Last updated: 2 October 2026
Why does an ice cube keep its shape while liquid water flows? Why can air be compressed much more easily than water? And how can the smell of perfume spread across a room even when nobody is deliberately moving the air?
These observations look unrelated until we describe matter at the particle level. Kinetic Particle Theory gives us a simple model: matter is made of tiny particles, and the behaviour we observe depends on how those particles are arranged, how they move and how much energy they have.
The aim of this topic is therefore not just to memorise three particle diagrams. You should be able to look at a property such as fixed shape, compressibility, melting or diffusion and explain why it occurs in terms of particles.
Try these five questions before reading the chapter. They will help you identify which particle ideas are already secure and which need more attention.
- 5 diagnostic questions
- Important particle misconceptions tested
- Instant explanation for every answer option
What is Kinetic Particle Theory?
Kinetic Particle Theory is a model that helps us explain the physical behaviour of matter. Although individual particles are far too small to see directly in ordinary classroom observations, we can use their behaviour to explain what we observe on a larger scale.
The model begins with several important ideas:
- matter is made of very small particles;
- there are spaces between the particles;
- the particles are continually moving;
- attractive forces act between particles; and
- temperature affects the average kinetic energy of the particles.
Kinetic energy is energy associated with motion. When temperature increases, the particles of a substance have greater average kinetic energy and therefore move, or vibrate, more vigorously.
What does “particle” mean?
The word particle is deliberately general. Depending on the substance, the particles may be atoms, molecules or ions.
For example, oxygen gas contains O₂ molecules, while a sample of a noble gas such as helium contains individual atoms. In this topic, however, our main concern is usually not the identity of each particle but how the particles are arranged and moving.
Particles in solids, liquids and gases
The three common states of matter differ because their particles have different arrangements and different freedom of movement.
| State | Arrangement | Movement | Effect of attractive forces |
|---|---|---|---|
| Solid | Particles are packed very closely in an orderly arrangement. | Particles vibrate about fixed positions. | Particles are held firmly in fixed positions. |
| Liquid | Particles are close together but arranged irregularly. | Particles move and slide past one another. | Attractions keep particles close but do not fix them in position. |
| Gas | Particles are very far apart and randomly arranged. | Particles move rapidly and randomly in all directions. | Attractions have very little effect while the particles are far apart. |
Solids: closely packed particles in fixed positions
In a solid, particles are packed closely together. Attractive forces keep each particle near a fixed position.
The word fixed does not mean that the particles are completely motionless. They continually vibrate about their positions.
Because the particles cannot move freely past one another, a solid maintains a fixed shape. Since the particles are already packed close together, a solid also has a fixed volume and is very difficult to compress.
A solid particle vibrates about a fixed position. It does not normally travel from one part of the solid to another.
Liquids: close together but able to move
Liquid particles are also close together, but they are not held in fixed positions. They can move around and slide past one another.
This explains why a liquid can flow and take the shape of its container.
However, the particles remain close together. A liquid therefore keeps an approximately fixed volume and is difficult to compress.
Inaccurate: Liquid particles are far apart because liquids can flow.
Better: Liquid particles are still closely packed, but they can move past one another because they are not held in fixed positions.
Gases: widely separated particles in rapid random motion
Gas particles are much farther apart than particles in solids or liquids. They move rapidly and randomly in all directions.
Because they can move freely throughout the available space, a gas has no fixed shape and no fixed volume. It spreads out to occupy the container in which it is placed.
Why are gases easily compressed?
There are large spaces between gas particles. When pressure is applied, the particles can be pushed closer together.
In solids and liquids, the particles are already close together. There is much less empty space available, so these states are much more difficult to compress.
When a gas is compressed, the individual particles do not shrink. The average distance between them becomes smaller.
Explaining physical properties using particles
Examination questions often give you a visible property and ask you to explain it using Kinetic Particle Theory. A strong answer links the observation to both the arrangement and movement of particles.
| Observation | Particle explanation |
|---|---|
| A solid keeps its shape. | Its particles are held in fixed positions and cannot move past one another. |
| A liquid flows. | Its particles can move and slide past one another. |
| A liquid has a fixed volume. | Its particles remain closely packed with little empty space between them. |
| A gas fills its container. | Its particles move rapidly and randomly throughout the available space. |
| A gas can be compressed. | Large spaces between gas particles allow them to be pushed closer together. |
Worked reasoning: why does a liquid take the shape of its container?
A good explanation can be built step by step:
- Liquid particles are closely packed.
- They are not held in fixed positions.
- They can therefore move and slide past one another.
- The particles can rearrange, so the liquid takes the shape of its container.
Writing only “because liquids can flow” does not explain the observation at the particle level. It simply repeats another property of liquids.
Changes of state
A substance can change between solid, liquid and gas when energy is transferred. During a change of state, the chemical identity of the substance does not change. What changes is the arrangement, movement and separation of its particles.
| Process | Change | Energy change |
|---|---|---|
| Melting | Solid → liquid | Energy is absorbed. |
| Boiling | Liquid → gas | Energy is absorbed. |
| Sublimation | Solid → gas | Energy is absorbed. |
| Freezing | Liquid → solid | Energy is released. |
| Condensation | Gas → liquid | Energy is released. |
Heating a solid
When a solid is heated, its particles gain kinetic energy. They vibrate more vigorously about their fixed positions.
If sufficient energy is supplied, the solid eventually reaches its melting point. The particles can then overcome enough of the attractive forces holding them in fixed positions.
Melting: solid to liquid
During melting, particles absorb energy and are no longer restricted to fixed positions. They become able to move past one another.
Importantly, the particles remain close together. Melting does not produce the widely separated arrangement of a gas.
Incorrect: During melting, the particles become far apart.
Correct: During melting, the particles remain close together but are no longer held in fixed positions, so they can move past one another.
Boiling: liquid to gas
As a liquid is heated, its particles gain kinetic energy and move more rapidly. At the boiling point, particles gain enough energy to overcome the attractive forces keeping them close together.
The particles separate and move freely as a gas.
Sublimation: solid directly to gas
During sublimation, a solid changes directly into a gas without first becoming a liquid.
The particles absorb sufficient energy to leave their fixed positions and become widely separated.
Freezing: liquid to solid
When a liquid is cooled, its particles lose kinetic energy and move more slowly. At the freezing point, attractive forces hold the particles in fixed positions.
The particles still vibrate, but they can no longer move freely past one another.
Condensation: gas to liquid
When a gas is cooled, its particles lose kinetic energy and move more slowly. Attractive forces can then bring the particles closer together.
The particles become closely packed but remain free to move past one another, forming a liquid.
Why temperature stays constant during melting and boiling
One of the most important ideas in changes of state is that a pure substance can continue absorbing energy while its temperature remains constant during melting or boiling.
Temperature is related to the average kinetic energy of the particles. Normally, heating makes particles move or vibrate faster, so the temperature rises.
During a change of state, however, the absorbed energy is used to overcome attractive forces between particles rather than to increase their average kinetic energy.
During melting and boiling, the energy supplied changes the arrangement and separation of the particles. The temperature remains constant until the change of state is complete.
Worked reasoning: a liquid boiling at its boiling point
Suppose a liquid has reached its boiling point and heating continues.
- Energy continues to enter the liquid.
- The particles use this energy to overcome attractive forces.
- More particles separate sufficiently to enter the gaseous state.
- Their average kinetic energy does not increase during the change of state, so the temperature remains constant.
- Once all the liquid has become gas, further heating can raise the temperature again.
Particles themselves do not expand when heated
Students sometimes draw larger circles when showing a heated substance. This gives the wrong impression.
Heating does not make the individual particles significantly larger. Instead, the particles gain kinetic energy and move or vibrate more vigorously. Their average separation may increase.
Incorrect: The particles expand when the substance is heated.
Correct: The particles gain kinetic energy and move or vibrate more vigorously; the average distance between particles may increase.
This distinction is important. When a whole substance expands, it is the spacing between particles that changes rather than the size of the particles themselves.
Diffusion: evidence that particles are moving
If particles were stationary, substances would remain exactly where they were placed unless something physically moved them.
Instead, particles can spread through liquids and gases even without deliberate stirring. This spreading is explained by their continual random motion.
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration as a result of their random motion.
What does “net movement” mean?
Individual particles are not programmed to travel only from high concentration to low concentration. They move randomly in different directions.
Suppose one side of a container initially contains many particles of a particular substance while the other side contains very few. Because there are more particles on the concentrated side, more particles will, overall, leave that region than enter it.
The result is a net movement from higher concentration to lower concentration.
Individual particles move in many directions, but when there is a concentration difference, the overall effect is movement from the more concentrated region towards the less concentrated region.
Does particle movement stop when concentration becomes uniform?
No. The particles continue moving randomly.
Once the substance is evenly distributed, there is no longer any net change in concentration because movement in opposite directions balances out.
Everyday examples of diffusion
Perfume spreading through air
When perfume is sprayed in one part of a room, perfume particles initially have a high concentration near the source.
Because the particles are continually moving, they spread through the surrounding air. Their concentration gradually becomes more even, allowing the smell to be detected farther away.
Colour spreading through water
If a coloured soluble substance is placed carefully into still water, the colour gradually spreads through the liquid even without deliberate stirring.
The coloured particles move randomly between water particles, producing a net movement away from the region of higher concentration.
Why diffusion is usually faster in gases than in liquids
Gas particles are widely separated and move rapidly through large spaces between particles. Diffusion through a gas therefore occurs relatively quickly.
In a liquid, particles are much more closely packed. A diffusing particle is surrounded by many neighbouring particles, so spreading is generally slower.
How temperature affects diffusion
Increasing temperature increases the average kinetic energy of particles. The particles move more rapidly, so they spread through the available space more quickly.
Example: colour spreading in warm and cold water
Imagine placing the same amount of coloured solution into equal volumes of warm and cold water.
Provided the comparison is carried out fairly, the coloured particles spread more quickly through the warmer water because particles have greater average kinetic energy at the higher temperature.
A good explanation therefore has two links:
How relative molecular mass affects diffusion
Temperature is not the only factor affecting the speed at which gaseous particles diffuse.
When gases are compared at the same temperature, particles with a lower relative molecular mass, Mᵣ, generally diffuse faster than particles with a higher Mᵣ.
At O-Level, you normally use this relationship qualitatively. You do not need a mathematical diffusion-rate equation to decide which of two gases diffuses faster.
Worked example: comparing NH₃ and CO₂
Consider ammonia, NH₃, and carbon dioxide, CO₂, at the same temperature.
NH₃ has the lower relative molecular mass.
Therefore:
Temperature itself affects particle speed. To make a meaningful comparison based on molecular mass, the gases should be considered under the same temperature conditions.
Common mistakes in Kinetic Particle Theory
| Common mistake | Why it is wrong | Better idea |
|---|---|---|
| Particles in a solid do not move. | Solid particles still possess kinetic energy. | They vibrate about fixed positions. |
| Liquid particles are far apart. | Liquid particles remain closely packed. | They are close together but can move past one another. |
| Gas particles become smaller when compressed. | The particles themselves do not shrink. | The average spacing between particles decreases. |
| Particles expand when heated. | The size of the individual particles does not increase. | Particles gain kinetic energy and their average separation may increase. |
| During melting, particles become widely separated. | Widely separated particles describe a gas. | Liquid particles remain close but can move past one another. |
| Temperature rises throughout boiling. | During boiling, absorbed energy is used to overcome attractive forces. | The temperature remains constant until the change of state is complete. |
| Every particle travels directly from high to low concentration. | Individual particles move randomly in many directions. | Diffusion is the net movement from higher to lower concentration. |
| Diffusion requires stirring. | Particles are already in continual motion. | Diffusion occurs even without deliberate stirring. |
| Particles stop moving when concentration becomes uniform. | Particle motion continues. | There is simply no further net change in concentration. |
| A gas with a larger Mᵣ diffuses faster. | At the same temperature, heavier gaseous particles generally diffuse more slowly. | Lower Mᵣ corresponds to faster diffusion. |
Kinetic Particle Theory checklist
The three states
- Can I describe the arrangement of particles in solids, liquids and gases?
- Can I describe how particles move in each state?
- Can I explain why solids have fixed shape and volume?
- Can I explain why liquids flow?
Particle explanations
- Can I explain why gases fill their containers?
- Can I explain why gases are easily compressed?
- Do I remember that solid particles still vibrate?
- Do I avoid saying that particles themselves expand?
Changes of state
- Can I identify melting, boiling, freezing, condensation and sublimation?
- Can I state whether energy is absorbed or released?
- Can I explain melting and boiling using attractive forces?
- Can I explain constant temperature during a change of state?
Diffusion
- Can I define diffusion using the phrase “net movement”?
- Can I explain diffusion using random particle motion?
- Can I explain why higher temperature increases diffusion rate?
- Can I compare gas diffusion rates using Mᵣ?
These ten questions go beyond simple definitions. Use particle arrangement, movement, energy and diffusion to reason through each situation.
- 10 mastery questions
- Application and misconception testing
- Instant explanation for every answer option
Frequently asked questions
Questions students often ask about Kinetic Particle Theory
These questions clarify some of the particle ideas that can be easy to oversimplify when looking at basic diagrams.
Are particles in a solid completely motionless?
No. Solid particles are held near fixed positions, but they still vibrate. Heating the solid increases their average kinetic energy, so the vibrations become more vigorous.
Are liquid particles much farther apart than solid particles?
No. Liquid particles are still closely packed. The important difference is that they are not held in fixed positions, so they can move past one another. This is why liquids flow while remaining difficult to compress.
Why does a gas occupy the whole container?
Gas particles are widely separated and move rapidly and randomly in all directions. They therefore spread throughout all the available space in the container.
Why can gases be compressed but liquids cannot be compressed easily?
Gases contain large spaces between particles, so pressure can push the particles closer together. Liquid particles are already closely packed, leaving relatively little empty space to remove.
Do the particles themselves become larger when a substance is heated?
No. Heating increases the average kinetic energy of the particles. They move or vibrate more vigorously, and their average separation may increase, but the particles themselves do not simply expand.
Why does the temperature not rise during melting or boiling?
Energy is still being absorbed, but during the change of state it is used to overcome attractive forces and change the arrangement of the particles. It is therefore not increasing their average kinetic energy, so the temperature remains constant until the change is complete.
Does diffusion mean every particle moves directly from high concentration to low concentration?
No. Individual particles move randomly in different directions. Because more particles are initially present in the high-concentration region, however, there is an overall or net movement towards the lower-concentration region.
Does diffusion stop once particles are evenly distributed?
The particles continue moving randomly. What disappears is the net change in concentration. Once the distribution is uniform, movement in opposite directions balances out.
Why does increasing temperature make diffusion faster?
Higher temperature means greater average kinetic energy. The particles therefore move more rapidly and spread through the available space more quickly.
How should I use Mᵣ when comparing the diffusion of gases?
Compare the gases at the same temperature. The gas with the lower relative molecular mass generally diffuses faster. At O-Level, this comparison is normally qualitative rather than requiring a mathematical diffusion formula.
Kinetic Particle Theory becomes much easier when you stop treating solids, liquids, gases, changes of state and diffusion as separate lists of facts. They all follow from the same idea: particles are continually moving, their arrangement and separation differ between states, and changing their energy changes how they behave. Once you learn to move from an observation to its particle explanation, many questions in this topic become reasoning questions rather than memory questions.