The kinetic theory of gases rests on a set of simplifying assumptions. NEET doesn't just test whether you can recite them — it tests whether you can identify which assumption breaks in a given scenario and what consequence follows.
The assumptions (NCERT Class 11 Physics Chapter 12, page 247):
Where aspirants lose marks:
The most common confusion is between elastic collisions and no intermolecular forces. These are separate assumptions. Elastic collisions mean kinetic energy is conserved during impact. No intermolecular forces means molecules travel in straight lines between collisions — no deflection, no potential energy between encounters. NEET distractors routinely swap these two or merge them into one statement.
A second frequent error: treating "point particles" as meaning molecules have zero mass. The assumption is about negligible size (volume), not mass. Each molecule retains its mass; it simply occupies negligible volume relative to the container.
Connection to the ideal gas equation: When all six assumptions hold, you can derive PV = NkT from first principles — pressure arises purely from momentum transfer during wall collisions. The formula F = (1/3)Nmv²_rms / V emerges directly. Any real-gas deviation (van der Waals correction, liquefaction at high pressure) traces back to one of these assumptions breaking down.
Watch-out: When a question says "ideal gas," it implicitly invokes all six assumptions. When it says "real gas at high pressure," assumptions 2 and 5 are violated — molecular volume matters and intermolecular attractions become significant.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following is an assumption of the kinetic theory of an ideal gas?
Answer: B. Perfectly elastic collisions — conserving total kinetic energy — is one of the six standard assumptions of kinetic theory (NCERT Class 11 Physics Chapter 12, page 247).
Why A is wrong: A is wrong because kinetic theory assumes no intermolecular forces between collisions. Van der Waals forces are a real-gas correction, not an ideal-gas assumption.
Why C is wrong: C is wrong because the assumption states molecular size is negligible compared to intermolecular separation — the opposite of what this option claims.
Why D is wrong: D is wrong because molecules undergo constant random motion, not fixed ordered paths. Random motion is a core assumption.
In kinetic theory, the assumption that gas molecules are "point particles" means:
Answer: A. The point-particle assumption refers to negligible size (volume), not mass. Each molecule retains its full mass but occupies a volume far smaller than the average distance between molecules (NCERT Class 11 Physics Chapter 12, page 247).
Why B is wrong: B is wrong because 'point particle' refers to negligible volume, not zero mass. Molecules must have mass to carry momentum and kinetic energy.
Why C is wrong: C is wrong because 'point particle' is a statement about spatial extent, not about energy states. A molecule at any temperature above 0 K has nonzero kinetic energy.
Why D is wrong: D is wrong because kinetic theory explicitly assumes molecules obey Newton's laws of motion — that is a separate assumption, not contradicted by the point-particle approximation.
According to kinetic theory assumptions, between successive collisions, an ideal gas molecule:
Answer: C. With no intermolecular forces acting between collisions and neglecting gravity at molecular scale, molecules travel in straight lines at constant speed between collisions — Newton's first law applied directly (NCERT Class 11 Physics Chapter 12, page 247).
Why A is wrong: A is wrong because the kinetic theory explicitly assumes no intermolecular forces between collisions. Attractive forces belong to the real-gas (van der Waals) model.
Why B is wrong: B is wrong because 'air resistance' is a macroscopic drag concept. Inside the gas itself, the only interactions are discrete collisions, not continuous drag.
Why D is wrong: D is wrong because the kinetic theory model neglects gravitational effects on individual molecular trajectories. Gravity plays no role in the standard derivation of pressure from molecular motion.
An ideal gas assumption states that collisions are perfectly elastic. This directly implies that during a collision:
Answer: D. A perfectly elastic collision conserves both total momentum and total kinetic energy. Momentum conservation holds for all collisions (Newton's third law); the additional constraint of elastic collisions is that kinetic energy is also conserved (NCERT Class 11 Physics Chapter 12, page 247).
Why A is wrong: A is wrong because it describes an inelastic collision. In kinetic theory, collisions are elastic, so kinetic energy IS conserved alongside momentum.
Why B is wrong: B is wrong because momentum conservation is never optional — it holds for all collisions by Newton's third law. You cannot have KE conserved while momentum is not.
Why C is wrong: C is wrong because momentum conservation is guaranteed by Newton's third law for any collision. Claiming neither is conserved contradicts fundamental mechanics.
Which pair of kinetic theory assumptions is violated when a real gas is compressed to very high pressure?
Answer: D. At very high pressures, molecules are forced close together. Molecular volume becomes significant relative to the container volume (violating the point-particle assumption), and intermolecular forces (attraction/repulsion) become non-negligible (violating the no-force assumption). These are the two corrections captured by the van der Waals equation (NCERT Class 11 Physics Chapter 12, page 247).
Why A is wrong: A is wrong because collisions remain approximately elastic and motion remains random even at high pressure. The dominant breakdowns are finite molecular volume and intermolecular forces.
Why B is wrong: B is wrong because while the point-particle assumption does break down at high pressure, elastic collisions are not the second violated assumption — intermolecular forces (not collision elasticity) are the issue.
Why C is wrong: C is wrong because collision duration and Newton's laws are not the assumptions that primarily break down at high pressure. The key failures are molecular volume and intermolecular forces.
A student claims: "In an ideal gas, molecules exert no forces on each other, therefore collisions between molecules cannot occur." The error in this reasoning is:
Answer: A. The assumption of no intermolecular forces means no attractive or repulsive forces act between collisions — molecules travel freely in straight lines. During the brief collision event itself, a short-range repulsive interaction occurs (modelled as instantaneous elastic impact). The student conflated "no forces between collisions" with "no forces ever" (NCERT Class 11 Physics Chapter 12, page 247).
Why B is wrong: B is wrong because kinetic theory explicitly states ideal gas molecules exert NO forces on each other between collisions. Claiming long-range forces exist contradicts the assumption.
Why C is wrong: C is wrong because collisions are central to the kinetic theory model — pressure itself is derived from molecular collisions with container walls.
Why D is wrong: D is wrong because molecules in an ideal gas do collide; the entire derivation of pressure depends on wall collisions, and intermolecular collisions determine mean free path.
If the assumption of negligible collision time were removed (i.e., collisions lasted a significant fraction of the time between collisions), which derived quantity of kinetic theory would be most directly affected?
Answer: C. Mean free path is defined as the average distance a molecule travels between successive collisions. If collision duration becomes significant, the molecule spends appreciable time in the collision process rather than travelling freely, directly altering the effective distance and time intervals used to calculate mean free path. Molar mass, mole count, and Avogadro's number are intrinsic properties unaffected by collision dynamics.
Why A is wrong: A is wrong because molar mass is an intrinsic molecular property determined by atomic composition, not by collision dynamics.
Why B is wrong: B is wrong because the number of moles is set by the amount of substance present, not by how long collisions last.
Why D is wrong: D is wrong because Avogadro's number is a fundamental constant defining the number of entities per mole — it does not depend on gas behaviour.
Two containers hold equal amounts of gas at the same temperature. Container X holds an ideal gas; Container Y holds a real gas at moderate pressure. Compared to Container X, the pressure in Container Y is:
Answer: B. At moderate pressures, the dominant real-gas correction is intermolecular attraction. As a molecule approaches the wall, it is pulled back slightly by neighbouring molecules, reducing the momentum it transfers to the wall. This lowers the measured pressure below the ideal-gas prediction. This is the basis of the van der Waals 'a' correction term: P_real ≈ P_ideal − a(n/V)² (NCERT Class 11 Physics Chapter 12, page 247 — deviation from ideal behaviour).
Why A is wrong: A is wrong because both containers hold the same gas (same molecular mass). Real-gas corrections come from intermolecular forces and finite molecular volume, not different masses.
Why C is wrong: C is wrong because pressure depends on molecular interactions as well as temperature. The ideal gas equation PV = nRT holds only when kinetic theory assumptions are satisfied. Real gases deviate.
Why D is wrong: D is wrong because finite molecular volume reduces the available free volume (effectively increasing pressure in the van der Waals model), but at moderate pressures, the attractive-force effect dominates, producing a net decrease in pressure.
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Given
• Gas: nitrogen (N₂) at standard temperature• Compression: volume reduced to V/200 (a 200× compression)• Student's model: ideal gas (all six kinetic theory assumptions applied)
Required
Identify the two kinetic theory assumptions that fail at extreme compression and describe the qualitative effect on pressure.
Concept
The ideal gas equation PV = nRT assumes: (1) molecular volume is negligible relative to the container, and (2) no intermolecular forces between collisions. At extreme compression, both assumptions break down because molecules are forced into close proximity.
Framework (van der Waals corrections as diagnostic)
The van der Waals equation corrects for both violations:• Finite molecular volume: effective free volume is (V − nb) < V. Molecules "use up" some container space.• Intermolecular attraction: effective pressure is reduced by a(n/V)².
These two corrections map directly to assumptions 2 (point particles) and 5 (no intermolecular forces).
Identification
Violated assumption 1: Negligible molecular volume (point-particle assumption). At V/200, the total molecular volume of N₂ becomes a significant fraction of the container volume.
Violated assumption 2: No intermolecular forces. At close range, van der Waals attractive forces between N₂ molecules become non-negligible, and at very close range, repulsive forces emerge.
Effect on pressure
• Finite volume effect: The available free space is less than V, so molecules hit walls more frequently than the ideal model predicts → pressure is higher than ideal at extreme compression (the nb correction).• Attractive force effect: Molecules approaching the wall are pulled back by neighbours → effective momentum transfer to the wall decreases → pressure is lower than ideal (the a/V² correction).
At extreme compression, the finite-volume effect typically dominates, so real pressure exceeds ideal-gas predictions.
Final answer
The two violated assumptions are: (i) negligible molecular volume, and (ii) no intermolecular forces. The net effect at extreme compression is that real pressure exceeds the ideal-gas prediction because finite molecular volume dominates over attractive-force reduction.
Note on constants: This is a qualitative problem. No numerical constants were used. In quantitative van der Waals problems, the constants 'a' and 'b' are gas-specific empirical values (not fundamental constants), and temperature T enters in kelvin.
Common trap
Aspirants often state only one violated assumption (usually intermolecular forces) and forget molecular volume. NEET questions on real-gas deviations frequently offer distractors that name only one correction. Both assumptions must be cited for full marks.
Similar NEET-style question
"At very high pressures, the compressibility factor Z = PV/nRT for a real gas is greater than 1. Which kinetic theory assumption is primarily responsible for Z > 1?" Answer: the finite molecular volume assumption — the repulsive/excluded-volume correction dominates at very high pressure, making PV > nRT.
(1) Gas consists of many identical molecules. (2) Molecules in random motion. (3) Collisions are elastic. (4) Total volume of molecules << container volume. (5) No intermolecular forces (except during collisions). (6) Newtonian mechanics applies.
-- NCERT Class 11 Physics, Ch. 12, p. 248More in Kinetic Theory: 2 exam traps and mistakes · 5 formulas · 4 question patterns from its other lessons.
No question in our NEET 2020–2025 set targets this topic directly.
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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