[Author © Dr WP Brown PhD: Doc Brown's chemistry exam revision notes on states of matter - physical properties of gases, liquids and solids, suitable for students of UK GCSE level and international IGCSE/O level chemistry courses, ~US grades 9-10 chemistry notes [page updated Nov 28th 2025]

INDEX of all my notes on the states of matter

The three states of matter are solid, liquid and gas.

Either melting and freezing can take place at the melting point, whereas boiling and condensing take place at the boiling point.

Evaporation can take place at any temperature from a liquid surface.

You can represent the three states of matter with a simple particle model.

In this model–diagrams, the particles are represented by small solid inelastic spheres (electron structure is ignored).

Kinetic particle theory can help to explain changes of state like melting, boiling, freezing and condensing.

The amount of energy needed to change state from solid to liquid or from liquid to gas depends on the strength of the forces between the particles of the substance.

These inter-particle forces may be relatively weak intermolecular forces (intermolecular bonding) or strong chemical bonds (ionic, covalent or metallic).

The nature of the particles involved depends on the type of chemical bonding and the structure of the substance.

The stronger the attractive forces between the particles the higher the melting point and boiling point of the substance

• WHAT ARE THE THREE STATES OF MATTER?

  • Most materials can be simply described as a gas, a liquid or a solid.

• WHY ARE THEY LIKE - WHAT STATE THEY ARE?

  • Just knowing isn't enough, we need a comprehensive theory of gases, that can explain their behaviour and make predictions about what happens e.g. if we change temperature or pressure.

• HOW CAN WE EXPLAIN HOW THEY BEHAVE?

  • We need a theoretical model  e.g. 'particle theory' that is supported by experimental evidence.

• CAN PARTICLE MODELS HELP US UNDERSTAND THEIR PROPERTIES and CHARACTERISTICS?

  • In a word, YES!

• WHY IS IT IMPORTANT TO KNOW THE PROPERTIES OF GASES, LIQUIDS AND SOLIDS?

  • It is important in the chemical industry to know about the behaviour of gases, liquids and solids in chemical processes e.g. what happens to the different states with changes in temperature and pressure.

• What is the KINETIC PARTICLE THEORY of gases, liquids and solids?

  • The kinetic particle theory of the states of matter is based on the idea of all materials existing as very tiny particles which may be individual atoms or molecules and the their interaction with each other either by collision in gases or liquids or by vibration and chemical bonding in solids.

• CAN WE MAKE PREDICTIONS BASED ON THEIR CHARACTERISTIC PROPERTIES?

  • This page introduces general physical descriptions of substances in the simplest physical (non–chemical) classification level i.e. is it a gas, liquid or a solid.

  • BUT, this web page also introduces 'particle models' in which a small circle represents an atom or a molecule i.e. a particular particle or simplest unit of a substance.

  • This section is quite abstract in a way because you are talking about particles you can't see individually, you just the 'bulk' material and its physical character and properties.

• Are there LIMITATIONS to the particle model?

  • Well, yes! e.g.

  • The particles are treated as simple minute inelastic spheres and just behave like minute snooker balls flying around, not quite true, but they do fly around non-stop at random!

  • Although the particles are assumed to be hard spheres and inelastic, in reality they are atoms, ions or molecules.

  • Apart from lone atoms, they can be all sorts of shapes and twist and bend on collision with other particles and when they react they split into fragments when bonds break.

  • The simple model assumes no forces between the particles, but this is untrue, the model takes little account of the forces between the particles, even in gases you get very weak intermolecular bonding forces.

  • The particle model takes no account of the actual size of the particles e.g. ions/molecules can be widely different in size e.g. compare an ethene molecule with a poly(ethene) molecule!

  • Neither does it take account of any space that may exist between the particles.

• A note on 'forces'

  • Forces between particles are mentioned on this page and some ideas will seem more abstract than others – but think about it ...

  • A gas spreads everywhere in a given space, so there can't be much attraction between the molecules/particles.

  • Something must hold liquid molecules together or how can a liquid form from a gas?

  • In fact between liquid molecules there are actually weak electrical forces of attraction called intermolecular forces, but they can't be strong enough to create a rigid solid structure.

  • However, in solids, these forces must be stronger to create the rigid structure.

  • Intermolecular forces are also called 'intermolecular bonds' BUT these are not the same as covalent, ionic or metallic bonds and are much weaker than these true chemical bonds.

Summary of the Kinetic Particle Theory of Matter – GCSE/IGCSE Level Revision Notes

Core Idea

The kinetic particle theory explains the properties of solids, liquids, and gases in terms of the arrangement, movement, and energy of particles.

Below are a list of situations which the kinetic particle model of matter tries to explain, with much success, but far from perfect.

1. States of Matter

• Solids
  • Particles: closely packed, regular arrangement
  • Movement: vibrate about fixed positions
  • Properties: fixed shape, fixed volume, not easily compressed

  • 7. Particle model of a solid and explaining the properties of solids, links to physics density notes

• Liquids
  • Particles: close together, irregular arrangement
  • Movement: slide past each other, more freedom than solids
  • Properties: fixed volume, no fixed shape, flow, not easily compressed

  • 5. Kinetic particle model of a liquid and properties explained, mention of Brownian motion

  • 6. Particle model of a liquid and diffusion experiments and associated phenomena

• Gases
  • Particles: far apart, random arrangement
  • Movement: rapid, random motion in all directions
  • Properties: no fixed shape or volume, easily compressed, expand to fill container

  • 3. A kinetic particle model of a gas, properties, pressure and the effect of temperature change

2. Changes of State

• Melting, boiling, condensation, freezing, sublimation explained by changes in particle energy and movement.
• Energy input/output changes kinetic energy → alters forces between particles.

• 8. Physical state changes - a summary diagram for gas <==> liquid <==> solid changes

• 9. Evaporation and boiling - explained using particle model and energy changes

• 10. Condensation and distillation - explained using particle model and energy changes

• 11. Melting and freezing state changes explained using the particle model and energy changes

• 12. The special case of sublimation - examples described and explained using particle model

3. Diffusion

• Particles move randomly → substances spread out.
• Faster in gases than liquids (particles further apart, weaker forces).

• 4. What is diffusion? Particle model and examples of demonstrating diffusion in gases

• 6. Particle model of a liquid and diffusion experiments and associated phenomena

4. Pressure in Gases

• Caused by particles colliding with container walls.
• Higher temperature → faster particles → more frequent, forceful collisions → higher pressure.

• 3. A kinetic particle model of a gas, properties, pressure and the effect of temperature change

Limitations of the Kinetic Particle Theory

• Simplified model: represents particles as spheres, ignores real atomic/molecular shapes.
• No account of forces: diagrams often omit attractive/repulsive forces between particles.
• Not scale-accurate: spacing and relative sizes are exaggerated for clarity.
• Doesn’t explain complex phenomena: e.g., solubility, chemical bonding, or why different substances have different melting/boiling points.
• Idealised assumptions: assumes perfectly elastic collisions and ignores intermolecular variations.

Exam Tips

• Use particle diagrams: draw clear, labelled diagrams for solids, liquids, gases.
• Link energy to movement: always mention kinetic energy when explaining changes of state.
• Pressure questions: connect temperature → kinetic energy → collision frequency/force → pressure.
• Diffusion explanations: compare particle spacing and forces in gases versus liquids.
• State properties: examiners expect you to connect arrangement + movement → observed properties.

Typical Misconceptions

• Thinking particles expand when heated → correction: particles move faster, spacing increases, but particles themselves do not expand.
• Believing liquids are easily compressed → correction: particles are close together, so liquids are not easily compressed.
• Assuming all gases diffuse at the same rate → correction: diffusion rate depends on particle mass (lighter gases diffuse faster).
• Forgetting that melting and boiling involve energy transfer → correction: energy is used to overcome forces, not to increase temperature during the change of state (plateau on heating curve).
• Confusing particle movement with bulk movement → correction: diffusion is due to random particle motion, not directed flow.

Typical exam board requirements

Revision Strategy:

• Practice drawing particle diagrams for each state.
• Memorise properties → particle explanation links.
• Revise limitations (often tested in evaluation-style questions).
• Use past papers to spot how each exam board phrases diffusion/pressure/state change questions.

Learning objectives for the kinetic particle model of gases, liquids and solids

Know what do we want or expect from a particle model of gases, liquids and solids.

Know that any particle theory must be supported from experimental observations and experiments to match the kinetic particle theory.

Know why it is so important to understand the particle nature of gases, liquids and solids.

Know, and be able to explain and describe how the kinetic particle theory of the states of matter is based on the idea of all materials existing as extremely tiny particles which may be individual atoms or molecules and the their interaction with each other either by collision in gases or liquids or by vibration in solids.

Be able to describe and explain some limitations to the particle model e.g. they are not simple tiny inelastic spheres of zero volume with no attractive forces between them e.g. in a gas or liquid.

BUT ...

(i) The particles in gases have a real volume, which is assumed to be zero when doing gas law calculations.

(ii) There are always weak intermolecular forces attractive forces between ALL particles in gases, liquids or solids in the order solids > liquids >> gases (put in main text)

All my UK GCSE level (~US grade 8-10) school chemistry revision notes