Ideal Gas equation and its Assumptions [duplicate]








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[Clarification of textbook concepts relating to "perfect", "ideal", and "real" gases](/questions/128088/clarification-of-textbook-concepts-relating-to-perfect-ideal-and-real-ga)

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The two assumptions are that the gases are points of mass that move, they have no volume and that there is no interaction between other molecules.


But even the points of mass can collide with each other so wouldn't that be considered interaction between molecules? 




Part of the assumptions set of an ideal gas is that there are no attractive or repulsive forces between them.


Also, part of the assumption is that the volume of molecules obtain a negligible amount when compared to the overall volume of the gas.


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There is no ideal gas in reality. If you consider the molecules as point masses, they have no volume by definition of a point. If point masses, or a better word, gas "particles" collide, their collision is perfectly elastic i.e., there is no loss of energy by either particle. When you go to advanced classes you will study other gas laws which do not rely on point masses but include molecular volume and molecular interactions. If you are interested see Van der Waals gas equation.


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IUPAC only defines an ideal gas phenomenologically:



> 
> Gas which obeys the equation of state pV=nRT...
> 
> 
> 


<https://goldbook.iupac.org/terms/view/I02935>


There's good reason for that, since there is no universally-agreed-upon microscopic definition of an ideal gas.


The definition I prefer is that an ideal gas consists of point masses that don't interact with each other, but only (elastically) with the walls of the container. [A monatomic ideal gas consists of point masses; a diatomic ideal gas particle consists of two point masses joined by a massless 2D connecter.]


An alternate definition is the one to which you alluded (and which can be found on, e.g., <https://en.wikipedia.org/wiki/Ideal_gas> ), which is that an ideal gas consists of point masses whose only interaction consists of purely elastic collisions: they extert no attractive or repulsive forces, except for the hard-sphere repulsive potential responsible for the elastic collision. 


The problem with the latter definition is that point masses have zero volume, and thus zero scattering cross-section. I.e., if they have no volume, they will never collide.


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