# Kinetic Particle Model, States of Matter and Gas Behaviour

> Physics · CIE IGCSE 2026-2028
> Source: https://www.owlsprep.com/study/cie-0625-u2-kinetic-particle-model-states-of/

This guide covers CIE IGCSE Physics 0625 2.1.1-2.1.3 syllabus content: kinetic particle model of the three states of matter, state changes, and gas behaviour for both Core and Extended tiers.

**Prerequisites:** [Basic particle nature of matter](https://www.owlsprep.com/study/cie-0625-u1-introduction-to-physics/); Fundamentals of temperature measurement

## Learning objectives

- Describe the particle arrangement, movement and properties of solids, liquids and gases
- Explain changes of state using the kinetic particle model
- Explain gas pressure in terms of particle collisions with container walls
- Describe the qualitative relationships between gas pressure, volume and temperature (Core)
- Apply Boyle's Law to calculate pressure or volume changes for fixed mass of gas at constant temperature (Extended)
- Describe absolute zero (−273 °C) as the lowest possible temperature and convert between °C and kelvin using T = θ + 273 (Core)
- Describe and explain Brownian motion as evidence for the kinetic particle model (Core)

## 1. Kinetic Particle Model of Solids, Liquids and Gases

All matter is made of tiny, constantly moving particles. The arrangement, movement and energy of these particles differ between solids, liquids and gases, explaining their distinct physical properties.

**Kinetic Particle Model** — A model that describes all matter as consisting of tiny particles in constant random motion, with behaviour determined by their energy and intermolecular forces.

| State | Particle Arrangement | Particle Movement | Key Properties |
| --- | --- | --- | --- |
| Solid | Regular, tightly packed lattice, fixed positions | Vibrate around fixed points, low kinetic energy | Fixed shape and volume, cannot be compressed |
| Liquid | Random, close together, no fixed positions | Slide past each other, medium kinetic energy | Fixed volume, takes shape of container, hard to compress |
| Gas | Random, very far apart, no structure | Move freely at high speed, high kinetic energy | No fixed shape/volume, easily compressed |

**Worked example:** Explain why a solid iron nail cannot be compressed, but a helium balloon can be squeezed to a smaller size.

1. Step 1: Describe the particle arrangement of solid iron:
2. Iron particles are tightly packed in a fixed lattice with almost no empty space between them.
3. Step 2: Explain why the nail cannot be compressed:
4. There is no space to push particles closer together, so the solid cannot change volume when squeezed.
5. Step 3: Describe the particle arrangement of gaseous helium:
6. Helium particles are very far apart with large amounts of empty space between them.
7. Step 4: Explain why the balloon can be compressed:
8. Squeezing reduces the empty space between gas particles, so the volume of the balloon decreases.

> **Exam tip:** Always link state properties directly to particle arrangement, movement and intermolecular forces in answers: vague responses without this link will not get full marks.

## 2. Changes of State and the Kinetic Model

When a substance is heated or cooled, the kinetic energy of its particles changes, leading to changes of state. These are reversible physical changes, so no new substance is formed.

- Melting: Solid → Liquid (heat absorbed, intermolecular forces weaken)
- Freezing: Liquid → Solid (heat released, intermolecular forces strengthen)
- Boiling/Evaporation: Liquid → Gas (heat absorbed, forces overcome completely)
- Condensation: Gas → Liquid (heat released, forces pull particles close)
- Sublimation: Solid → Gas directly (e.g. dry ice, solid iodine)

**Worked example:** Explain why the temperature of melting ice stays constant at 0°C even when heat is continuously added by a Bunsen burner.

1. Step 1: State what happens to added heat energy during melting:
2. Heat energy is used to weaken the intermolecular forces holding water particles in their fixed solid lattice positions.
3. Step 2: Link to temperature:
4. None of the added energy increases the average kinetic energy of the particles. Since temperature is a measure of average particle kinetic energy, the temperature stays constant until all ice has melted.

> **Exam tip:** Do not confuse evaporation and boiling: evaporation occurs only at the liquid surface at any temperature, while boiling occurs throughout the liquid at its fixed boiling point.

## 3. Core Gas Behaviour: Pressure and Temperature

Gas pressure is caused by the constant collision of fast-moving gas particles with the walls of their container. The pressure of a fixed mass of gas depends on its temperature and volume.

**Gas Pressure** — The total force exerted per unit area by gas particles colliding with the walls of their container.

- Fixed volume, heated gas: Particles gain kinetic energy, move faster, collide with walls more often and with greater force → pressure increases
- Constant temperature, compressed gas: Particles have less space to move, collide with walls more often → pressure increases

**Worked example:** A sealed aerosol can is left in direct sunlight, causing its internal temperature to rise. Explain why the can might burst if it gets too hot.

1. Step 1: Identify fixed variables:
2. The can is sealed, so mass and volume of the gas inside are fixed.
3. Step 2: Describe the effect of rising temperature on gas particles:
4. Increasing temperature raises the kinetic energy of gas particles, so they move faster.
5. Step 3: Link to pressure change:
6. Faster particles collide with the can walls more frequently and with greater force, increasing internal gas pressure.
7. Step 4: Explain bursting:
8. If the pressure exceeds the strength of the can material, the can will burst.

## 4. Temperature, Absolute Zero and the Kelvin Scale

The temperature of an object is linked to the average kinetic energy of its particles. Heating an object makes its particles move faster (more kinetic energy); cooling it makes them move more slowly (less kinetic energy).

**Absolute zero** — The lowest possible temperature, −273 °C, at which the particles have the least (minimum) kinetic energy. It is the zero of the kelvin scale (0 K).

Because temperatures cannot fall below absolute zero, scientists also use the **kelvin (K)** temperature scale, which starts at absolute zero. A change of 1 K is the same size as a change of 1 °C, so you convert between the two scales just by adding or subtracting 273. In the equation below, $T$ is the temperature in kelvin and $\theta$ is the temperature in degrees Celsius.

$$T = \theta + 273$$

**Worked example:** (a) Convert 27 °C to kelvin. (b) Convert 200 K to degrees Celsius.

1. Step 1: To change °C to K, add 273:
2. (a) T = 27 + 273 = 300 K
3. Step 2: To change K to °C, subtract 273:
4. (b) θ = 200 − 273 = −73 °C

> **Exam tip:** You must be able to recall and use T (in K) = θ (in °C) + 273 without a formula sheet: to go from °C to K add 273, and from K to °C subtract 273.

## 5. Brownian Motion: Evidence for the Particle Model

If you look at smoke particles in air under a microscope (a smoke cell), the smoke specks are seen to move in a continuous, random, jerky, zig-zag path. This random motion of small particles suspended in a fluid is called Brownian motion.

**Brownian motion** — The random, jerky (zig-zag) motion of small particles suspended in a liquid or gas.

Brownian motion happens because the visible microscopic particles (e.g. smoke) are constantly bombarded by the many tiny, fast-moving, invisible particles of the surrounding gas or liquid. These collisions happen randomly and unequally from different directions, so the microscopic particle is knocked first one way and then another, giving its random path. The random motion of these suspended particles is important evidence for the kinetic particle model - it shows that a fluid is made of tiny particles in constant, random motion.

> **note**
>
> Extended only: a light, fast-moving gas or liquid molecule can still move a much larger, heavier microscopic particle when they collide. Use the terms 'atoms' or 'molecules' for the small fluid particles, kept distinct from the larger 'microscopic particles' that you actually see moving.

**Worked example:** Smoke is placed in a small glass cell and viewed under a microscope with a bright light shining across it. Describe what is seen and explain the observation.

1. Step 1: State what is observed:
2. Bright specks of light (the smoke particles) are seen moving continuously in random, jerky, zig-zag paths.
3. Step 2: Explain the cause:
4. The smoke particles are being hit by many fast-moving, invisible air particles. The collisions are random and come unequally from different directions, so each smoke particle is pushed first one way and then another.
5. Step 3: Link to the model:
6. This random motion is evidence that the air is made of tiny particles in constant, random motion, supporting the kinetic particle model.

> **Exam tip:** Be clear about which particles are which: the specks you can see are the large microscopic particles (e.g. smoke), and the things hitting them are the much smaller, invisible air (gas) particles.

## 6. Extended: Boyle's Law

For Extended tier candidates, you need to recall and apply the inverse relationship between pressure and volume of a fixed mass of gas at constant temperature, known as Boyle's Law.

**Boyle's Law** — For a fixed mass of gas at constant temperature, the pressure of the gas is inversely proportional to its volume.

$$p \\propto \\frac{1}{V}$$

$$p_1 V_1 = p_2 V_2$$

Where $p_1$ and $V_1$ are initial pressure and volume, and $p_2$ and $V_2$ are final pressure and volume.

**Worked example:** A fixed mass of gas has a volume of 200 cm³ at a pressure of 100 kPa. The gas is compressed to a volume of 50 cm³ at constant temperature. Calculate the new pressure of the gas.

1. Step 1: List known values:
2. $p_1 = 100$ kPa, $V_1 = 200$ cm³, $V_2 = 50$ cm³, $p_2 = ?$
3. Step 2: Apply Boyle's Law equation:
4. $$p_1 V_1 = p_2 V_2$$
5. Step 3: Rearrange to solve for $p_2$:
6. $$p_2 = \\frac{p_1 V_1}{V_2}$$
7. Step 4: Substitute values and calculate:
8. $$p_2 = \\frac{100 \\times 200}{50} = 400 \\text{ kPa}$$

> **Exam tip:** Ensure units for pressure and volume are the same on both sides of the Boyle's Law equation: you do not need to convert to SI units as long as they are consistent.

## Common pitfalls

- **Wrong:** Describing solid particles as 'not moving' instead of vibrating around fixed positions.
  - Why it fails: All particles have kinetic energy even at low temperatures, so they are never completely stationary.
  - Correct: State that solid particles vibrate around fixed positions with very low kinetic energy.
- **Wrong:** Claiming temperature rises as heat is added to melting ice.
  - Why it fails: Heat energy is used to break intermolecular bonds during state changes, not to increase particle kinetic energy (which determines temperature).
  - Correct: Explain temperature stays constant during state changes as energy is used to overcome intermolecular forces.
- **Wrong:** Forgetting Boyle's Law only applies to fixed mass of gas at constant temperature.
  - Why it fails: The inverse pressure-volume relationship only holds if no gas is added/removed and temperature does not change.
  - Correct: Always state the conditions (fixed mass, constant temperature) when applying or explaining Boyle's Law.
- **Wrong:** Claiming gas pressure is caused by particles repelling each other.
  - Why it fails: Gas pressure arises from collisions of particles with container walls, not repulsion between particles.
  - Correct: Link gas pressure directly to the frequency and force of particle collisions with container walls.
- **Wrong:** Swapping initial and final pressure/volume values in Boyle's Law calculations, leading to lower pressure after compression.
  - Why it fails: Compression reduces volume, so pressure must increase.
  - Correct: Sense-check your answer: if volume decreases, pressure should rise, and vice versa.

## Cheatsheet

| Concept | Core Summary | Extended Summary |
| --- | --- | --- |
| Solids | Tightly packed particles, vibrate in fixed positions, fixed shape/volume | Same as Core |
| Liquids | Particles close, slide past each other, fixed volume, no fixed shape | Same as Core |
| Gases | Particles far apart, move freely, no fixed shape/volume, compressible | Same as Core |
| State Changes | Temperature constant during change, energy used to adjust intermolecular forces | Same as Core |
| Gas Behaviour | Pressure increases with temperature (fixed volume) or reduced volume (fixed temp) | Same as Core + Apply $p_1V_1 = p_2V_2$ for fixed mass at constant temperature |

## What's next

Now that you have mastered the kinetic particle model and gas behaviour, you are ready to progress to more advanced thermal physics topics in the CIE IGCSE 0625 syllabus. Next, you will learn about thermal energy transfer processes, which build directly on your understanding of particle movement and energy. You can also practice exam-style structured questions on this topic to reinforce your learning, and review thermal physics required practicals if you are preparing for Paper 3/4 assessments.

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