Liquid And Solid State

THE LIQUID STATE

$\bullet$The liquid molecules are relatively close together.

$\bullet$The intermolecular forces of attraction in case of liquids are much larger than in gases.

$\bullet$Unlike gases, liquids have a definite volume although no definite shape (similarity with gases).

$\bullet$The molecules are in constant random motion.

$\bullet$The average kinetic energy of molecules in a given sample is proportional to the absolute temperature.

$\bullet$Guldberg's rule: Normal boiling point $(T_b)$of the liquid is nearly two – third of its critical temperature $(T_c)$.when both are expressed on the absolute scale.

$T_b=\frac{2}{3}{T}_c$

$\bullet$Trouton's rule : The molar heat of vapourisation of a liquid expressed in Joules divided by normal b.p. of the liquid on the absolute scale is approximately equal to 88.

$\frac{\Delta H_{\nu }}{T_{b.p.}}=88J{K}^{-1}mo{l}^{-1}$

Evaporation

Evaporation is the spontaneous change in which a liquid changes into vapours at the surface of liquid. Evaporation occurs at all temperatures. Evaporation increases with increase in surface area, increase in temperature and decrease in intermolecular attractive forces. In contrast to evaporation, boiling takes place at a definite temperature and it involves bubble formation below the surface. Evaporation produces cooling.

Vapour Pressure

Vapour pressure of a liquid at any temperature may be defined as the pressure exerted by the vapour present above the liquid in equilibrium with the liquid at that temperature. The magnitude of vapour pressure depends upon the nature of liquid and temperature.

$\bullet$Non – polar or less polar liquids have fairly high vapour pressure due to weak forces of attraction (e.g. $CC{l}_4,CHC{l}_3,$ether etc). Polar liquids (e.g. water, alcohols, etc.) have lower vapour pressure because of strong dipole – dipole interaction between their molecules.

$\bullet$Vapour pressure of a liquid is constant at a given temperature. Further the vapour pressure of a liquid increases with increase in temperature. When the vapour pressure of the liquid is equal to the external pressure (normal pressure or 1 atm pressure) acting upon the surface of the liquid, the bubbles increase in size and escape freely; the temperature at which this happens is called the boiling point of the liquid. In case the external pressure is more than the atmospheric pressure, more heat will be required to make the vapour pressure equal to the external pressure and hence higher will be the boiling point. In case, the external pressure is low (as on the top of a mountain), the boiling point of the liquid decreases. This explains why a liquid boils at a lower temperature on the top of a mountain. (where pressure in low) than on the sea shore.

$\bullet$Substances having high vapour pressure (e.g. petrol) evaporate more quickly than substances of low vapour pressure. (e.g. motor oil)

SURFACE TENSION

Surface Tension of a liquid is defined as the force acting at right angles to the surface along one centimeter length of the surface. It is represented by the greek letter gamma, $\gamma .$

Due to surface tension molecules tend to leave the surface, i.e. the surface of the liquid tends to contract to the smallest possible area for a given volume of the liquid. Further for a given volume of the liquid, sphere has the minimum surface area. This explains why the drops of a liquid are spherical.

Thus it is apparent that in order to increase its surface area, force must be exerted to overcome the surface tension. In other words, work will have to be done to increase the surface area. Thus the surface tension of a liquid is defined as the work (energy) required to expand the surface of a liquid by unit area. Mathematically,

$surfacetension=\frac{Workdone}{Changeinarea}$

Thus surface tension of a liquid may also be defined as the force in dynes necessary to rupture its surface along one centimeter length. In SI units it is defined as the force in newtons $(1newton={10}^{5}dynes)$ required to rupture 1 meter length of the surface of a liquid.

Thus the units of surface tension are dynes per cm (or Newtons per metre, $N{m}^{-1}$in SI system).

Surface tension of a liquid is measured with the help of apparatus called stalgmometer.

The surface tension of a liquid decreases with increase of temperature and becomes zero at its critical temperature (where the surface of separation between liquid and its vapour disappears). The decrease in surface tension with increase of temperature is due to the fact that with increase of temperature, the kinetic energy of the molecule (and hence the speed of molecules) increases and hence the intermolecular forces of attraction decreases.

Surface tension in everyday life

$\bullet$Cleansing action of soap and detergents is due to their property of lowering the interfacial tension between water and greasy substances. Thus soap solution due to its lower surface tension can penetrate into the fibre to surround the greasy substances and wash them away.

$\bullet$ Efficacy of tooth pastes, mouth washes and nasal jellies is partly due to the presence of substances having lower surface tension. Lowering of interfacial tension helps these preparations to spread evenly over the surface with which they come in contact thereby increasing the efficiency of their antiseptic action.

VISCOSITY

It is well known that all liquids do not flow with the same speed. Some liquids like water, alcohol, ether, etc. flow very rapidly while some one like glycerine, honey, castor oil, etc. flow slowly. This indicates that every liquid has some internal resistance to flow. This internal resistance to flow possessed by a liquid is called its viscosity. The liquids which flow rapidly have low internal resistance and hence are said to be low viscous, i.e. their viscosity is less. On the other hand, the liquids which flow slowly have high internal resistance and hence are said to be more viscous, i.e. their viscosity is high. This force of friction which one part of the liquid offers to another part of the liquid is called Viscosity.

Coefficient of viscosity may be defined as the force per unit area required to maintain unit difference of velocity between two parallel layers in the liquid, one unit apart. In C.G.S. units, it is expressed in dynes per square centimeter$(dynesc{m}^{-2})$. This unit is called a poise after the name of Poiseulle who pioneered the study of viscosity. Low values of viscosity are expressed in centipoise $({10}^{-2}poise)$and millipoise$({10}^{-3}poise)$. In S.I. units, viscosity is expressed in $N{m}^{-2}s$ or Pas (pascal second).

$1Pas=1N{m}^{-2}s=({10}^{5}dyn){({10}^{+2}cm)}^{-2}s=10dync{m}^{-2}s=10poise$

The viscosity of a liquid generally decreases with rise in temperature. With increase of temperature, the kinetic energy of the molecules of the liquid increases and hence the liquid starts flowing faster, i.e. the viscosity decreases. The decrease in viscosity is found to be about 3% per degree rise of temperature.

Viscosity in everyday life

$\bullet$Lubricating oils are graded according to their viscosity. A good quality or 'all – weather' lubricating is one whose viscosity and hence lubricating property does not change much with increase of temperature. Such oils are obtained by adding long chain coiling polymers to the oil. As temperature rises, the polymer particles tend to uncoil and thus increase the viscosity of oil thereby compensating for the decrease of viscosity of the oil with rise of temperature.

$\bullet$The condition of high blood pressure and thus strain on heart may also be explained on the property of viscosity. In arteriosclerosis (hardening of arteries), arterial walls contract resulting in decrease of diameter of capillaries.

Narrow capillaries offer resistance to the flow of blood due to viscosity with the result greater force is needed to make blood flow through capillaries. This results in a condition of high blood pressure and strain on the heart.

$\bullet$The increased blood circulation required during fever is supplied by its temperature dependence properly (recall that rise of every degree centigrade temperature decreases viscosity of blood by about 3%). Thus lowering of viscosity results in a more rapid flow of blood without any extra strain on the heart.

$\bullet$In case of asphyxia, concentration of CO in blood increases resulting in swelling of corpuscles which then increases viscosity of blood.

LIQUID CRYSTALS

In a temperature range just above the melting point, crystals of certain substances can exist in a definite pattern as in solid but can flow like a liquid. Such crystals are called liquid crystals. When white light falls on a liquid crystal, it reflects only one colour, and as the temperature is changed it reflects different light. Thus, liquid crystals can be used to detect even small temperature changes. There are two important types of liquid crystals namely nematic liquid crystals (needle like) and smectic (soap like) liquid crystals.

THE SOLID STATE

Solids are rigid and have definite shapes. They also possess definite volume which is independent of the volume of the vessel.

Solids possess higher densities and are almost incompressible. All these characteristic properties are due to strong inter particle forces, smaller inter particle spares and restricted motion of the particles in the solid state.

The temperature at which solid changes into liquid state at normal pressure is called melting point of the substance.

Classification of Solids

Solids are broadly classified into two types: Crystalline solids and amorphous solids.

Distinction between Crystalline and Amorphous Solids

Classification of Solids based on different binding forces

Crystalline solids can be classified into different categories depending upon the type of constituent particles and the nature of attractive forces operating between them. Various categories are:

$\bullet$Atomic solids

$\bullet$Molecular solids

$\bullet$Ionic solids

$\bullet$Covalent solids

$\bullet$Metallic solids

Atomic Solids

In these solids the constituent particles are atoms. These closely packed atoms are held up by London dispersion forces. Some examples are crystals of noble gases. Such solids are very soft, possess very low melting points and poor conductors of heat and electricity.

Molecular Solids

In these solids, the constituent particles which pack up together are molecules of the substance. These molecules may be non – polar (dipole moment = 0) such as $C{O}_2,I_2,CC{l}_4$etc. or they may be polar (dipole moment > 0) like $H_{2}O,HCl,HF$etc.

$\bullet$In case of non – polar molecules, the attractive forces operating between the molecules are Vander Waal forces (also called dispersion forces). The example of such solids are : dry ice (Solid$C{O}_2)$, iodine (crystals).

$\bullet$In case of polar molecules, the attractive forces operating between the molecules in solid state are dipole – dipole forces. The examples of such solids are : solid$S{O}_2$, solid HCl. However, in some solids with polar molecules, the interparticle forces are hydrogen bonds. The examples of such solids are ice$(H_{2}O)$; solid hydrogen fluoride (HF); solid ammonia$(N{H}_3)$, etc.

Characteristics of Molecular Solids

Some of the general characteristics of molecular solids are :

$\bullet$They are generally soft.

$\bullet$Their melting points are low to moderately high. The melting points of solids with non – polar molecules are relatively low whereas solids with polar molecules have moderately high melting points.

$\bullet$They are generally bad conductors of heat and electricity.

$\bullet$They have generally low density.

Ionic Solids

In ionic solids, the constituent particles are ions of opposite charges. Each ion is surrounded by a definite number of ions of opposite charge. The number of ions that surround a particular ion of opposite charge its called co – ordination number of the ion. For example, in sodium chloride crystal each sodium ion $(N{a}^{+})$is surrounded by six chloride ions. Hence coordination number of $N{a}^{+}$ is 6. At the same time each chloride ion is surrounded by six $N{a}^{+}$ ions. Therefore the co – ordination number of $C{l}^{-}$

ion is also 6. However, in calcium fluoride crystal each $C{a}^{2+}$ ion is surrounded by eight fluoride $(\stackrel{-}{F})$ ions and each $F^{-}$ ion is surrounded by four $C{a}^{2+}$ ions. Thus, in $Ca{F}_2$crystal co – ordination numbers of $C{a}^{2+}$ and $F^{-}$ ions are respectively 4 and 8. The interparticle forces in ionic solids are ionic bonds operating between the ions of opposite charges some examples of ionic solids are : sodium chloride (NaCl) ; ceasium chloride (CsCl), zinc sulphide (ZnS), calcium fluoride$(Ca{F}_2)$, etc.

Characteristics of Ionic Solids

Some common characteristics of ionic solids are as follows:

$\bullet$They are hard, brittle and have low volatility.

$\bullet$They have high melting points.

$\bullet$They are poor conductors of electricity in solid state, however they become good conductors of electricity in molten state or in dissolved state.

$\bullet$They are generally soluble in polar solvents like water.

Covalent Solids

In these types of solids the constituent particles are atoms of same or different elements connected to each other by covalent bond network. For example, in diamond only carbon atoms constitute the covalent network while carborundum covalent bond network is constituted by silicon and carbon atoms. Obviously, the interparticle forces operating in these solids are covalent bonds. These solids are also called network solids because the covalent bonds extend in three dimensions forming a giant interlocking structure. Some examples of covalent solids are :

Diamond, silicon carbide, aluminium nitrite etc.

Characteristics of Covalent Solids

Some common characteristics of covalent solids are :

$\bullet$They are very hard. Diamond is the hardest naturally occurring substance.

$\bullet$They have very high melting points.

$\bullet$They are poor conductors of heat and electricity.

$\bullet$They have high heats of fusion.

Metallic Solids

In these type of solids, the constituent particles are metal atoms. The interparticle forces in these solids are metallic bonds. In the metallic crystals the metal atoms occupy the fixed positions but their valence electrons are mobile. The close packed assembly of metal kernels (part of metal atom without valence electrons) remain immersed in the sea of mobile valence electrons. The attractive force between the kernels and mobile valence electrons is termed as metallic bond.

Characteristics of Metallic Solids

The common characteristics of metallic solids are as follows:

$\bullet$They generally range from soft to very hard.

$\bullet$They are malleable and ductile.

$\bullet$They are good conductors of heat and electricity.

$\bullet$They possess bright lustre.

$\bullet$They have high melting and boiling points.

$\bullet$They have moderate heats of fusion.

The summary of classification of solids on the basis of interparticle forces is given in

Classification of Solids on the Basis of Binding Forces

Intermolecular forces or Vander Waal's forces

Intermolecular forces or vander Waals' forces originate from the following three types of interactions.

$\bullet$Dipole – Dipole interactions: In case of polar molecules, the vander waals' forces are mainly due to electrical interaction between oppositively charged ends of molecules (Fig. 1. a) called dipole – dipole interactions. For example, gases such as $N{H}_3,S{O}_2,HCl,HF$etc.have permanent dipole moments as a result of which there is appreciable dipole – dipole interactions between the molecules of these gases. The magnitude of this type of interaction depends upon the dipole moment of the molecule concerned. Evidently, greater the dipole moment, stronger is the dipole – dipole interactions. Because of these attractive forces, these gases can be easily liquefied.

Interaction between molecules

$\bullet$Dipole – Induced dipole Interactions: A polar molecule may sometimes polarize a non – polar molecule which lies in its vicinity and thus induces polarity in that molecule just as a magnet induces magnetic polarity in a neutral piece of iron lying close by. The induced dipole then interacts with the dipole moment of the first molecule and thereby the two molecules are attracted together (Fig. 1. b). The magnitude of this interaction, evidently depends upon the magnitude of the dipole moment of the polar molecule and the polarizability of the non – polar molecule. The solubility of inert gases in $H_{2}O$increases from He to Rn due to a corresponding increase in magnitude of the dipole – induced dipole interactions as the polarizability of the noble gas increases with increase in size from He to Rn.

$\bullet$Momentary dipole – induced dipole interactions: The electrons of neutral molecules keep on oscillating w.r.t. the nuclei of atoms. As a result, at a given instant, one side of the molecule may have a slight excess of electrons relative to the opposite side. Thus a non – polar molecule may become momentarily self – polarized. This polarized molecule may induce a dipole moment in the neighbouring molecule. These two induced dipoles then attract each other (Fig. 1. c). These momentary dipole – induced dipole attractions are also called London forces or dispersive forces. The magnitude of these forces depends upon the following:

(i) Size or molecular mass: The melting points and boiling points of non – polar molecules increase as the size or molecular mass of the molecule increases. For example, the m.p. and b.p. of alkanes, halogens, noble gases etc. increase as the molecular mass of the molecule increases.

(ii) Geometry / Shape : For example, isomer n – pentane has higher boiling point than neo – pentane because the former is zig – zag chain with larger sites of contact and hence large intermolecular forces whereas the latter is nearly spherical and hence has less contact and weaker forces.

Illustration1. Why liquid have a definite volume but no definite shape?

Solution: This is because the intermolecular forces are strong enough to hold the molecule together but not so strong as to fix them into definite positions (as in solids) instead, they possess fluidity and hence no definite shape.

Illustration 2. Why diethyl ether has higher vapour pressure than ethyl alcohol at a particular temperature.

Solution: This is because the intermolecular forces of attraction in ethyl alcohol are stronger than those present in diethyl ether.