Topic 2.1: Kinetic Particle Model of Matter

The kinetic particle model explains matter by describing how particles behave in solids, liquids, and gases.
This topic forms the foundation for understanding temperature, pressure, and gas laws later.

2.1.1 States of Matter

The Three States of Matter

Matter exists in solids, liquids, and gases. Their properties depend entirely on how the particles are arranged and how they move.

1. Solids

Properties:

• Fixed shape
• Fixed volume
• Cannot be compressed

Particles in solids:

• Arranged in a regular pattern
• Packed very close together
• Can only vibrate about fixed positions
• Strong attractive forces

Example:
Ice, metal, wood.

2. Liquids

Properties:

• No fixed shape (take the shape of the container)
• Fixed volume
• Cannot be compressed

Particles in liquids:

• Irregular arrangement
• Close together but with spaces to move past each other
• Move randomly
• Weaker attractive forces than solids

Example:
Water, oil.

3. Gases

Properties:

• No fixed shape
• No fixed volume (expand to fill any container)
• Easily compressed

Particles in gases:

• Far apart
• Move randomly at high speed
• Almost no attractive forces

Example:
Air, oxygen, carbon dioxide.

Changes of State

You must know these terms:

Change	Name
Solid → Liquid	Melting
Liquid → Solid	Freezing / Solidification
Liquid → Gas	Evaporation / Boiling
Gas → Liquid	Condensation

Gas → Solid and Solid → Gas are not required for IGCSE Core (these are deposition and sublimation).

Key Exam Tip

State changes occur because particles gain or lose kinetic energy.
Higher temperature → more particle motion → easier to break forces → melting/boiling.

2.1.2 The Particle Model

1. Particle Structure: Arrangement, Separation, Motion

State	Arrangement	Separation	Motion
Solid	Regular	Very close	Vibrate only
Liquid	Irregular	Close	Move around each other
Gas	Random	Far apart	Fast, random motion

A particle diagram typically shows:

• Solids: tightly packed dots
• Liquids: close but not ordered
• Gases: widely spaced dots

2. Motion of Particles & Temperature

• Temperature is a measure of average kinetic energy of particles.

• Higher temperature → particles move faster.

• Lower temperature → particles move slower.

Absolute Zero

• Lowest possible temperature = –273°C = 0 K (kelvin)

• Particles have minimum kinetic energy (they do NOT stop completely in reality, but have minimal motion)

Exam Tip:
Temperature in physics often uses kelvin, not °C.
Always add +273 to convert °C → K.

3. Gas Pressure (Particle Collision Theory)

Gas pressure is caused by particles colliding with the walls of the container.

• Faster particles = more collisions = higher pressure

• More particles in same space = more collisions = higher pressure

4 & 5. Brownian Motion

Brownian motion is the random, zig-zag movement of tiny particles suspended in a fluid (liquid or gas).

Cause:
They are being hit unevenly by fast-moving molecules around them.

Example:
Smoke particles in air under a microscope move randomly.

This provides evidence for:

• Particles exist
• Particles move randomly
• Particles collide with each other

Exam Tip:
A common question asks:
“Explain Brownian motion.”
Answer:
“Tiny particles move randomly because they are struck unevenly by fast-moving molecules of the fluid.”

6. Forces & Distance Between Particles

The physical properties of solids, liquids, and gases depend on:

• How far apart particles are

• How strong the forces between them are

• How much kinetic energy they have (motion)

Examples:

• Solids are hard and fixed because particles are close with strong forces.

• Gases diffuse quickly because particles are far apart with weak forces.

7 & 8. Pressure Explained Using Forces

Pressure = Force / Area

Gas particles exert pressure because:

• They collide with surfaces

• Each collision produces a tiny force

• Many collisions create a total force

• Gas pressure = total force per unit area

Microscopic particles (dust, pollen) can be moved by:

• Fast-moving gas molecules

• Fast-moving liquid molecules

Use correct terms:

Correct	Meaning
Atoms/molecules	Actual gas or liquid particles
Microscopic particles	Larger particles seen under microscope (e.g., smoke particles)

2.1.3 Gases and the Absolute Temperature Scale

1. Effects on Gas Pressure (Qualitative)

(a) Changing temperature at constant volume

• Higher temperature → particles move faster → more frequent and harder collisions → higher pressure

• Lower temperature → slower particles → fewer collisions → lower pressure

Example:
Heating a sealed can causes pressure to rise.

(b) Changing volume at constant temperature

• Decrease volume → particles collide more often → higher pressure

• Increase volume → collisions reduce → lower pressure

Example:
Compressing air in a syringe increases pressure.

2. °C and Kelvin Conversion

Formula:

T(K) = θ(°C) + 273

Examples:

1. 27°C → K = 27 + 273 = 300 K

2. 0°C → K = 273 K

3. –50°C → K = –50 + 273 = 223 K

Exam Tip:
Kelvin never uses the degree symbol °.
Write 300 K not 300°K.

3. Gas Law: pV = constant (Boyle’s Law) – Supplement

For a fixed mass of gas at constant temperature:

pV = constant

Meaning:

• Pressure and volume are inversely proportional.
• If volume increases, pressure decreases.
• If volume decreases, pressure increases.

Formula Triangle:

p1​V1 ​= p2​V2​

Example Problem:

A gas has pressure 200 kPa and volume 3 m³.
If the volume is halved to 1.5 m³ (temperature constant), find new pressure.

Graphical Representation

The graph of p vs V is a curve (hyperbola):

• As V increases → p decreases

• As V decreases → p increases

A p against 1/V graph is a straight line.

Exam Tip:
They might ask:
“Sketch the graph of p against V.”
Remember it is NOT a straight line.

Summary Table

Concept	Key Idea
Particle motion	Higher temperature → faster particles
Absolute zero	–273°C, particles have minimum kinetic energy
Gas pressure	Caused by particle collisions
Brownian motion	Random movement due to collisions with fluid molecules
pV = constant	Pressure and volume inversely related

Quick Exam Tips

• ALWAYS convert °C → K when doing gas calculations.

• When describing particle motion, always say random movement (for liquids and gases).

• In solids, NEVER say “particles do not move”—they vibrate.

• In Brownian motion questions, mention uneven collisions.

• For pressure explanations, use keywords:
  collision frequency, force on surface, force per unit area.

• Draw neat particle diagrams if asked.