UNIT LIOUIDS
Structure
Introduction
Objectives
Comparison of Liquids with Gases and Solids
Structure of Liquids
Surface Tension and Viscosity
Vaporization
Vapour Pressure
Boiling Point
rout on's' Rule
Liquid Crystals
Summary
Terminal Questions
Answers
In Unit 2, we discussed the characteristics of ideal gases. We assumed that there is no
attractive or repulsive interaction between the individual molecules. In Unit 3, this
treatment was modified to account for the behaviour of real gases'at low-temperatures and
high pressures and to explain the liquefaction of gases. Finite size of the gaseous molecules
and their weak interaction were recognised. In Unit 5, we are going to study the strong
interactions in a solid crystal and the orderly arrangement of particles in it. In this unit, we
will discuss the characteristics of liquids in contrast to those of gases and solids. Our aim is
not to list the properties of liquids but to correlate these to the intermolecular interactions.
We will describe the features of a model proposed for the structure of liquids. We shall
explain the correlation between the intermolecular forces and the properties of liquids such
as surface tension, viscosity, vapour pressure, boiling point and molar enthalpy of
vaporization. Finally we will briefly study liquid crystals, their types and their applications.
Objectives
After studying this unit, you should be able to :
explain the structure of liquids,
state the significance of surface tension and viscosity of liquids,
discuss the qualitative dependence of vapour pressure, boiling point and molar enthalpy
: of vaporization of liquids on the molecular interactions,
state and explain Trouton's rule, and
discuss the types of liquid crystals and their applications.
4.2 COMPARISON OF LIQUIDS WITH GASES AND
SOLIDS
We can obtain a liquid by heating a solid or by cooling a gas under certain conditions.
Therefore, liquid state is in between solid and gaseous states. In a solid, the particles have
only vibrational motion about their equilibrium positions. The strong intermolecular forces
present in a solid crystal are responsible for the restricted motion of the particles and their
orderly arrangement.
As a result, a solid has a definite shape. In contrast to this, the molecules in a gas are free to
move randomly and have a disorderly arrangement. The gases can expand or contract to
conform to the volume of the vessel. Hence, the gases have no definite shape or volume.
The characteristics of a liquid lie between the extremes of a gas and a solid. The particles in
a liquid are free to move from one point to another. In this respect, it resembles a gas. The
ability of a liquid to flow enables it to assume the shape of its container. Yet it never
expands or contracts to fill the container and thus resembles a solid. Let usmow examine
the structural aspects of liquids.
, Liquids
'4;3 STRU.CTUREOF LIOUIDS
The particles in a liquid are not as much orderly as in a solid; also not as much disorderly as
in a gas. To establish this, we cite the following three pieces of evidence :
Volume Change During Fusion and Vaporization
A pure solid melts to give a liquid at a sharp'temperature. This process is called fusion. It is
generally seen that during fusion, volume increases by 10%.This implies that a substance Water and a few other substances
retains its orderliness to a considerable extent during fusion. On the contrary, in the ?re exceptional in having a lower
conversion of a liquid into vapour at its boiling point (known as vaporiiation), the volume volume per unit mass (and highey
increases 100-1000 fold. This large increase in volume during vaporization indicates that density) in liquid state than in solid
state. We shall discuss this aspect in
the particles are changed into a more disorganised state. the unit on phase equilibria.
Molar Enthalpies of Fusion and Vaporization The state of a substance under g i m
The amount of heat required at constant pressure to convert one mole of a solid into liquid temperature and pressure is decided
at its melting point is called molar enthalpvm .
- of fusion (A@,.). Similarlv. the amount of heat
r
by the forces
operating in a substance. Fusion,
required at &"stant pressure to convert one mole of ;liquidinto its vapour at its boiling
vaporizatio~etc. are dependent upaa
point is called the molar enthalpy of vaporization (@v.p). The values of A@,,, and the external forces (such as pressun)
boiling points (BP) are given in Table 4.1 for some substances. It is seen that A&., is larger applied on a substance.
than A@,,, for all the substances. It requirks more heat to convert a liquid into vapour than
to convert a solid into a liquid. It seems'reasonable to assume that a large heat absorption Heat absorbed by a substance at
constant pressure at its melting or
during change of state is associated with inorease in disorder. On this assumption, we can boiling point is used, not to increw
think that a liquid has considerable measure of orderly arrangement as compared to a gas. the temperature but to increase its
disorderliness. In the language of
fable 4.1 :Molar Enthalpks of Fusion (m-)t a d Va~*fion (w-I m b g Poinis (BP) of the thermodynamia, such heat
Substnnces absorption during change of strte
increases the entropy ofthe
Substance &JW mol-' &&A mol-' BP/K substance. We shall discuss this in .
Unit 8. Some correlations regarding
Methane 1.O 8.2 111.5 AH0.., are given in Secs.4.5 and 4.6.
Etbane 2.9 14.5 184.4
X-Ray Diffraction by Liquids
In the next unit, we shall study that the X-ray diffraction by a solid crystal gives rise to sharp X-ray diffraction is the scattering o[
diffraction pattern. The sharpness of diffraction pattern is an indication of the orderly X-rays from a regular array of
arrangement of atoms or ions in the crystal lattice. Gases, on the other hand, do not give atoms, molecules or ions,
rise to diffraction lines with X-rays. This is again due to the random arrangement and
movement of molecules in a gas. Liquids do give diffraction patteras with X-rays, although
Voids
the lines are diffuse (i.e., not quite sharp). The diffuse diffraction pattern makes it clear that
the order in the arrangement of particles is o ~ l ypartial but not total. Experimental data
indicate that the first few neighbours.of Cparticle in a liquid are at fairly well-defined
distances; the neighbours farther away are randomly distributed. This means that the
arrangement of particles in a liquid exhibits short range order and long-range disorder. The
number of nearest neighbours around the particles in different regions of a liquid is not the
same. A rnodel fof the structure of liquids is shown in Fig. 4: 1.
The main aspects of this model are summarised below :
The particles in a liquid are fairly close.
These particles have higher kinetic energy (and hence, speed) compared to those in a E tg. J. I : I\ niodcl for the
solid. structure of liquid,
Because of their speed, the individual particles occupy more space, and a liquid is less
dense than the corresponding solid.
To explain the relativedensities of liquids and solids, it is further assumed that there are
some voids between the.molecules.
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