ChemistryICSE Class 8 · Matter

Kinetic Molecular Theory of Matter: Postulates, States and Change of State (ICSE Class 8 Chemistry)

Q: What is the kinetic molecular theory?

It is a model in which matter is made of tiny particles with spaces and attractions between them, moving constantly with kinetic energy that rises with temperature.

Q: How does the kinetic theory explain change of state?

Heating increases kinetic energy and overcomes the attraction, increasing the space (solid to liquid to gas); cooling reverses this.

Q: Why can gases be compressed?

Because there are large spaces between gas particles that can be reduced under pressure.

Updated 17 October 2026 · Draft, not yet fact-checked

The kinetic molecular theory says matter is made of tiny particles with spaces between them, which attract each other and are in constant random motion, with kinetic energy that rises with temperature. Solids, liquids and gases differ in the space between particles, the attraction between them and how they move; heating or cooling changes the state.

Notice particles are packed and fixed in a solid, close but sliding in a liquid, far apart in a gas; heat speeds them up, cooling slows them.

Illustration drawn for QuestionBuddy.

Understand Kinetic Molecular Theory and Change of State

Postulates of the kinetic molecular theory.
1. Matter is made of very small particles (atoms or molecules).
2. There are spaces between the particles (inter-particle space).
3. The particles attract each other (inter-particle force of attraction), more strongly when they are close.
4. The particles are in constant random motion, so they have kinetic energy.
5. The kinetic energy increases with temperature: heated particles move faster.
6. Moving particles collide with each other and with the walls of the container.

Solids, liquids and gases.

Feature	Solid	Liquid	Gas
Inter-particle space	least	more	very large
Attraction	strongest	weaker	weakest
Motion	vibrate about fixed positions	move about, slide past	move freely and fast
Collisions	—	frequent	very frequent, also with walls
Shape and volume	both fixed	fixed volume	neither fixed

Gases can be compressed easily because of the large spaces between particles; solids hardly at all. Diffusion (the smell of perfume spreading) happens because particles move and mix.

Change of state.

Heating (melting, boiling): particles gain kinetic energy and move faster; they overcome the attractions and the space increases: solid → liquid → gas.
Cooling (condensation, freezing): kinetic energy falls; attraction pulls them closer; the space decreases: gas → liquid → solid.

Gas pressure. In a sealed container, heating makes particles collide harder and more often with the walls, so the pressure rises.

Kinetic Molecular Theory and Change of State: formulas and key facts

Postulates: Tiny particles
Spaces between them
Forces of attraction
Constant random motion
Kinetic energy rises with temperature
Collisions; Six statements.
Order of space: $so l i d < l i q u i d < g a s$; Attraction: solid > liquid > gas.
Heating and cooling: Heating: faster particles, more space | Cooling: slower particles, less space; Change of state.

How to solve Kinetic Molecular Theory and Change of State questions

Answer every "explain" question with three words: space, attraction, motion.

1Name the state(s) involved.
2Describe the inter-particle space and the attraction in each.
3Describe the motion (vibrating, sliding, free).
4For heating or cooling, say what happens to kinetic energy and to the attraction.
5Link to the observed property (compressibility, fixed shape, pressure).

Kinetic Molecular Theory and Change of State in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Scent across a room

Suppose someone opens a perfume bottle in one corner of a still room, and in a minute people near the other wall smell it.

Explain using the kinetic theory.

Try it first, then show the solutionSolution

Perfume particles move randomly and fast.
They move into the spaces between air particles and spread through the room: diffusion.

Answer: Random motion and spaces between particles cause diffusion.

Memory hook: Particles never rest.

Hot-day tyre

Imagine a sealed cycle tyre left in the Sun on a hot day feels harder.

Explain the rise in pressure.

Try it first, then show the solutionSolution

Heating increases the kinetic energy of the air particles.
They hit the walls harder and more often, so the pressure rises.

Answer: More energetic collisions with the walls increase pressure.

Memory hook: Hot particles, hard hits.

Tricks to remember Kinetic Molecular Theory and Change of State

Short memory aids. Say each one aloud once, then use it on a question the same day.

SAM
Space, Attraction, Motion: three words for every state.
Use it when: Explanations.
Heat frees
Heat makes them move faster and break free.
Use it when: Melting.
Gases compress
Big spaces mean gases squeeze; solids do not.
Use it when: Compressibility.

Kinetic Molecular Theory and Change of State: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: A postulate · 1 mark · MCQBoard level

Which of the following is NOT a postulate of the kinetic molecular theory?
(a) particles are in constant motion
(b) there are spaces between particles
(c) particles are at rest at room temperature
(d) particles attract each other

Try it first, then show the solutionSolution

Particles are always in motion, so (c) is not a postulate.

Answer: (c)

Example 2: Which postulate? · 2 marks · Short answerBoard level

A drop of ink spreads through a glass of water without stirring. Name the process and the two postulates of the kinetic theory that explain it.

Try it first, then show the solutionSolution

The process is diffusion. (½)
Particles are in constant random motion. (¾)
There are spaces between the particles into which the ink particles move. (¾)

Answer: Diffusion; constant motion; spaces between particles.

Example 3: Compare the states · 3 marks · Short answerHarder board level

Explain using the kinetic theory: (a) gases are easily compressed but solids are not, (b) liquids flow but solids do not.

Try it first, then show the solutionSolution

(a) Gas particles are far apart with large spaces, so they can be pushed closer; in solids they are already close together. (1½)
(b) In a liquid the particles can slide past each other; in solids they only vibrate in fixed positions. (1½)

Answer: Space and movement.

Example 4: Melting on the particle model · 3 marks · Short answerHarder board level

Explain what happens to the particles, the kinetic energy, the attraction and the inter-particle space when ice melts to water.

Try it first, then show the solutionSolution

The particles gain kinetic energy and move faster. (1)
They overcome the forces of attraction holding them in fixed positions. (1)
The particles are no longer held in fixed positions and can slide past one another, so they move more freely and the space between them generally increases (slightly). Note: ice is a special case, because water is denser than ice, but for the ICSE answer state that the space increases on melting.

Answer: KE increases; attraction overcome; particles slide past each other; space increases (slightly)

Example 5: Perfume and temperature · 4 marks · Case-basedHOTS / competency

A drop of ink spreads faster in hot water than in cold water, and the smell of a perfume spreads across a room. (a) Name the process. (b) Explain the faster spreading in hot water. (c) Why is it fastest in gases? (d) Which postulate explains the pressure on the walls of a gas cylinder?

Try it first, then show the solutionSolution

(a) Diffusion. (1)
(b) Hot water particles have more kinetic energy and move faster, so they mix faster. (1)
(c) Gas particles have the weakest attraction and the most space and move freely. (1)
(d) Moving particles collide with the walls of the container. (1)

Answer: (a) diffusion (b) higher KE (c) free motion (d) collisions with walls

Where students lose marks in Kinetic Molecular Theory and Change of State

The slips examiners see most often, and what to write instead.

Mistake: Saying particles stop in solids
They vibrate.
Wrong working
"Particles in a solid do not move."
Right working
"Particles in a solid vibrate about fixed positions."
Do this: Use "vibrate".
Mistake: Mixing up attraction order
Solids have the strongest attraction.
Wrong working
"Gases have the strongest attraction."
Right working
"Gases have the weakest."
Do this: Space and attraction are opposite.
Mistake: Explaining without particles
The question asks for the kinetic theory.
Wrong working
"Because gas is light."
Right working
"Because the particles are far apart."
Do this: Talk about particles.

Exam tips

Use the table of states in your answer.
Quote kinetic energy and temperature for heating.
Say "collide with the walls" for gas pressure.
Give an example (perfume, ink) for diffusion.

Practice questions on Kinetic Molecular Theory and Change of State

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · Very short answerBoard level
What does the kinetic energy of the particles depend on?
Hint
Think of heat.
Answer
The temperature: it increases when the substance is heated.
Q2 · 2 marks · Short answerHarder board level
Arrange solids, liquids and gases in increasing order of (a) inter-particle space, (b) force of attraction.
Hint
Opposite orders.
Answer
(a) solid < liquid < gas. (b) gas < liquid < solid.
Q3 · 3 marks · Short answerHarder board level
Why does a gas fill its whole container but a liquid does not?
Hint
Attraction and motion.
Answer
Gas particles have very weak attraction, large spaces and move freely and fast in all directions, so they spread to fill all the space. Liquid particles attract enough to stay together with a fixed volume.
Q4 · 2 marks · Short answerHarder board level
A balloon placed in sunlight swells slightly. Use the kinetic theory to explain.
Hint
Think of collisions with the walls.
Answer
The air particles gain kinetic energy and collide harder and more often with the walls, so the balloon expands.
Q5 · 2 marks · Assertion-reasonHOTS / competency
Assertion (A): Gases have no fixed volume.
Reason (R): The attraction between gas particles is very strong.
(a) A and R true, R explains A
(b) A and R true, R does not explain A
(c) A true, R false
(d) A false, R true
Hint
Compare attraction in gases.
Answer
A is true; R is false because the attraction between gas particles is very weak. So (c).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which state has the most space?Show answer
Gas.
2.Particles in a solid?Show answer
Vibrate about fixed positions.
3.What increases with temperature?Show answer
Kinetic energy of the particles.
4.Cause of gas pressure?Show answer
Collisions with the walls.
5.What is diffusion?Show answer
Spreading of particles by random motion.

Kinetic Molecular Theory and Change of State: frequently asked questions

What is the kinetic molecular theory?

It is a model in which matter is made of tiny particles with spaces and attractions between them, moving constantly with kinetic energy that rises with temperature.

How does the kinetic theory explain change of state?

Heating increases kinetic energy and overcomes the attraction, increasing the space (solid to liquid to gas); cooling reverses this.

Why can gases be compressed?

Because there are large spaces between gas particles that can be reduced under pressure.