The Gas is the term applied to the state of any Substance of which the Evaporation from the Liquid state is complete.Examples are Air,Oxygen,Nitrogen etc.
Vapour may be defined as a partially Evaporated Liquid and consist of the pure gaseous state together with the particles of Liquid in suspension."Superheating" is the process where Dry Vapour is heated further.This Superheated Vapours approximately behaves like a Gas.Example of Vapours used for Engineering purpose are Wet Steam,Ammonia etc.
A Perfect Gas or an idle Gas is one which strictly obeys all Gas Laws under all conditions of Temperature and Pressure.Actually no Gas known is perfect but within the Temperature limits of Applied Thermodynamics many Gas like Hydrogen,Oxygen and even Air may be regarded as Perfect Gas.
The Pressure(p) exerted by the Gas,the Volume(v) occupied by it and its Temperature (T) are three variables control the Physical properties of a Gas.
Any one variable can be calculated by applying the Gas Law if other two variables are given.These are not applicable for Vapours.
Important Laws of Perfect Gases.
01)Boyle's Law:-This Law states that "The Volume of a given Mass of a Gas varies inversely as its absolute Pressure,provided the Temperature remains constant.
The deviation can be overlooked in case of Air,Hydrogen and Nitrogen etc.expect at very low Temperature or at very high Pressure.
02)Charles's Law:-This Law states that"The volume of a given Mass of a Gas varies directly as its absolute Temperature provided the Pressure is kept constant".
03)Joule's Law:-This Law states that "The internal Energy of a given quantity of a Gas depends on the Temperature".
When a Gas expands without doing external work and without taking in or giving out Heat,Its Temperature does not change.
04)Regnault's Law:-This Law states that "The two specific Heat at constant Pressure and specific Heat at constant Volume of a Gas do not change with the change of Temperature and pressure.
05)Avogadro's Law:-This Law states that "Equal Volume of different Gases at the same Temperature and Pressure contains equal number of Molecules".