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 State Variable: The measurable properties by which a state can be
expressed.
Example – pressure, volume, temperature, composition (no. of moles)
 State Function: The properties of the system which depends upon state

Example – Internal energy, enthalpy, entropy, Gibb’s free energy.
 Path Function:The properties of the system which depends upon the path

or manner by which the change has been achieved.
Example – Work and heat
 Thermodynamic process:The operation by which a system changes from
one state to another.

(i)Isothermal Process:The process which takes place at constant
temperature.
(ii)Isobaric Process:The process which takes place at constant pressure

(iii)Isochoric Process:The process which takes place at constant volume.
(iv)Adiabatic Process:The process which takes place without exchange of
heat energy.

(v)Cyclic Process:The process in which the system returns to its original
state after a series of changes.
(vi)Reversible Process:The process in which the change is carried out

infinitesimally slowly so that the system and the surrounding always remain
in equilibrium.
(vii)Irreversible Process:The process in which the change is carried out
rapidly so that the system does not get any chance to remain in equilibrium

with the surrounding.
 Zeroth law of Thermodynamics:It states that if two systems are in thermal

equilibrium with a third system, they are also in thermal equilibrium with
each other.
 Internal energy(U):
It is the total energy of the system. It is the sum of kinetic and potential

energy of the system. It is the heat change at constant volume.
Characteristics of Internal energy:
(i)It is an extensive property.

(ii)It is a state function.
(iii)It does not depend upon path.