Liquid State
In the liquid state the molecules are in contact with each other. The forces of attraction
between the molecules are strong enough to hold them together. The molecules are
able to move past one another through available intermolecular spaces. Most of the
physical properties of liquids are actually controlled by the strengths of intermolecular
attractive forces.
Intermolecular forces in liquids
Intermolecular forces in liquids are collectively called ‘van der Waals’ forces. These
forces are essentially electrical in nature and result from the attraction of charges of
opposite sign. The principal kinds of intermolecular attractions are:
1. Dipole-dipole attractions
Dipole-dipole attractions exist between molecules that are polar. This requires the
presence of polar bonds and an unsymmetrical molecule. These molecules have a
permanent separation of positive and negative charge.
In the illustration the H end of HCl has a partial positive charge (δ+) and the Cl end has a
partial negative charge (δ-). The H atom in one molecule is attracted to the Cl in a
neighbor. The intermolecular force is weak compared to a covalent bond, but this
dipole-dipole interaction is one of the stronger intermolecular attractions.
2. London dispersion forces (temporary dipole-induced dipole attraction)
The London dispersion force is the weakest intermolecular force; arise from temporary
charge development that occurs spontaneously in all compounds, even in neutral
symmetrical atoms.
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PHRM 101: Physical Pharmacy I
The electrons are mobile, and at any one instant they might find themselves towards
one end of the molecule, making that end δ-. The other end will be temporarily short of
electrons and so becomes δ+ (temporary dipole).
As another neutral molecule approaches, its electrons will tend to be attracted by the
slightly positive end of the temporary dipole. This sets up an induced dipole in the
approaching molecule, which is orientated in such a way that the δ+ end of one is
attracted to the δ- end of the other. Attractions between opposite partial charges of
neighboring induced dipoles cause atoms to stick together for a very short time.
3. Hydrogen bonding
A chemical bond between an electronegative atom and a hydrogen atom bonded to
another electronegative atom. These bonds are generally stronger than ordinary dipoledipole and dispersion forces, but weaker than true covalent and ionic bonds. Hydrogen
bonding is a unique type of intermolecular attraction. There are two requirements:
1. Covalent bond between an H atom and either F, O, or N. These are the three
most electronegative elements.
2. Interaction of the H atom in this kind of polar bond with a lone pair of electrons
on a nearby atom like F, O, or N.
Hydrogen Bond Donor: A hydrogen atom attached to an electronegative atom (usually
F, O or N) is a hydrogen bond donor. The electronegative atom attracts
the electron cloud from around the hydrogen nucleus and leaves the hydrogen atom
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PHRM 101: Physical Pharmacy I
with a positive partial charge. Because of the small size of hydrogen relative to other
atoms and molecules, the resulting charge, though only partial, is stronger.
Hydrogen Bond Acceptor: A hydrogen bond results when this strong partial positive
charge attracts a lone pair of electrons from an electronegative atom, which becomes
the hydrogen bond acceptor. Greater electronegativity of the hydrogen bond acceptor
will create a stronger hydrogen bond.
Applications for Hydrogen Bonds
 Hydrogen bonds occur in inorganic molecules, such as water. Hydrogen bonding
in water contributes to its unique properties, including its high boiling point and
surface tension.
 The two complementary strands of DNA are held together by hydrogen bonds
between complementary nucleotides (A&T, C&G).
 Intermolecular hydrogen bonding is partly responsible for the secondary,
tertiary, and quaternary structures of proteins and nucleic acids.
Physical properties of liquid state
a) Vapour pressure
The process by which molecules of a liquid go into the gaseous state (vapor) is called
vaporization or evaporation. The reverse process whereby gas molecules become liquid
molecules is called condensation.
If the liquid is placed in a closed vessel, the molecules with high kinetic energies escape
into space above the liquid. As the number of molecules in the gas phase increases,
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PHRM 101: Physical Pharmacy I
some of them strike the liquid surface and are recaptured (condensation). A stage
comes when the rate of evaporation exactly equals the rate of condensation. Thus a
dynamic equilibrium is established between the liquid and the vapour at the given
temperature.
Liquid ↔ Vapour
So, the vapour pressure of a liquid can be defined as the pressure exerted by the vapour
in equilibrium with the liquid at a fixed temperature.
Effect of Temperature on Vapour Pressure: If the temperature of the liquid is increased,
the vapour pressure will increase. This is so because at higher temperature more
molecules in the liquid will have larger kinetic energy and will break away from the
liquid surface.
Relation between Vapour Pressure and Boiling Points: The boiling point of the liquid
can be defined as the temperature at which the vapour pressure of the liquid is equal to
the atmospheric pressure (760 torr or 1 atm).
Normally water will boil at 100°C at sea level. At this temperature liquid water turns to
gas. Applying more heat to an open pot of water will only increase the rate that liquid
water turns to vapour but will not increase the temperature of the liquid.
If the pressure is decreased (e.g., at higher elevations) the temperature that water boils
will be decreased (since it is easier for water molecules to escape the surface).
With no place to escape if heat is applied to the closed container the molecules in the
gas state will increase velocity. This will increase the pressure on the surface of the
liquid and temperature of the water system will then increase. Because the effective
cooking temperature is higher in the pressure cooker as high as 120°C; cooking time can
drop substantially.
b) Surface tension (γ)
Surface tension is an effect within the surface layer of a liquid that causes the layer to
behave as an elastic sheet. The surface tension is defined as ‘the amount of force (Nm)
necessary to expand the surface (m2) of a liquid by one unit.’
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PHRM 101: Physical Pharmacy I
Surface tension is caused by the attraction between the molecules of the liquid, due to
various intermolecular forces. In the bulk of the liquid each molecule is pulled equally in
all directions by neighbouring liquid molecules, resulting in a net force of zero. At the
surface of the liquid, the molecules are pulled inwards by other molecules deeper inside
the liquid, but there are no liquid molecules on the outside to balance these forces. All
of the molecules at the surface are therefore subject to an inward force of molecular
attraction which can be balanced only by the resistance of the liquid to compression.
Thus the liquid squeezes itself together until it has the lowest surface area possible
(spherical shapes).
Effect of Temperature on Surface Tension: Surface tension decreases with increase in
temperature. When temperature increases, there is an increase in kinetic energy of
liquid molecules (KE ∝ T), thereby decreasing intermolecular forces. It results in
decrease in the inward pull functioning on the surface of the liquid.
c) Viscosity (η)
Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a
moving fluid. A fluid with large viscosity resists motion because its molecular makeup
gives it a lot of internal friction. A fluid with low viscosity flows easily because its
molecular makeup results in very little friction when it is in motion.
Let us consider two adjacent moving layers of a liquid. Let these be separated by a
distance dx and have a velocity difference dv. The force of friction (F) resisting the
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PHRM 101: Physical Pharmacy I
relative motion of the two layers is directly proportional to the area A and the velocity
difference dv, while it is inversely proportional to the distance between the layers, dx.
That is,
FA
dv
dx
dv
dx
F dx
or η  
A dv
or F  ηA
Where η is the proportionality constant. It is known as the coefficient of viscosity or
simply viscosity of a liquid. η has a specific value for a given liquid at the same
temperature. It may be defined as: the force of resistance per unit area which will
maintain unit velocity difference between two layers of a liquid at a unit distance from
each other.
Units of Viscosity: In CGS system the unit of η is expressed as g cm–1 s–1. It is called poise
(P). In practice smaller units centipoise (10–2 poise) and millipoise (10–3 poise) are used.
Measurement of Viscosity: Viscosity of a liquid can be determined with the help of
Poiseuille’s equation. This expression which governs the flow of a liquid through a
capillary may be written as:
η
πPr 4 t
8lV
Where, t = time of flow of liquid,
V = volume of the liquid,
P = pressure-head (height of a column of fluid necessary to develop a specific pressure
difference)
r = radius of the tube
l = its length.
The experimental measurement of P, r, l and V offers considerable difficulty. Therefore,
it is not possible to find the absolute coefficient of viscosity (η) straight away from
Poiseulle’s equation.
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PHRM 101: Physical Pharmacy I
Ordinarily, the viscosity of a liquid is determined with respect to that of water. This is
called ‘Relative viscosity’. Let t1 and t2 be the times of flow of a fixed volume (V) of the
two liquids through the same capillary. The expression for relative viscosity (η 1/ η2):
η1 πP1r 4 t 1
Pt
8lV


 11
4
η2
8lV
πP2 r t 2 P2 t 2
Since the pressure-head is proportional to density (d) of the liquid,
η1 d1 t 1

η2 d 2 t 2
Ostwald Viscometer
The apparatus commonly used for the determination of relative viscosity of a liquid is
known as Ostwald viscometer. The right-hand limb is essentially a pipette with two
calibration marks A and B. A length of capillary tube joins the pipette to a bulb C in the
left-hand limb.
A definite volume of liquid (say 25 ml) is poured into the bulb C with a pipette. The
liquid is sucked up near to the top of the right-limb with the help of rubber tubing
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PHRM 101: Physical Pharmacy I
attached to it. The liquid is then released to flow back into the bulb C. The time (t) from
A to B is noted with a stopwatch.
Then the apparatus is cleaned and the experiment repeated with water, taking about
the same volume. The time of flow of water (tw) from A to B is recorded. The density of
the liquid (d) and water (dw) are determined with the help of a pyknometer. The relative
viscosity coefficient is calculated from the expression
dt
η

ηw d w t w
Knowing the value of the viscosity coefficient of water (η w) at the temperature of the
experiment, the absolute viscosity coefficient (η) of the given liquid can be found.
[The viscosity of water at 20°C is 1.002 centipoise or 0.01002 poise
Density of water at 20°C is 0.998 g/cm3 or 998.2 Kg/m3]
Problem: In an experiment with Ostwald viscometer, the times of flow of water and
ethanol are 80 sec and 175 sec at 20°C. The density of water = 0.998 g/cm3 and that of
ethanol = 0.790 g/cm3. The viscosity of water at 20°C is 0.01008 poise. Calculate the
viscosity of ethanol.
Solution: Substituting values in the expression
η  ηw
dt
dwtw
η  0.01008 
0.790  175
 0.01747 poise
0.998  80
Problem: In an experiment with Ostwald viscometer, pure water took 1.52 minutes to
flow through the capillary at 20°C. For the same volume of another liquid of density 0.80
g/cm3 the flow time was 2.25 minutes. Find the relative viscosity of the liquid and its
absolute viscosity in centipoise. Density of water at 20°C is 0.9982 and absolute viscosity
of water is 1.005 centipoise.
Solution: Substituting values in the expression
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PHRM 101: Physical Pharmacy I
dt
η

ηw d w t w
η
0.80  2.25

 1.184
η w 0.9982  1.52
η  1.184  1.005  1.19 centipoise
Thus the relative viscosity of the liquid is 1.184 and its absolute viscosity is 1.19
centipoise.
Effect of temperature on viscosity of a liquid: In general, the viscosity decreases with
increase in temperature. It has also been found that there is 2% decrease in viscosity for
every increase in one degree of temperature of the liquid.
d) Refractive index (n)
The refractive index of a substance is defined as the ratio of the velocity of light in
vacuum or air, to that in the substance.
Velocity of light in air/vaccum
n
Velocity of light in substance/medium
When a ray of light passes from air into a liquid, its direction is changed. This change of
direction is called refraction. The refractive index of the liquid with respect to air is given
by Snell’s Law.
n
Sin i
Sin r
Where i = angle of incidence
r = angle of refraction
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PHRM 101: Physical Pharmacy I
e) Optical activity
A beam of ordinary light consists of electromagnetic waves oscillating in many planes.
When passed through a polarizer (e.g., a Polaroid lens), only waves oscillating in a single
plane pass through. The emerging beam of light having oscillations in a single plane is
said to be plane polarized.
When plane-polarized light is passed through certain organic compounds, the plane of
polarized light is rotated. A compound that can rotate the plane of polarized light is
called optically active. This property of a compound is called optical activity. Organic
molecule having a certain carbon atom to which four different moieties are attached,
rendering the molecule asymmetric (chiral carbon), are all optically active.
 Levorotatory: A compound which rotates the plane-polarized light to the left
(anticlockwise) and designated by ‘l’ (Latin: Laevus = left). Rotation to the left is
given a minus sign (-). For example, (-)-lactic acid is levorotatory.
 Dextrorotatory: A compound that rotates the plane-polarized light to the right
(clockwise) and designated by ‘d’ (Latin: Dexter = right). Rotation to the right is
given a plus sign (+). For example, (+)-lactic acid is dextrorotatory.
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