Particle Model Overview

This lesson provides a comprehensive overview of the entire Particle Model of Matter topic as required by the AQA GCSE Physics specification (4.3). It brings together all the key concepts from the previous lessons — density, changes of state, internal energy, specific latent heat, and gas pressure — into a single revision resource. Use this lesson to consolidate your understanding and identify any gaps in your knowledge before the exam.

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The Particle Model: Key Principles

The particle model (also called the kinetic model) describes all matter as being made up of tiny particles (atoms or molecules) that are in constant motion. The behaviour of these particles explains the properties of solids, liquids, and gases.

Principle	Description
All matter is made of particles	Atoms or molecules that are too small to see
Particles are in constant motion	Vibrating (solids), moving around each other (liquids), or moving freely (gases)
Particles have kinetic energy	Related to their speed and the temperature of the substance
Forces of attraction exist between particles	Strongest in solids, weakest in gases
Energy can be transferred to or from particles	By heating, cooling, or doing work on them

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States of Matter: Complete Comparison

graph TD
    subgraph Solid["SOLID"]
        S1["Particles in regular,<br/>closely packed arrangement"]
        S2["Vibrate around<br/>fixed positions"]
        S3["Strong forces of<br/>attraction between particles"]
        S4["Fixed shape and volume"]
        S5["Cannot be compressed"]
        S6["High density"]
    end
    subgraph Liquid["LIQUID"]
        L1["Particles close together<br/>but irregularly arranged"]
        L2["Move around<br/>each other"]
        L3["Weaker forces than solid<br/>but still significant"]
        L4["No fixed shape,<br/>but fixed volume"]
        L5["Cannot be compressed<br/>(almost)"]
        L6["Medium density"]
    end
    subgraph Gas["GAS"]
        G1["Particles far apart<br/>and randomly arranged"]
        G2["Move rapidly in<br/>all directions"]
        G3["Very weak forces<br/>between particles"]
        G4["No fixed shape<br/>or volume"]
        G5["Can be compressed<br/>easily"]
        G6["Low density"]
    end

    style Solid fill:#3498db,color:#fff
    style Liquid fill:#2ecc71,color:#fff
    style Gas fill:#e74c3c,color:#fff

Exam Tip: When describing the three states of matter, always mention three things about the particles: their ARRANGEMENT (regular or irregular), their MOVEMENT (vibrate, slide, or move freely), and the FORCES between them (strong, weaker, or very weak). These are the three aspects the mark scheme rewards.

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Key Equations Summary

Equation	Meaning	On Equation Sheet?
p = m / V	density = mass / volume	No — must be memorised
E = m x L	energy for change of state = mass x specific latent heat	Yes — given in exam
E = m x c x change in T	energy for temperature change = mass x specific heat capacity x temperature change	Yes — given in exam
p x V = constant [H]	Boyle's Law: pressure x volume is constant at constant temperature	Yes — given in exam (Higher only)
p = F / A	pressure = force / area	Yes — given in exam

How to Use These Equations

Scenario	Equation to Use
Finding the density of an object	p = m / V
Calculating energy to melt or boil a substance	E = m x L
Calculating energy to heat a substance (no state change)	E = m x c x change in T
Finding the new pressure when volume changes at constant temperature [H]	p1 x V1 = p2 x V2

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Changes of State: Complete Summary

Change	From	To	Heating or Cooling?	Energy	What Happens to Particles
Melting	Solid	Liquid	Heating	Absorbed	Particles overcome some forces; move from fixed positions
Freezing	Liquid	Solid	Cooling	Released	Particles form bonds; settle into regular arrangement
Boiling	Liquid	Gas	Heating	Absorbed	Particles overcome all forces; separate completely
Condensation	Gas	Liquid	Cooling	Released	Particles form bonds; come closer together
Evaporation	Liquid	Gas	At any temperature	Absorbed	Fastest particles escape from the surface
Sublimation	Solid	Gas	Heating	Absorbed	Particles go directly from fixed positions to free movement

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Internal Energy: Complete Picture

graph TD
    A["INTERNAL ENERGY<br/>= Total KE + Total PE<br/>of all particles"] --> B["KINETIC ENERGY<br/>Energy of particle motion<br/>Related to TEMPERATURE"]
    A --> C["POTENTIAL ENERGY<br/>Energy stored in bonds<br/>between particles<br/>Related to STATE"]

    B --> D["Heating without<br/>state change:<br/>KE increases<br/>Temperature rises"]
    C --> E["Heating during<br/>state change:<br/>PE increases<br/>Temperature constant"]

    style A fill:#2c3e50,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#8e44ad,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#e74c3c,color:#fff

Key Points About Internal Energy

• Internal energy = total kinetic energy + total potential energy of all particles.
• Temperature is a measure of the average kinetic energy of particles (not total energy).
• When a substance is heated and its temperature rises, kinetic energy increases.
• When a substance changes state, potential energy changes while kinetic energy remains the same — temperature is constant.
• Absolute zero (0 K = -273 degrees C) is the lowest possible temperature, where particles have minimum kinetic energy.

Exam Tip: Internal energy, temperature, and heat are three different concepts. TEMPERATURE is the average kinetic energy of particles. INTERNAL ENERGY is the total kinetic + potential energy of all particles. HEAT (or heating) is the PROCESS of transferring energy. Make sure you use these terms correctly in the exam.

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Specific Latent Heat: Key Facts

• Specific latent heat of fusion (Lf): energy to change 1 kg from solid to liquid (or vice versa) at the melting point.
• Specific latent heat of vaporisation (Lv): energy to change 1 kg from liquid to gas (or vice versa) at the boiling point.
• Lv is always greater than Lf because all intermolecular forces must be overcome during vaporisation, whereas only some are overcome during fusion.
• The equation E = m x L applies only during a change of state (not during heating or cooling).

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Gas Pressure: Key Facts

For All Students (Foundation and Higher)

• Gas pressure is caused by collisions of gas particles with the container walls.
• Increasing temperature at constant volume increases pressure (particles move faster, collide more frequently and with more force).
• Gas pressure increases when there are more frequent and/or more forceful collisions.

For Higher Tier Students Only [H]

• Boyle's Law: pressure is inversely proportional to volume at constant temperature: p1 x V1 = p2 x V2.
• Decreasing volume at constant temperature increases collision frequency (same speed, less distance to travel).
• Compressing a gas does work on the gas, increasing its internal energy and temperature.

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Density: Key Facts