Latent Heat, Vapour pressure, Factor affecting vapour pressure, Surface area, Types of molecule, Temperature and Intermolecular forces, Sublimation Critical point

1. IIIrd Semesester B. pharmacy
Physical Pharmaceutics-I
Unit-II
State of matter and properties
of matter (Part-2)
(Latent Heat, Vapour pressure and Sublimation.)
Miss. Pooja D. Bhandare
(Assistant professor)
Kandhar college of pharmacy

2. LATENT HEAT
• In latent heat, energy absorbed or released by a substance during a change in
physical state (phase) that occurs without changing its temperature.
• The heat required to convert a solid into a liquid or vapour or a liquid into a
solid without change of temperature.
• The latent heat normally expressed as the amount of heat ( in unit of joules or
calories) per mole or unit mass of the substance undergoing a change of state.
• Examples are latent heat of fusion and latent heat of vaporization involved in
phase changes i.e a substance condensation or vaporization at a specific
temperature and pressure

3. VAPOUR PRESSURE
• The vapour pressure of a liquid is the equilibrium pressure of a vapour above its
liquid ( or solid); that is, the pressure of vapour resulting from evaporation of a
liquid (or solid) above a sample of the liquid (or solid) in closed container.
• The vapour pressure of a liquid varies with
its temperature, as the following graph show
for water.
• As the temperature of liquid or solid increases
its vapour pressure also increases.
Conversely, Vapour pressure decreases as
the temperature decreases.

4. • At the point when the rate of evaporation is equal to the rate of condensation of
the equilibrium the pressure created by vapour on the inner wall of container is
called as vapour pressure of liquid.
• Examples: Substance Vapour pressure at 25°C
Diethyl ether 0.7 atm
Bromine 0.3 atm
Ethanol 0.08 atm
Water 0.03 atm

5. FACTOR AFFECTING VAPOUR PRESSURE
1. Surface area :
Vapour pressure is independent of the surface area.
2. Types of molecules:
If the intermolecular forces between molecules are relatively strong, the vapour pressure will be
relatively low.
3. Temperature:
At a high temperature, more molecule have a enough energy to escape from the liquid or solid.
At a lower temperature, fewer molecule have sufficient energy to escape from the liquid or solid.
4. Intermolecular Forces:
Those liquid in which the intermolecular forces are weak show high vapour pressure

6. SUBLIMATION CRITICAL POINT
• Definition: Sublimation is the change of state from a solid to a gas without passing
through the liquid state
• Carbon dioxide is an example of a material that easily undergoes sublimation.
• Phase diagram: It is a representation of the states of matter: Solid, Liquid or gas as a
function of temperature and pressure.

7. • In triple fusion curve B is the point where all phase of matter all in equilibrium
this point is called triple fusion point.
• The temperature at which the vapor pressure of the solid phase of a compound is
equal to the total pressure of the gas phase in contact with it: analogous to the
boiling point of the liquid.
• Phase Rule: Phase rule is a rule relating the possible number of phase,
constituent, degree of freedom in a chemical system. This Rule was proposed by
J. Willard Gibbs in 1876.

8. • The phase can be define as any homogeneous part of a system having all the
physical and chemical properties.
• The properties are identical throughout.
• A system may consist of one phase or more than one phase.
1. A system containing only liquid water is a single-phase system (P=1)
2. A system containing liquid water and steam ( a gas) is a two-phase system
(P=2)
3. A system containing liquid water, steam and solid ice is three phase system

9. • For a system at equilibrium the phase rule relates:
P + F = C + 2
Where,
P = Number of phase that can coexist
F = Number of components making up the phases
C = Number of independent variables or degree of freedom.

10. Thank
you!