States of Matter

Summary: The kinetic particle theory explains the properties and behaviour of solids, liquids, and gases in terms of particle arrangement, movement, and energy. Changes of state involve energy changes at constant temperature. Diffusion provides direct evidence for particle motion. Tags: igcse chemistry particulate-nature Created: 2026-07-14 Last Updated: 2026-07-17

The states of matter are the distinct physical forms in which all substances can exist — solid, liquid, and gas — each defined by a characteristic arrangement, separation, and motion of particles. The kinetic particle theory explains these states: the balance between the kinetic energy of particles and the strength of the forces between them determines whether a substance holds a fixed shape, flows, or expands to fill its container. Matter converts between states through changes of state — melting, boiling, evaporation, freezing, and condensation — which are physical (reversible) changes involving energy transfer at constant temperature. These energy plateaus are visible on heating and cooling curves, where all supplied or released energy goes into breaking or forming interparticle forces rather than changing temperature. The behaviour of gases under changing temperature and pressure is explained by the same theory (see Gas Laws), and diffusion provides direct observable evidence that particles are in constant random motion. Understanding states of matter underpins nearly every other topic in chemistry, from atomic structure to rates of reaction.

The Three States of Matter

All matter is made of particles (atoms, molecules, or ions) in constant motion. The kinetic particle theory states that the amount of energy and the forces between particles determine the state of matter.

The Kinetic Particle Theory in Detail

The kinetic particle theory makes three key assertions:

1. All matter is made of tiny particles (atoms, molecules, or ions).
2. These particles are in constant random motion.
3. The amount of motion (kinetic energy) increases with temperature.

Evidence for the kinetic particle theory:

• Diffusion (see below): particles moving from high to low concentration — observed in gases and liquids.
• Brownian motion: the random, jerky movement of smoke particles or pollen grains viewed under a microscope — caused by collisions with invisible air/water particles.
• Expansion on heating: particles gain kinetic energy and move further apart (except during phase changes).
• Gas pressure: the force exerted by gas particles colliding with container walls.

Changes of State

Changes of state are physical changes — no new substances are formed, and the process is reversible.

Solid ⇌ Liquid ⇌ Gas
  (melting)   (boiling/evaporation)
  (freezing)  (condensation)
    ⇌  (sublimation)  ⇌

Key substances that sublime (IGCSE examples):

• Iodine (I2): grey-black solid → purple vapour on gentle heating
• Carbon dioxide (CO2): solid (dry ice) → gas at -78 degrees C at atmospheric pressure — used in fog machines, shipping frozen goods
• Ammonium chloride (NH4Cl): appears to sublime but actually thermally dissociates: NH4Cl(s) ⇌ NH3(g) + HCl(g), then recombines on cooling

Heating and Cooling Curves

A heating curve plots temperature against time as a substance is heated. The shape reveals the states and changes of state.

Key features:

• Horizontal sections (plateaus): a change of state is occurring. Temperature remains constant because all the energy supplied is used to overcome the forces between particles (breaking bonds/intermolecular forces), NOT to raise the temperature. At the melting point, energy breaks the forces holding particles in fixed positions. At the boiling point, energy breaks the remaining forces between particles completely.
• Sloping sections: a single state is being heated. Temperature increases as particles gain kinetic energy.

Cooling curve: the reverse process. Horizontal sections occur at the freezing point and condensation point (same temperatures as melting and boiling points respectively).

The melting point and boiling point are read from the horizontal plateau on the heating/cooling curve. Pure substances have sharp melting/boiling points (short horizontal sections). Impure substances melt/boil over a range of temperatures.

Explaining Properties Using Kinetic Particle Theory

Expansion on heating:

• When heated, particles gain kinetic energy and vibrate/move more vigorously.
• They push each other further apart.
• The substance expands (increased volume).
• This explains: expansion gaps in bridges and railway tracks, why hot air rises (less dense), thermometers.

Compressibility of gases:

• In gases, particles are far apart with large empty spaces between them.
• Applying pressure forces the particles closer together.
• Gases are highly compressible.
• Solids and liquids are incompressible because particles are already close together with very little empty space.

Gas pressure:

• Gas particles are in constant rapid random motion.
• They collide with the walls of their container.
• Each collision exerts a tiny force on the wall.
• The cumulative effect of billions of collisions per second = gas pressure.
• Increasing temperature → particles have more kinetic energy → collisions are more frequent and more energetic → pressure increases (at constant volume).
• Decreasing volume → particles collide with walls more frequently → pressure increases (at constant temperature).

Diffusion

Definition: Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, resulting from the random motion of particles. Diffusion occurs in liquids and gases (and very slowly in solids over geological timescales).

Evidence for particle motion: Diffusion can only be explained if particles are in constant random motion. The spread of perfume across a room, or the spread of ink in water, is direct evidence.

The ammonia and hydrogen chloride experiment (classic IGCSE practical):

Setup: A long glass tube with cotton wool plugs at each end. One plug is soaked in concentrated ammonia solution (NH3(aq)); the other in concentrated hydrochloric acid (HCl(aq)). The ends are stoppered.

Observations:

• A white ring of ammonium chloride (NH4Cl) forms inside the tube.
• The ring forms closer to the HCl end of the tube.

Equation: NH3(g) + HCl(g) → NH4Cl(s)

Why the ring forms closer to the HCl end:

• Ammonia (NH3, Mr = 17) is a lighter gas than hydrogen chloride (HCl, Mr = 36.5).
• Lighter particles diffuse faster than heavier particles at the same temperature (Graham’s law: rate of diffusion is inversely proportional to the square root of the molar mass).
• In the same time, NH3 particles travel further than HCl particles.
• They meet closer to the HCl source, where the ring forms.

Factors affecting the rate of diffusion:

Changes of State in Terms of Particle Theory (Summary)

• Melting: Solid particles, vibrating in fixed positions, absorb energy. Vibrations become more vigorous until particles overcome the forces holding them in place. They become free to move past each other (liquid). Temperature does NOT rise during melting — all energy goes into overcoming interparticle forces.
• Boiling: Liquid particles, moving past each other, absorb energy. Movement becomes more vigorous until particles overcome all remaining forces. They escape the liquid entirely and move freely in all directions (gas). Temperature does NOT rise during boiling.
• Freezing: Liquid particles lose energy. Movement slows until forces between particles lock them into fixed positions, forming a regular solid lattice.
• Condensation: Gas particles lose energy and come close enough for forces to pull them together as a liquid.

Key Points

• Solid: fixed shape/volume, particles vibrate in fixed positions, strong forces, regular arrangement
• Liquid: fixed volume, no fixed shape, particles slide past each other, weaker forces, random arrangement
• Gas: no fixed shape or volume, particles move rapidly, negligible forces, random arrangement
• Melting and boiling: temperature stays constant because energy breaks interparticle forces, not to raise temperature
• Sublimation: solid → gas without passing through liquid (e.g., I2, CO2)
• Diffusion: net movement of particles from higher to lower concentration due to random motion
• NH3 + HCl → NH4Cl (white ring); ring forms closer to HCl end because NH3 (Mr 17) diffuses faster than HCl (Mr 36.5)
• Lighter particles diffuse faster at the same temperature
• Higher temperature = faster diffusion (more kinetic energy)
• Gas pressure arises from particles colliding with container walls

Key Concepts from Past Papers

• Diffusion: the net movement of particles from a region of higher concentration to a region of lower concentration
• Sublimation: the change of state directly from solid to gas (without passing through the liquid state)
• Kinetic particle theory: the model that describes all matter as being made of particles in constant motion, where the amount of energy and forces between particles determines the state
• During melting/boiling, temperature remains constant because energy is used to overcome forces between particles
• NH3 is lighter than HCl, so it diffuses faster
• Gas pressure is caused by particles colliding with the walls of the container

Keywords from Past Papers

point, melting, boiling, between, liquid, lower, solid, higher, octane, nerol, ora, points, temperature, first, gas

Related Notes

• Solid — Properties of solids
• Liquid — Properties of liquids
• Gas — Properties of gases
• Particle Arrangement — Particle separation, arrangement and motion in the three states
• Kinetic Particle Theory — The theory underpinning states of matter
• Melting / Boiling / Evaporation / Freezing / Condensation — Changes of state
• Heating Curve / Cooling Curve — Interpreting temperature-time graphs
• Gas Laws — Effects of temperature and pressure on gas volume
• Diffusion
• Relative Molecular Mass — Effect of Mr on diffusion rate

Sources

• OpenStax Chemistry 2e — Chapter 10: Liquids and Solids, Rice University (free, CC BY 4.0)
• BBC Bitesize GCSE Chemistry — States of Matter, BBC (free educational resource)
• Cambridge IGCSE Chemistry 0620 — Syllabus Section 1: States of matter, Cambridge Assessment International Education
• CK-12 Chemistry for High School — States of Matter, CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

• 0620/32 Feb/March 2018: Q33(b)(iii) (1m), Q33(b)(iii) (1m)
• 0620/32 Feb/March 2019: Q77(a)(iii) (1m), Q77(a)(iii) (1m)
• 0620/32 Feb/March 2021: Q33(a)(iii) (2m), Q33(a)(iii) (2m)
• 0620/32 Feb/March 2022: Q66(a)(ii) (2m), Q66(a)(ii) (2m)
• 0620/32 May/June 2018: Q33(c)(i) (1m), Q55(b)(ii) (1m), Q77(a)(i) (1m) (+1 more)
• 0620/32 May/June 2020: Q44(a)(iii) (2m), Q77(c)(ii) (2m), Q44(a)(iii) (2m) (+1 more)
• 0620/33 May/June 2016: Q33(a)(iii) (1m), Q33(a)(iii) (1m)
• 0620/33 May/June 2017: Q55(e)(iii) (0m), Q55(e)(iii) (0m)
• 0620/33 May/June 2019: Q55(b)(i) (2m), Q55(b)(i) (2m)
• 0620/33 May/June 2021: Q33(a)(ii) (2m), Q33(a)(ii) (2m)
• 0620/33 May/June 2022: Q33(c)(ii) (0m), Q33(c)(ii) (2m)
• 0620/33 May/June 2023: Q22(a)(iii) (2m), Q22(a)(iii) (2m)

Common Misconceptions