What is an ideal gas? Why do the real gases show deviations from ideal behaviour?

Explanation

(i) A gas which obeys the ideal gas equation `PV=nRT`, at all temperature and pressure is called an ideal or perfect gas.
(ii) Following two assumptions are used to drive the ideal gas equation.
(iii) The size of the gas molecules is negligibly small.
(iv) There is no force of attraction amongst the molecules of a gas.
(v) No real or actual gas fulfills the above conditions. Hence the behaviour of a real gas differs from that of an ideal gas.
(vi) At low pressure are valid and some above assumptions are valid and some real gases like `H_(2),O_(2),H_(e)` etc. almost behave like an ideal gas.