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Foundation3 past questions

Kinetic theory and molecular motion

The kinetic theory explains the behavior of gases in terms of the motion and collisions of their molecules, relating properties like pressure, temperature, and kinetic energy.

Why this shows up in the exam

NEET tests your understanding of how molecular motion leads to macroscopic gas properties and the assumptions of kinetic theory.

How NEET tests this

Assertion–reason · 2 QsDirect recall

Learn the idea

Kinetic theory links the microscopic random motion of gas molecules to macroscopic quantities like pressure and temperature; the key insight is that the average translational kinetic energy of every ideal gas molecule is (3/2) kᵦ T, independent of the gas’s identity.

🧠 Memory hook: ENVL – Elastic, No volume, No forces, Large number – the four pillars that make kinetic theory work like a ping‑pong ball game inside a box.

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • Gas molecules are point particles with negligible volume
  • Collisions between molecules and with walls are perfectly elastic
  • No intermolecular forces act except during collisions
  • Pressure P = (1/3)(N m v_rms²)/V
  • Average translational KE per molecule = (3/2) kᵦ T
  • For an ideal gas PV = N kᵦ T

How to approach it

  1. 1Read the statement and decide which kinetic‑theory relation is needed (pressure, KE, rms speed, etc.)
  2. 2Write down the exact NCERT formula that connects the required quantity with T, N, m or V
  3. 3Apply the assumptions: elastic collisions, no volume, no forces – to justify using the ideal‑gas form
  4. 4Solve algebraically, then check units and whether the answer depends on gas type (it should not for average KE)

Worked example — watch it click

Assertion A: Average kinetic energy per molecule of any ideal monoatomic gas is 3/2 kBT. Reason R: Average kinetic energy depends only on temperature and is independent of the nature of the gas.

  • ✅Both Assertion and Reason are true, and Reason is the correct explanation of assertion.
  • B)Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
  • C)Assertion is true, but Reason is false.
  • D)Both Assertion and Reason are false.

The concept behind this problem

The worked example asks you to connect the universal formula (3/2 kᵦ T) with the reason that this energy depends only on temperature, testing whether you recognise that the kinetic‑theory derivation removes any dependence on the nature of the gas.

Step by step

  1. 1Assertion: For a monoatomic ideal gas, average KE per molecule = (3/2)kᵦT.
  2. 2This is correct from kinetic theory.
  3. 3Reason: Average KE depends only on temperature and is independent of the nature of the gas.
  4. 4This is also correct - the average translational KE per molecule is (3/2)kᵦT for all ideal gases regardless of their identity.
  5. 5The Reason correctly explains why the Assertion holds: the formula depends only on T, not on the gas type.

Watch out

Students often think the kinetic‑energy formula varies with the gas type, forgetting that for ideal gases it is solely a function of temperature.

Common slip-ups that cost marks

  • •Confusing total kinetic energy of the whole sample with average kinetic energy per molecule
  • •Using the 5/2 kᵦ T factor (which belongs to diatomic gases) for a mono‑atomic gas
  • •Assuming collisions are inelastic – the theory requires perfectly elastic collisions

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 3

Assertion A: In case of collision of the gas molecules in a given amount of gas, the total kinetic energy is conserved. Reason R: All collisions of the gas molecules in a given amount of gas are elastic.

Push further

More challenging

10 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 10

Two containers, A and B, hold equal masses of an ideal monoatomic gas. Container A is at 200 K and Container B is at 400 K. Which container has molecules with higher average speed?

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