STATES OF MATTER - NCERT

1 136 CHEMISTRYUNIT 5 After studying this unit you will beable to explain the existence of differentstates of MATTER in terms ofbalance between intermolecularforces and thermal energy ofparticles; explain the laws governingbehaviour of ideal gases; apply gas laws in various real lifesituations; explain the behaviour of realgases; describe the conditions requiredfor liquifaction of gases; realise that there is continuity ingaseous and liquid state; differentiate between gaseousstate and vapours; and explain properties of liquids interms of OF MATTERINTRODUCTIONIn previous units we have learnt about the propertiesrelated to single particle of MATTER , such as atomic size,ionization enthalpy, electronic charge density, molecularshape and polarity, etc. Most of the observablecharacteristics of chemical systems with which we arefamiliar represent bulk properties of MATTER , , theproperties associated with a collection of a large numberof atoms, ions or molecules.

2 For example, an individualmolecule of a liquid does not boil but the bulk of water molecules have wetting properties;individual molecules do not wet. Water can exist as ice,which is a solid; it can exist as liquid; or it can exist inthe gaseous state as water vapour or steam. Physicalproperties of ice, water and steam are very different. Inall the three STATES of water chemical composition of waterremains the same , H2O. Characteristics of the threestates of water depend on the energies of molecules andon the manner in which water molecules aggregate. Sameis true for other substances properties of a substance do not change withthe change of its physical state; but rate of chemicalreactions do depend upon the physical state. Many timesin calculations while dealing with data of experiments werequire knowledge of the state of MATTER . Therefore, itbecomes necessary for a chemist to know the physicalThe snowflake falls, yet lays not longIts feath ry grasp on Mother EarthEre Sun returns it to the vapors Whence it came,Or to waters tumbling down the rocky O Connor2021-22137 STATES OF MATTER laws which govern the behaviour of MATTER indifferent STATES .

3 In this unit, we will learnmore about these three physical STATES ofmatter particularly liquid and gaseous begin with, it is necessary to understandthe nature of intermolecular forces, molecularinteractions and effect of thermal energy onthe motion of particles because a balancebetween these determines the state of FORCESI ntermolecular forces are the forces ofattraction and repulsion between interactingparticles (atoms and molecules). This termdoes not include the electrostatic forces thatexist between the two oppositely charged ionsand the forces that hold atoms of a moleculetogether , covalent intermolecular forces are knownas van der waals forces, in honour of Dutchscientist Johannes van der waals (1837-1923), who explained the deviation of realgases from the ideal behaviour through theseforces. We will learn about this later in thisunit. van der waals forces vary considerablyin magnitude and include dispersion forcesor london forces, dipole-dipole forces, anddipole-induced dipole forces.

4 A particularlystrong type of dipole-dipole interaction ishydrogen bonding. Only a few elements canparticipate in hydrogen bond formation,therefore it is treated as a separatecategory. We have already learnt about thisinteraction in Unit this point, it is important to note thatattractive forces between an ion and a dipoleare known as ion-dipole forces and these arenot van der waals forces. We will now learnabout different types of van der waals dispersion Forces or london ForcesAtoms and nonpolar molecules are electricallysymmetrical and have no dipole momentbecause their electronic charge cloud issymmetrically distributed. But a dipole maydevelop momentarily even in such atoms andmolecules. This can be understood as we have two atoms A and B in theclose vicinity of each other (Fig. ). It mayso happen that momentarily electronic chargedistribution in one of the atoms, say A ,becomes unsymmetrical , the charge cloudis more on one side than the other (Fig.)

5 Band c). This results in the development ofinstantaneous dipole on the atom A for a veryshort time. This instantaneous or transientdipole distorts the electron density of theother atom B , which is close to it and as aconsequence a dipole is induced in theatom B .The temporary dipoles of atom A and B attract each other. Similarly temporary dipolesare induced in molecules also. This force ofattraction was first proposed by the Germanphysicist Fritz london , and for this reasonforce of attraction between two temporaryFig. forces or london forcesbetween is known as london force. Anothername for this force is dispersion force. Theseforces are always attractive and interactionenergy is inversely proportional to the sixthpower of the distance between two interactingparticles ( , 1/r6 where r is the distancebetween two particles). These forces areimportant only at short distances (~500 pm)and their magnitude depends on thepolarisability of the - Dipole ForcesDipole-dipole forces act between the moleculespossessing permanent dipole.

6 Ends of thedipoles possess partial charges and thesecharges are shown by Greek letter delta ( ).Partial charges are always less than the unitelectronic charge ( 10 19 C). The polarmolecules interact with neighbouringmolecules. Fig (a) shows electron clouddistribution in the dipole of hydrogen chlorideand Fig. (b) shows dipole-dipole interactionbetween two HCl molecules. This interactionis stronger than the london forces but isweaker than ion-ion interaction because onlypartial charges are involved. The attractiveforce decreases with the increase of distancebetween the dipoles. As in the above case herealso, the interaction energy is inverselyproportional to distance between polarmolecules. Dipole-dipole interaction energybetween stationary polar molecules (as insolids) is proportional to 1/r3 and thatbetween rotating polar molecules isproportional to 1/r 6, where r is the distancebetween polar molecules.

7 Besides dipole-dipole interaction, polar molecules caninteract by london forces also. Thuscumulative effect is that the total ofintermolecular forces in polar Induced Dipole ForcesThis type of attractive forces operate betweenthe polar molecules having permanent dipoleand the molecules lacking permanent dipole of the polar moleculeinduces dipole on the electrically neutralmolecule by deforming its electronic cloud(Fig. ). Thus an induced dipole is developedin the other molecule. In this case alsointeraction energy is proportional to 1/r6where r is the distance between twomolecules. Induced dipole moment dependsupon the dipole moment present in thepermanent dipole and the polarisability of theelectrically neutral molecule. We have alreadylearnt in Unit 4 that molecules of larger sizecan be easily polarized. High polarisabilityincreases the strength of (a) Distribution of electron cloud in HCl a polar molecule, (b) Dipole-dipoleinteraction between two HCl moleculesFig.

8 - induced dipole interactionbetween permanent dipole and induceddipoleIn this case also cumulative effect ofdispersion forces and dipole-induced dipoleinteractions Hydrogen bondAs already mentioned in section ( ); this isspecial case of dipole-dipole interaction. Wehave already learnt about this in Unit 4. This2021-22139 STATES OF MATTERis found in the molecules in which highly polarN H, O H or H F bonds are present. Althoughhydrogen bonding is regarded as being limitedto N, O and F; but species such as Cl mayalso participate in hydrogen bonding. Energyof hydrogen bond varies between 10 to 100kJ mol 1. This is quite a significant amount ofenergy; therefore, hydrogen bonds arepowerful force in determining the structure andproperties of many compounds, for exampleproteins and nucleic acids. Strength of thehydrogen bond is determined by the coulombicinteraction between the lone-pair electrons ofthe electronegative atom of one molecule andthe hydrogen atom of other diagram shows the formation ofhydrogen F H F + + Intermolecular forces discussed so far areall attractive.

9 Molecules also exert repulsiveforces on one another. When two moleculesare brought into close contact with each other,the repulsion between the electron clouds andthat between the nuclei of two molecules comesinto play. Magnitude of the repulsion rises veryrapidly as the distance separating themolecules decreases. This is the reason thatliquids and solids are hard to compress. Inthese STATES molecules are already in closecontact; therefore they resist furthercompression; as that would result in theincrease of repulsive THERMAL ENERGYT hermal energy is the energy of a body arisingfrom motion of its atoms or molecules. It isdirectly proportional to the temperature of thesubstance. It is the measure of averagekinetic energy of the particles of the matterand is thus responsible for movement ofparticles. This movement of particles is calledthermal FORCES vsTHERMAL INTERACTIONSWe have already learnt that intermolecularforces tend to keep the molecules together butthermal energy of the molecules tends to keepthem apart.

10 Three STATES of MATTER are the resultof balance between intermolecular forces andthe thermal energy of the molecular interactions are veryweak, molecules do not cling together to makeliquid or solid unless thermal energy isreduced by lowering the temperature. Gasesdo not liquify on compression only, althoughmolecules come very close to each other andintermolecular forces operate to the , when thermal energy of moleculesis reduced by lowering the temperature; thegases can be very easily of thermal energy and themolecular interaction energy of a substancein three STATES is depicted as follows :We have already learnt the cause for theexistence of the three STATES of MATTER . Nowwe will learn more about gaseous and liquidstates and the laws which govern thebehaviour of MATTER in these STATES . We shalldeal with the solid state in class THE GASEOUS STATEThis is the simplest state of our life we remain immersed inthe ocean of air which is a mixture of spend our life in the lowermost layer ofthe atmosphere called troposphere, which isheld to the surface of the earth by gravitationalforce.