  Ideal gas is the most fundamental and simple system in thermodynamics, which
has extensive applications in energy research and engineering. By reviewing the
physical concept of ideal gas, it is found that the current understanding of
ideal gas is still inappropriate and ambiguous, making it challenging to reveal
the essential difference between ideal and real gases. Therefore, the macro and
microscopic definitions and properties of ideal gas need to be
reconceptualized. On the microscopic level, an ideal gas is a hypothetical
collection of classical masses in irregular motion, which does not involve
quantum mechanics; on the macroscopic level, any real gas cannot strictly obey
the equation of state of ideal gas. Moreover, the heat capacity is a constant
which should be an endogenous attribute of ideal gas model in order to unify
the macroscopic and microscopic definitions of ideal gas. Finally, according to
the law of heat capacity, the real gas can be divided into three categories:
far-ideal, near-ideal, and quasi-ideal. This makes it easier to perform
thermodynamic calculations with the assistance of ideal gas. Among them, the
far-ideal gas heat capacity is a function of temperature and pressure, the
near-ideal gas heat capacity is a single-valued function of temperature, and
the quasi-ideal gas heat capacity is a constant.
