading them to "sink" on the surface. (vi) Liquids tend to minimize their surface area due to surface tension, which explains that the droplets form spheres. influences various. tension (vii) Surface phenomena, such as, capillary action, wetting of surfaces and formation of droplets and bubbles. Factors affecting Surface tension (i) Temperature: Surface tension typically decreases with increasing temperature. (i) Intermolecular forces: Stronger intermolecular forces lead to higher surface tension. Table 5.2: Surface tension of some liquids at 20°C in Nm Si. No. Liquid 1. Water H2O 2. Ethyl alcohol CH3CH2ОH 3. Methyl alcohol CHOH 4. Acetone (CH3)2CO 5. Benzene C6H6 6. Carbon tetrachloride CCl4 Viscosity of liquids Definition: Viscosity is the resistance by a liquid to its flow. Explanation • The property that describe thickness or stickiness of a liquid. • Liquid with higher viscosity are thicker and more resistance to flow. • Liquid with low viscosity are thinner and less resistant to flow. • Honey is more viscous than H20. • Stronger intermolecular forces results in high viscosity. Units of viscosity: In international system (SI), unit of viscosity is in Kg m! system, the unit of viscosity is poise (P). Fig: Surface tension in liquids Fig: Mosquitoes can float on surface on water due to high surface tension. Surface tension (10-3 Nm) 72.75 22.75 22.61 23.70 28.85 26.95 . In CGS The Factors affecting viscosity (i) An increase of temperature increases the average kinetic energy of molécules. This allows the molecules to overcome the attractive forces. This lowers the viscosity. (i) The stronger the intermolecular forces, the greater the viscosity of liquids because liquid molecules cannot move around each other freely, so the resistance to flow increases. Quick Check 5.4 (a) Arrange the following liquids is increasing order of surface tension, given reason: Acetone (CHCOCH), ethanol (CHsOH), methoxy methane (dimethyl ether, CH3OCH3) Ans. Despite of numerical values, as per practical evidences the increasing surface tension order is: CHOCH3 < CHCOCH3 < CzHsOH As we know that ethanol has strong hydrogen bonding than others and low volatility it shows more surface tension (with B.P 78.3°C) Acetone has surface tension less than ethanol but more than dimethyl ether, with strong inter-molecular forces due to acetone's structure. (b) Why do you think tetrachloro methane (carbon tetrachloride, CCl4) has higher viscosity than chloroform (CHCI) but less than ethanol (CHsOH)? Ans. Tetrachloromethane (CCl4) has London forces and chloroform has dipole-dipole forces. But due to the larger sizes of CCl4 molecules it has strong forces as compared to CHCk and showing higher viscosity. While comparing with ethanol (CHsOH) that has strong hydrogen bond, the molecules of CCl4 have lower viscosity. The viscosity of honey is higher than water, explain why? Ans. The viscosity of honey is far more than water due to the strong intermolecular forces depending upon different factors like water content, temperature and specific composition of honey (e.g., different flower sources). (d) Which of the following is more viscous: glycerine (CH_ОНСНОНСН›ОН) or hexane (C6H14)? Why? Ans. Glycerine has more viscosity than hexane (C.H14) because of strong intermolecular forces. Glycerine has hydrogen bonding and London forces as well while the molecules of hexane have only London forces.

Evaporation Definition: Evaporation is the spontaneous conversion of a liquid into vapor at any temperature. Evaporation causes cooling • When high energy molecules leave the liquid and low energy molecules are left behind, the temperature of the liquid falls. • The heat moves from the surrounding to the liquid and then the temperature of surrounding also falls. • This phenomenon helps to understand the evaporation causes cooling. One can feel cool after bath • A person after bath feels a sense of cooling due to evaporation of water from his body when exposed to air. • The molecules of H2O take away the energy of body. • Earthenware vessels keep water cooler under the same phenomenon. • As a result earthenware vessels are porous. Earthenware vessels keep water cool • The water molecules having sufficient kinetic energy to overcome inter molecular forces come out of these pores and evaporate. • This process of evaporation keeps on taking place and energy required for this process comes from the liquid • The average kinetic energy of remaining water molecules decreases which results in decreases in temperature of the liquid. As a result earthenware vessels keep water cool. vapor pressure Definition: The pressure exerted by the liquid vapor in equilibrium with its liquid at a given temperature is called vapor pressure. Explanation • The molecules of a liquid which leave the open surface are mixed up with air above the liquid. • If the vessel is open these molecules go on leaving the surface of liquid: • If we close the system, the molecules of liquid start gathering above the surface. • These molecules collide with the walls of the container, and also with the surface of the liquid as well • There are chances that these molecules are recaptured by the surface of liquid. This process is called condensation. • 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 figure. evaporation Liquid condensation Factors affecting vapor pressure • The magnitude of vapor pressure does not depend upon the amount of liquid in the container or the volume of container. • It does not depend on surface area of a liquid. The larger surface area presents a larger target for returning the molecules, so the rate of condensation also increases. • The vapor pressure decreases with increase in intermolecular forces of compounds and vice verse. • The vapor pressure of liquid increases by increasing temperature. Vacuum vapor 11111 1111.. Liquid Liquid (a) Fig: Attainment of equilibrium when the evaporation of liquid is carried in an evacuated closed vessel (a) Initial state, with evacuated space above the liquid, (b) intermediate state, and (c) equilibrium state, when the rate of evaporation is equal to the rate of condensation. : vapor vapor Liquid (b) (c) Quick Check 5.5 (a) Which of the liquids in each of the following pairs has a higher vapor pressure? (wii) Mercury, water (i) Alcohol, glycerine (i) Petrol, kerosene, Ans. (i) Alcohol, Glycerine: Alcohol has more vapor pressure than glycerine. (i) Petrol, Kerosene oil: Petrol has more vapor pressure than kerosene oil. (wii) Mercury, Water: Water has more vapor pressure than mercury. (b) Which one in each of the following pairs is more viscous: Glycerine or kerosene? Ans. Glycerine is more viscous than kerosene oil because of strong forces like hydrogen bonding while kerosene oil is non-polar and it has weak London dispersion forces. (c) Separate portions of acetone and water at the same temperature are poured on your hands. The acetone feels colder. Account for this in terms of attractive forces. Ans. Acetone is non-polar and it evaporates quickly as compared to water due to weak intermolecular forces. That's why when it poured on hands and one can feels colder. (d) Why evaporation gets faster at higher temperatures? Ans. Evaporation gets faster at higher temperatures. This is because of, when temperature is increased then K.E also increased, which enhances vapor pressure and faster the process of evaporation. (e) Why do we feel cool near the bank of a river? Ans. We feel cool near the bank of a river mainly due to evaporation. As water from the river evaporates, it absorbs heat from the surroundings, causing the air temperature to drop. This process, known as evaporative cooling, creates a cooling effect.

Boiling Point Definition: The temperature at which the vapor pressure of a liquid becomes equal to its external pressure is called boiling point. Explanation • When a liquid is heated, its vapor pressure goes on increasing. • A stage reaches when the vapor pressure of the liquid becomes equal to the atmospheric or external pressure. At this temperature the liquid starts boiling. • The reason for this is that the bubbles of vapor which are formed in the interior of the liquid have greater internal pressure than atmospheric pressure on the surface of liquid. • This makes the bubbles to come out of the liquid and burst at the surface. A constant of bubbles comes out at the boiling point of the liquid. Graph of boiling point • When a liquid is heated, the kinetic energy of its molecules increases. This causes the increase of the temperature. • At the boiling point, the kinetic energy of the molecules becomes maximum. • Any further heating at this stage will not increase the temperature. • The further heating is utilized to break the intermolecular forces and convert the liquid into its vapor. • The boiling points of some commonly ( available liquids at one atmospheric pressure are shown in the table 5.3. • The variation of vapor pressure of water, ethyl alcohol, ethylene glycol and diethyl ether with temperature is shown in figure. • The graphs show that the liquids reach unto their boiling points when their vapor pressures are equal to 760 torr at sea level Factors affecting boiling points of liquids The boiling point of a liquid is affected by the factors given below. (i) Strength of intermolecular forces Stronger the intermolecular forces, lower will be the vapor pressure and higher will be the boiling point. Higher boiling point of H2O indicates stronger intermolecular forces than that of ethanol and methanol, table 5.3. Table 5.3: Boiling points of some common liquids at 760 torr (1 atm) Formula =50 B.P (°C) liquids 118.50 CH:COOH Acetic acid 56.00 Acetone CH3COCH3 80.15 Benzene С6нь Carbon 46.30 CS2 disulphide (ii) External pressure • When vapor pressure of a liquid becomes equal to the external pressure then the liquid boils. • It means that when external pressure is changed, its boiling point will also change. • When the external pressure is high the liquid requires greater amount of heat to equalize its vapor pressure to external pressure, so, boiling point is raised • At a lower external pressure, a liquid absorbs less amount of heat and it boils at a lower temperature. • Water shows boiling point of 120°C at 1489 torr pressure and boils at 25°C at 23.7 torr. • Water boils at 98°C at Murree hills due to external pressure of 700 torr while at the top of Mount Everest water boils at only 69°C at 323 torr. • Increase the external pressure artificially on the surface of boiling water by using a pressure cooker. • Pressure cooker is a closed container. The vapor of water formed is not allowed to escape. • It exerts more pressure on the water surface in the cooker and the boiling temperature increases. • As more heat is absorbed in water, so food is cooked quickly under increased pressure. Normal boiling points 78:3°C/ 100°C 34.6°C 800 4760 Diethyl Water 600 /Ethyl alcohol ether (ethanol) sou 400 - 200 Vapo Ethylene glycol 80 100 20 40 60 0 Temperature Fig: vapor pressures (torr) of four common liquids shown as a function of temperature Formula B.P ("C) Liquids CC14 76.50 Carbon tetrachloride Ethanol 78.30 C¿H2OH 218.00 CioH& Naphthalene Water H2O 100.00 Quick Check 5.6 (a) Why food cooking is difficult in the areas with high altitudes? Ans. At high altitude the external pressure is low and the vapor pressure of the food mixture is also low. It creates difficulty in the food cooking. (b) The food cooks faster in the pressure cooker, explain. Ans. The food is cooked in the pressure cooker more quickly due to the high vapor pressure as it is a closed container and it helps to attain high temperature quickly. (c) Why the boiling point of water (100°C) is higher than that of ethanol (78°C), although both have hydrogen bonds? Ans. The boiling point of water is 100°C and that of ethanol is 78°C. In water molecules, there are more hydrogen bonding as compared to ethanol. Each water molecule can form two hydrogen bonds with two hydrogen atoms and two hydrogen bonds with oxygen atoms tetrahedrally

Energetic of phase changes Physical change of matter is always accompanied by an energy change. The change in energy is the quantitative measurement of the difference in the strength of intermolecular forces. Molar heat of fusion (AHr) Definition: The amount of heat absorbed by one mole of a solid to melt it into the liquid at its melting point at 1 atmospheric pressure is called molar heat of fusion. Example: →H0(6) 4H, =4.6 kJmol Explanation • Heat of fusion plays a significant role in the study of glaciers and ice sheets. • Heat of fusion determines the energy required for ice to melt. • As water has high heat of fusion, ice requires a lot of energy to melt. • Due to the low melting, glaciers gather mass which limits their flow. • It causes accumulation of ice on the earth poles, known as polar ice caps. • Polar ice caps are important in regulating the earth's temperature by reflecting the sunlight. • These ice caps also help regulate sea levels by storing fresh water ice. • Both the ice caps and icebergs are stable due to the high heat of fusion of water • If the heat of fusion of water were low, melting of glaciers and ice caps would result in drastic changes in the sea and earth's ecosystems. Molar heat of vaporization (AHv) Definition: The amount of heat absorbed by one mole of a liquid to convert it into one mole of vapor at its boiling point at 1 atmospheric pressure is called molar heat of vaporization. Example: When one mole H2O is converted to vapor at 100°C, then heat absorbed is 40.6 kJ mol H2O) →H,0(g) AH, =40.6kJmol Dependence on inter molecular forces • The molar heats of vaporization depends upon the strength of intermolecular forces. • NH3 and HC only need 21.7 and 15:6 kJ mol-1 respectively to become vapor at their boiling points. The difference is due to the stronger intermolecular forces in water. Effect of molar heat of vaporization and fusion on matter particles • The molar heat of fusion and vaporization affect the particles that make up matter by providing them with enough energy. • The particle move away from one another and change the state of the substances from solids to liquids to gases respectively. • Particles gain more freedom to move and rotate in the liquid and gas phase.

Solids Definition: Solids are those substances which are rigid, hard, have definite shape and definite volume. The atoms, ions, and molecules that make up a solid are closely packed. They are held together by strong cohesive forces. The constituent atoms, ions or molecules of solids cannot move at random. General properties of solids (i) Compression of solids • The atoms, molecules or ions of a solid substance are closely packed. • The particles of solids cannot move closer to each other unlike gases. • It is true for metallic solids that their atoms are spherically symmetrical. • When such particles are arranged, they can give birth to hexagonal and cubic close-packed structures. • Outer boundaries of constituent particles touch each other but due to their spherical shapes, they cannot occupy more than 74% space. • It means the compression of solids is not possible. (ii) Expansion of solids • The expansion is the property in which spaces between constituent particles are increased. • In case of solids the forces of attractions are so strong that increase of temperature hardly affects their relative positions. • The particles in a solid have vibrational motions about their mean positions. • Increase in temperature of solids hardly increases their volume. • There are parameters like coefficient of linear and cubic expansions of various solids, but these are negligible as compared to liquids and gases. (wii) Motion of particles in solids • The constituent particles of a solid do not undergo translatory motion, and neither rotational ones. • They only vibrate about their mean positions. • These vibrations become more intense at higher temperature. (iv) Inter-particle space in solids • The interparticle spaces in solids are far less than liquids. • The forces among their particles are stronger. • Due to closely packed atoms, molecules and ions, solids are mostly hard, have high melting points and high stability. (v) Inter-particle forces in solids • The particles (atoms, ions, molecules) are held together by ionic, covalent, metallic or van der Waals forces. • These forces are strong enough to fix the particles at their places thus allowing these particles to just vibrate about their mean positions. (vi) Kinetic energy based on KMT • Kinetic energy is due to the motion of constituent particles of a solid. Solid particles have only vibratory motion and they do not have translational or rotational motion. • The only kinetic energy that solids possess is vibrational kinetic energy.

Properties of crystalline solids (i) Geometrical shape • All the crystalline solids have a definite, distinctive geometrical shape due to definite and orderly arrangement of atoms, ions or molecules in three-dimensional space. • For a given crystal, the interfacial angles, at which the surfaces intersect, are always the same no matter in which shape they are grown. • The faces and angles remain characteristic even when the material is ground to a fine powder. (ii) Melting points Crystalline solids have sharp melting points. They can be identified from their definite melting points. (wii) 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 give crystalline solid. • The value of this angle varies from one solid to another solid. (iv) Growing of a crystal • When we have a saturated or super saturated solution of a crystalline material in a suitable solvent, it can give us same types of crystals by arranging atoms, ions or molecules. • This happens through the process called growth of crystals. • This can be done by slow evaporation of the solvent or by seeding process from saturated solution. Example: The solubility of sodium thiosulphate (NazS2O3) in water at 1,00°C is 231g/100 • At room temperature solubility is 50g/100 cm?. In case, we have saturated solution of NazS203 at 100°C, on cooling slowly no growth of crystal happens. It means that super saturated solution is not in equilibrium with solid substance. Now, if small crystal of NazS203 is added to super saturated solution, the crystallization happens rapidly. (v) Habit of a crystal • The shape of a crystal in which it usually grows is called habit of a crystal. • If the conditions for growing a crystal are maintained, then the shape of the crystal always remains the same. • If the conditions are changed the shape of the crystal may change. Example: A cubic crystal of NaCl becomes needle like when 10% urea is present in its solution as an impurity.

Crystalline solids Definition: The solids which have definite regular and three dimensional geometric shapes are called crystalline solids. Example: Diamond, sodium chloride, ice, etc. Crystal Lattice: The regular arrangement of ions, atoms, or molecules in three dimensional space is called the crystal lattice. Amorphous solids Amorphous solids, contrary to the crystalline solids, do not possess regular three dimensional geometrical shapes. Example: The amorphous solids are glass, wood amorphous sulphur (plastic sulphur), charcoal, coal, coke, etc. Properties of amorphous solids (i) The amorphous solids can have small regions where orderly arrangement of particles is found, but they do not have long range of regularity. The regions where orderly arrangement of particles is present are known as crystallites. (ii) Amorphous solids like glass melts over a wide temperature range. (wii) They can be molded and blown to form different shapes. (iv) Amorphous solids do not have definite value of heat of fusion. Difference between crystalline and amorphous solids are given following in the table. Table 5.4: Comparison of amorphous and crystalline solids Crystalline solids Property of solids Geometry Crystalline solid characteristic geometrical shapes. Crystalline solids melt sharply at Melting of solid their melting points. Directional Crystalline solids are anisotropic Amorphous solids are isotropic. character of the in nature. It means that their Their properties do not depend properties depend upon the upon the properties direction along which measurements are made. Order of particles Crystalline solids have long-range in solid order. Quick Check 5.7 (a) Why solids have very low compressibility and expansion? Ans. Molecules, atoms or ions (particles) of solids are very close to each other due to strong forces, they have very low value of compression and also show very small value of expansion due to strong forces. (b) What is meant by habit of a crystal? Ans. The shape of a crystal in which it usually grows is called habit of a crystal. The shape Amorphous solids show Amorphous solids generally appear in lump or in a fine powdered form. Amorphous solids do not have sharp melting points and they melt over a range of temperature. direction the measurement. In amorphous solids, long-range order is absent. of NaCl is cubic and it is the habit of NaCl. (c) Why solids do not undergo translatory motion? Ans. Solids do not show translatory motion because of closed packed arrangement of solid particles. They only show vibratory motion about their mean position.

Liquid crystals Definition: The liquid crystalline state exists between two temperatures that is melting temperature and clearing temperature. Example: The cholestryl benzoate forms liquid crystals between 145°C and 179°C. Its melting temperature is 145°C and clearing temperature is 179°C. Explanation • The distinction between liquids and solids is clear-Cu2+. • The phase transition between them is always sharply defined • At certain temperature, many substances exist in a phase that is neither fully liquid nor fully solid. • The molecules in these substances can move around, as in viscous liquids, but have a restricted range of motion, as in solids. These substances are called liquid crystals. Crystalline solid → Cloudy liquid (liquid crystal) → Clear liquid • In most liquid crystals, the molecules have a rigid, rod-like shape with a length four to eight times greater than their diameter. • When packed together, the molecules tend to orient with their long axes roughly parallel, like logs in a stack of firewood. • Individual molecules can migrate through the fluid • They can spin around their long axis, but they can't rotate end over end. General properties of liquid crystals The general properties of liquid crystals are as follows: (i) Parallel ordered arrangement. (wii) Flow like liquids (v) Somewhat rigid Uses of liquid crystals in daily life: The main application area of liquid crystals is in electro- optic devices. These are electrically controlled devices that modulate light in a desired way. Liquid crystals have many uses in daily life. (i) In diagnostics • Special liquid crystal devices can be used to diagnose the tumors and infections in the human body. • This is because often physical problems, such as tumors, have a different temperature than the surrounding tissue. • When cholesteric liquid crystals are applied to the breast, a tumor is located because of being warmer than the other parts. • This technique is successfully applied to diagnose breast cancer in the early stage. (ii) Elongated, rod-like and linear. (iv) Show viscosity like liquids (vi) Always anisotropic (ii) Characteristic higher temperature determination • Liquid crystal temperature sensors can also be used to find faulty connections on a circuit board by detecting the characteristic higher temperature. • As temperature sensors, they can be used in thermometers. (wii) Liquid crystal displays (LCDs) • The most common application of liquid crystal technology is liquid crystal displays (LCDs). • Liquid crystal screens are oscillograph and TV, laptops, cell phone displays. • Liquid crystals are used in TV displays, computer screens, calculators and watches, etc. • As new properties and types of liquid crystals are investigated and researched, these materials are sure to gain increasing importance in industrial and scientific applications. Quick Check 5.8 (a) Name the properties of liquid crystals in which they resemble solids. Ans. Some of the properties of liquid crystals that resembled with solids are: (veii) Have parallel ordered arrangement (ix) Elongated, rod like and linear (x) Show optical properties (xi) Somewhat rigid (X11) Anisotropic 1) Somon the properties of liquid crystal it which the resible grids. • They show fluidity • They show viscosity • They fill containers (c) Which property of liquid crystals make it possible to use them in temperature sensing devices? Ans. Liquid crystals are temperature sensors, so they can be used to find faults in the electrical circuits by detecting the characteristic high temperature. They can also be used in thermometers.