What you'll learn
This topic covers the particle model of matter, which explains the behaviour of solids, liquids and gases. You'll learn how particles are arranged and move in different states, how substances change state, and how to interpret data about physical changes. This foundational topic underpins much of GCSE Chemistry and appears frequently in exam questions worth 2-6 marks.
Key terms and definitions
Particle model — a scientific model that describes all matter as made up of tiny particles (atoms, molecules or ions) that behave differently depending on the state of matter
State of matter — the physical form of a substance: solid, liquid or gas, determined by particle arrangement and energy
Diffusion — the spreading out of particles from an area of high concentration to an area of low concentration, occurring in liquids and gases
Sublimation — the direct change of state from solid to gas without passing through the liquid state
Melting point — the temperature at which a solid changes to a liquid; a pure substance has a sharp, specific melting point
Boiling point — the temperature at which a liquid changes to a gas throughout the bulk of the liquid, not just at the surface
Conservation of mass — during state changes, no particles are created or destroyed, so total mass remains constant
Kinetic energy — the energy possessed by particles due to their movement; increases with temperature
Core concepts
The particle model and states of matter
All substances exist as solids, liquids or gases. The particle model explains the properties of each state:
Solids:
- Particles arranged in a regular, fixed pattern
- Particles very close together
- Particles vibrate about fixed positions but cannot move from place to place
- Strong forces of attraction between particles
- Definite shape and volume
- Cannot be compressed
- Cannot flow
Liquids:
- Particles close together but not in a fixed arrangement
- Particles can move around each other
- Medium forces of attraction between particles
- No definite shape (takes the shape of its container)
- Definite volume
- Cannot be compressed significantly
- Can flow
Gases:
- Particles far apart
- Particles move randomly at high speeds in all directions
- Very weak forces of attraction between particles
- No definite shape or volume (fills the container)
- Can be compressed
- Can flow
- Much lower density than solids or liquids
The state of a substance at room temperature depends on the strength of forces between its particles and the particle energy.
Changes of state
When substances change state, the particles themselves don't change — only their arrangement and movement change. Energy must be transferred to or from the substance.
State changes that require energy input (heating):
- Melting — solid to liquid
- Boiling/evaporation — liquid to gas
- Sublimation — solid to gas (e.g. carbon dioxide, iodine)
State changes that release energy (cooling):
- Freezing — liquid to solid
- Condensation — gas to liquid
During a state change, temperature remains constant even though energy is being transferred. This energy breaks or forms forces between particles rather than increasing kinetic energy.
Key principles:
- State changes are physical changes, not chemical changes
- The process is reversible
- Mass is conserved — no particles are lost or gained
- The substance has the same chemical properties before and after
Heating and cooling curves
A heating curve shows how temperature changes when a substance is heated at a constant rate.
Features of a heating curve:
- Diagonal sections show temperature increasing as kinetic energy of particles increases
- Horizontal (flat) sections show state changes occurring at constant temperature
- First flat section is the melting point
- Second flat section is the boiling point
- Pure substances have sharp melting and boiling points (horizontal lines)
- Mixtures melt and boil over a range of temperatures (sloped transitions)
Cooling curves show the reverse process, with the same principles applying.
Diffusion
Diffusion occurs because particles move randomly. It happens in both liquids and gases but not in solids (where particles cannot move from place to place).
Factors affecting rate of diffusion:
Temperature:
- Higher temperature = faster diffusion
- Particles have more kinetic energy so move faster
Relative molecular mass:
- Lighter particles diffuse faster than heavier particles
- At the same temperature, lighter particles move faster
- Example: ammonia (Mr = 17) diffuses faster than hydrogen chloride (Mr = 36.5)
State:
- Diffusion is faster in gases than liquids
- Gas particles are further apart and move more freely
- Liquid particles must move around each other
Real-world examples:
- Smell of perfume spreading across a room
- Food colouring spreading through water
- Bromine gas spreading through air
Limitations of the particle model
The simple particle model taught at GCSE has limitations:
- Assumes all particles are solid spheres (reality: atoms have internal structure)
- Doesn't show the forces between particles accurately
- Doesn't account for the size of particles or space between them precisely
- Cannot explain all properties of substances (e.g. different types of bonding)
Despite these limitations, the particle model successfully explains:
- Properties of solids, liquids and gases
- State changes
- Diffusion
- Pressure in gases
- Conservation of mass
Using state symbols in equations
Chemical equations use state symbols to indicate the physical state of each substance:
- (s) — solid
- (l) — liquid
- (g) — gas
- (aq) — aqueous (dissolved in water)
Example: H₂O(s) → H₂O(l) represents ice melting to water
State symbols are essential for accurately representing chemical and physical changes and must be included when specified in exam questions.
Worked examples
Example 1: Explaining diffusion (3 marks)
Question: A student places a drop of blue food colouring in the centre of a beaker of water. After one hour, the water throughout the beaker is pale blue. Explain this observation using ideas about particles.
Mark scheme answer:
- The food colouring particles move randomly (1 mark)
- The particles spread out from an area of high concentration to low concentration / diffusion occurs (1 mark)
- This continues until particles are evenly distributed throughout the water (1 mark)
Examiner note: Use scientific terminology like "diffusion" and "particles" rather than vague descriptions. Link particle movement to concentration differences.
Example 2: Interpreting a heating curve (4 marks)
Question: The graph below shows the temperature of a pure substance as it is heated.
[Imagine a typical heating curve with two flat sections at 80°C and 150°C]
(a) State the melting point of this substance. (1 mark) (b) State the boiling point of this substance. (1 mark) (c) Explain why the temperature does not increase during melting. (2 marks)
Mark scheme answer: (a) 80°C (1 mark) (b) 150°C (1 mark) (c)
- Energy is used to break/weaken forces between particles (1 mark)
- Rather than increasing the kinetic energy of the particles / increasing particle movement (1 mark)
Examiner note: For explanation questions, always link energy transfer to what's happening at particle level.
Example 3: Comparing states of matter (6 marks)
Question: Compare the arrangement and movement of particles in a solid, liquid and gas. (6 marks)
Mark scheme answer:
Solid:
- Particles in a regular arrangement/pattern (1 mark)
- Particles very close together (1 mark)
- Particles vibrate about fixed positions (1 mark)
Liquid:
- Particles close together but randomly arranged (1 mark)
- Particles can move around each other (1 mark)
Gas:
- Particles far apart (1 mark)
- Particles move randomly in all directions / move quickly (1 mark)
[Any 6 points from above]
Examiner note: "Compare" means you must discuss both similarities and differences. Use comparative language like "whereas" and "in contrast to."
Common mistakes and how to avoid them
Confusing particles with atoms — not all particles are atoms; they can be molecules (groups of atoms bonded together) or ions. Use "particle" as the general term unless the question specifies atoms.
Thinking particles get bigger when heated — particles themselves don't change size when heated. They gain kinetic energy and move faster, taking up more space due to increased movement.
Believing particles disappear during state changes — mass is conserved because particles are not lost. If water seems to "disappear" when boiling, it has become water vapour (gas), not vanished.
Saying particles touch in gases — in gases, particles are far apart and only collide briefly. Don't draw or describe gas particles as touching except during collisions.
Using "molecules" for all substances — ionic compounds and metals don't form molecules. Use "particles" as the general term.
Forgetting that diffusion occurs in liquids — diffusion happens in both gases and liquids, just faster in gases. Don't limit diffusion to gases only in your answers.
Exam technique for "C1: Particles"
"Describe" questions require you to state observations or features without explanation (typically 1 mark per feature). For state changes: "The particles become further apart" scores a mark; adding "because..." wastes time unless "explain" is asked.
"Explain" questions need reasons linked to particle behaviour. Use "because," "so," or "therefore" to link cause and effect. Structure: state what happens, then why it happens at particle level (usually 2-3 marks).
Drawing particle diagrams — use circles to represent particles; make them touching for solids/liquids and well-spaced for gases; show regular patterns for solids; draw at least 6-8 particles to clearly show arrangement; use the same size particles throughout.
Mark allocation guides depth — a 1-mark question needs a simple statement; a 4-mark question requires developed explanation with multiple linked points about particle behaviour, arrangement and movement.
Quick revision summary
The particle model describes matter as made of tiny particles in constant motion. Solids have particles in fixed positions vibrating in place; liquids have particles moving around each other; gases have particles far apart moving randomly. State changes involve energy transfer to break or form forces between particles without changing the particles themselves. Diffusion occurs when particles spread from high to low concentration, faster in gases than liquids and at higher temperatures. Mass is conserved during all physical changes.