Q.Gases deviate from ideal behavior at high pressure. Which of the following statement(s) is correct for non-ideality?

Step 1. Ideal-gas behaviour rests on two assumptions: negligible molecular volume and no intermolecular attraction. At high pressure, gas density rises sharply, so molecules are forced much closer together.

Step 2. Check each option: (a) collisions becoming "enormous" is not the textbook's reasoning for non-ideality.

(b) molecules do not start moving in only one direction under pressure -- their motion stays random.

(c) is the opposite of what actually happens: at high pressure the molecules' own volume becomes significant relative to the container, not insignificant.

Step 3. (d) matches the actual physical cause: closer molecules feel each other's attractive pull far more strongly, which is exactly what the van der Waals pressure correction $an^2/V^2$ accounts for.

Recall the two kinetic-theory assumptions ideal gases rely on, and identify which one fails as molecules are forced closer together at high pressure.

• Picking (c), which states the volume effect backwards -- at high pressure the molecules' own volume becomes significant, not insignificant.
• Confusing this qualitative reasoning question with an equation-recall question.