What is meant by a gas being 'non-ideal'?

A gas being 'non-ideal' refers to its failure to perfectly obey the ideal gas law, which states that the behavior of gas can be described by the equation PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the ideal gas constant, and T is temperature. In reality, most gases do not conform to this law under all conditions, especially at high pressures or low temperatures. Deviations from ideal behavior can occur due to factors such as intermolecular forces and the finite volume of gas particles, which ideal gas theory generally overlooks.

In contrast, the other options describe characteristics that do not accurately define non-ideal gases. A non-ideal gas can still be compressible; it simply behaves differently than expected under certain conditions. Similarly, the temperature of a gas does not categorically define it as non-ideal, as non-ideal behavior can be observed at various temperatures. Lastly, the composition of a gas does not inherently determine its ideal or non-ideal status; a gas can consist of one type of particle and still exhibit non-ideal behavior due to intermolecular interactions. Thus, the fundamental concept of non-ideality is closely tied to deviations from the ideal gas law