Sec 3 Chemistry — Kinetic Particle Theory
Prepared by Miss Clarissa Ng · www.clartutors.com
Part A · How the theory explains what you see
1 What the Kinetic Particle Theory Tells Us
Kinetic particle theory is the model we use to explain why solids, liquids and gases behave in different ways.
- Every substance is made of very small particles that cannot be seen directly, even under a microscope.
- There are forces of attraction holding the particles together — the more strongly they are pulled together, the harder it is to separate them.
- The particles are never still: they have kinetic energy and are always in random motion.
Link it up: the whole topic comes from one idea — how strongly the particles are held together compared with how much kinetic energy they have. Strong attraction with little energy gives a solid; weak attraction with lots of energy gives a gas.
2 The Particulate Nature of Matter
Scientists use a simple particle model, called the particulate nature of matter, to picture what is happening inside a substance. In each state, three things are different:
- how the particles are arranged,
- how the particles move,
- how much energy the particles have.
The model gives us this picture of the three states of matter:
| Solid | Liquid | Gas | |
|---|---|---|---|
| Diagram | |||
| Arrangement of particles | Packed very tightly together in a regular, orderly arrangement. | Packed closely together, but in a disorderly arrangement. | Very far apart from one another, in a disorderly arrangement. |
| Space between particles | Very little — the particles are almost touching. | Little space. | A lot of space. |
| Forces of attraction | Very strong. | Strong, but weaker than in a solid. | Very weak. |
| Movement of particles | Vibrate and rotate about fixed positions. | Free to slide past one another within the liquid. | Free to move quickly and randomly in all directions. |
| Energy of particles | Energy of the particles increases from solid → liquid → gas. As temperature rises, the average kinetic energy of the particles rises, so they move more quickly. | ||
Why it matters: strong forces plus little energy explain why a solid keeps its own shape and volume, a liquid keeps its volume but takes the shape of its container, and a gas fills whatever container it is in.
Part B · What happens when energy goes in or out
3 Temperature and Thermal Energy
Temperature is a measure of the average kinetic energy of the particles in a substance.
- When a substance absorbs thermal energy, some of that energy is converted into kinetic energy of its particles. The average kinetic energy rises, so the temperature rises.
- When a substance releases thermal energy, the average kinetic energy of its particles falls, so the temperature falls.
Careful — a common trap: while a substance is actually changing state, the temperature stays the same even though thermal energy is still being absorbed. The energy is being used to overcome the forces of attraction between the particles, not to speed them up.
This is why the heating and cooling curves later in these notes have flat sections.
4 Changes of State
A substance changes state when it absorbs thermal energy from the surroundings or releases thermal energy to them.
Changes of state, and whether thermal energy is absorbed or released
| Change of state | What happens | Thermal energy |
|---|---|---|
| Melting | Solid → liquid | Absorbed from the surroundings |
| Freezing | Liquid → solid | Released to the surroundings |
| Boiling / evaporation | Liquid → gas | Absorbed from the surroundings |
| Condensation | Gas → liquid | Released to the surroundings |
| Sublimation | Solid → gas, without becoming a liquid first | Absorbed from the surroundings |
| Vapour deposition | Gas → solid, without becoming a liquid first | Released to the surroundings |
Remember the pattern: moving to a state with more energy (solid → liquid → gas) takes thermal energy in; moving back down gives thermal energy out.
5 Evaporation and Boiling
Evaporation is the change of a substance from the liquid to the gaseous state at a temperature between its melting point and its boiling point.
- When a liquid absorbs thermal energy, its particles gain kinetic energy, slide past one another more quickly and move further apart.
- Evaporation happens when particles at the surface of the liquid have enough kinetic energy to overcome the forces of attraction holding them to their neighbours, so they escape into the air as vapour.
- Since only the fastest surface particles escape, the liquid left behind has a lower average kinetic energy — which is why evaporation cools the liquid.
Volatile liquids
Volatile liquids are liquids with a boiling point slightly above room temperature, such as ethanol and petrol. They vaporise readily at room temperature because their particles need only a little energy to escape.
Evaporation compared with boiling
| Evaporation | Boiling |
|---|---|
| Happens at any temperature between the melting point and boiling point of the substance | Happens at one fixed temperature — the boiling point |
| A relatively slow process | A relatively fast process |
| Happens only at the surface of the liquid | Happens throughout the liquid |
6 Sublimation and Vapour Deposition
Only some substances change state in this way.
- When heated gently, these substances sublime: they change directly from solid to gas, without melting into a liquid first.
- When their vapour is allowed to cool, they undergo vapour deposition: they change directly from gas back to solid, without condensing into a liquid first.
- Common examples: iodine, dry ice (solid carbon dioxide) and naphthalene, the substance found in mothballs.
7 Expansion and Contraction
Expansion is the increase in volume of a substance when it is heated.
When a solid is heated, it absorbs thermal energy from the surroundings. This energy becomes kinetic energy of its particles, so the particles vibrate and rotate more quickly about their fixed positions and move further apart — the solid takes up more space.
Contraction is the decrease in volume of a substance when it is cooled.
When a solid is cooled, it releases thermal energy to the surroundings. The kinetic energy of its particles falls, so the particles vibrate and rotate more slowly and move closer together — the solid takes up less space.
Key point: during expansion and contraction, the number and size of the particles do not change — only the spacing between them changes. This is why the mass of the substance stays the same.
Part C · What proves the particles are moving
8 Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, until equilibrium is reached.
- Particles in a gas or liquid move randomly and take up any space available to them, until the substance is evenly mixed.
- Everyday picture: open a bottle of perfume in a room — you smell it across the room a moment later, because the perfume particles spread themselves through the air.
What changes the rate of diffusion
| Factor | What happens to the rate of diffusion |
|---|---|
| Temperature | As temperature increases, the rate of diffusion increases — the particles have more kinetic energy and move faster. |
| Mass of the particles | As the mass of the particles increases, the rate of diffusion decreases — heavier particles move more slowly. |
Exam habit: if a question asks you to compare how fast two gases spread, compare their particle masses — the lighter one spreads faster at the same temperature.
Part D · Following it on a graph
9 The Heating Curve
The graph shows how the temperature of a substance changes as it is heated steadily over time.
Heating curve — the flat sections are the changes of state
| Section | What the particles are doing |
|---|---|
| A → B |
|
| B → C |
|
| C → D |
|
| D → E |
|
| E → F |
|
10 The Cooling Curve
The graph shows how the temperature of a substance changes as it is cooled steadily over time — it is the heating curve in reverse.
Cooling curve — the flat sections are condensation and freezing
| Section | What the particles are doing |
|---|---|
| P → Q |
|
| Q → R |
|
| R → S |
|
| S → T |
|
| T → U |
|
How to score in curve questions: say what the slope means (rising or falling = change in average kinetic energy, so temperature changes), and say what the flat part means (change of state, energy used to overcome or released as the forces of attraction change). Name the change of state at each flat part.
11 Put It Together — Exam-Style Question
A student drops some ice into a beaker and heats it gently, stirring. She records the temperature every 30 seconds with an alcohol-in-glass thermometer.
[1](a) Name the change of state taking place while the temperature stays at 0 °C.
[2](b) Explain, in terms of the particles, why the temperature does not rise while the ice is melting.
[2](c) Describe how the arrangement and movement of the particles differ between the ice and the water.
[2](d) Once all the ice has melted the temperature rises again, then stays at 100 °C. Name the change of state at that point and explain why the temperature stops rising.
[1](e) The thermometer is accurate to ±0.5 °C. State the reading you would record for the boiling point.
Model answers.
(a) Melting (solid → liquid) at its melting point.
(b) The energy supplied is used to overcome the forces of attraction holding the particles in fixed positions, instead of raising their average kinetic energy — so the temperature stays at 0 °C until all the ice has melted.
(c) In ice the particles are packed closely in an orderly arrangement and can only vibrate about fixed positions. In water they are still close together but the arrangement is disorderly and they can slide past one another.
(d) Boiling (liquid → gas) at its boiling point. The energy now separates the particles completely — they move very far apart with a disorderly arrangement — so again it is used to overcome forces of attraction rather than to raise the temperature.
(e) 100.0 °C — quote the reading to the precision the thermometer gives.
(a) Melting (solid → liquid) at its melting point.
(b) The energy supplied is used to overcome the forces of attraction holding the particles in fixed positions, instead of raising their average kinetic energy — so the temperature stays at 0 °C until all the ice has melted.
(c) In ice the particles are packed closely in an orderly arrangement and can only vibrate about fixed positions. In water they are still close together but the arrangement is disorderly and they can slide past one another.
(d) Boiling (liquid → gas) at its boiling point. The energy now separates the particles completely — they move very far apart with a disorderly arrangement — so again it is used to overcome forces of attraction rather than to raise the temperature.
(e) 100.0 °C — quote the reading to the precision the thermometer gives.
★ Chapter Concept Map
Kinetic Particle Theory
All matter is made of tiny particlestoo small to be seen directly
KINETIC
PARTICLE
THEORY
PARTICLE
THEORY
Forces of attraction hold particles togethervery strong in a solid · very weak in a gas
Particles have kinetic energy and are in constant random motionthe higher the temperature, the faster they move
Solidregular, orderly arrangement · vibrate about fixed positions
STATES OF
MATTER
MATTER
Liquidclosely packed, disorderly arrangement · slide past one another
Gasvery far apart, disorderly → move quickly and randomly in all directions
EVIDENCE
FOR THE
THEORY
FOR THE
THEORY
Diffusionparticles spread from higher to lower concentration — faster when hot, slower when heavy
Expansion and contractionparticles move further apart or closer · number and size stay the same, so mass is conserved
CHANGES
OF STATE
OF STATE
Melting · boiling · sublimationthermal energy absorbed — overcome forces of attraction
Freezing · condensation · vapour depositionthermal energy released — forces of attraction strengthen
CURVES
Heating and cooling curvessloping = temperature changes · flat = change of state
During any change of state the temperature stays the same, because the energy absorbed or released goes into changing the forces of attraction between particles — not their average kinetic energy.