A real gas molecule has a shape and a finite size. An ideal gas molecule (imaginary) is a point with no shape and it occupies no space. A real gas molecule interacts with others. An ideal gas molecule reacts totally independent of all others.
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Nothing, if you are at "room temperature" (20-25C) and atmospheric pressure. However, if you are at temperatures far below 0C and very high pressures, then real gases start to depart from ideal behavior. Also, if you deal with gases like sulfur dioxide, ammonia, and steam, then things go haywire in a hurry. There are whole books of "steam tables" that tell you how water vapor behaves non-ideally.
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An ideal gas is just theoretical. A real gas is just that: Real.