An ideal gas is an imaginary substance that obeys the relation Pv = RT.
At low pressures and high temperatures, the density of a gas decreases, and the gas behaves as an ideal gas under these conditions.
In the range of practical interest, many familiar gases such as air, nitrogen,
oxygen, hydrogen, helium, argon, neon, carbon dioxide, and even heavier
gases such as krypton can be treated as ideal gases with negligible error (often
less than 1 percent).
Dense gases such as water vapor in steam power plants and refrigerant vapor in refrigerators, however, should not be treated as ideal gases.