Which of the following is NOT a characteristic of an ideal gas?

The correct answer is that gas molecules exert attractive forces between each other. In the ideal gas model, it is assumed that the gas molecules do not attract or repel each other; they are considered to behave independently. This is foundational to the behavior of ideal gases, where the only interactions that are considered are those occurring during collisions, which are treated as perfectly elastic, meaning no kinetic energy is lost during these interactions.

In contrast, the other statements align with the characteristics of an ideal gas. It is indeed assumed that gas molecules have zero volume for the purposes of calculations, as their size is negligible compared to the distance between them. Additionally, collisions between gas molecules are considered completely elastic, allowing for the conservation of kinetic energy. Lastly, the average kinetic energy of gas molecules is directly proportional to the temperature of the gas, a key principle in understanding gas behavior under varying thermal conditions.

Therefore, the presence of attractive forces among gas molecules contradicts the ideal gas assumptions, making this the only characteristic that does not apply to an ideal gas.