The particle model explains why solids, liquids and gases behave so differently, using just three ideas: how close together the particles are, how they’re arranged, and how much energy they have.
1. The Particle Model
State
Arrangement
Motion
Energy
Solid
Regular, tightly packed
Vibrate about fixed positions
Lowest
Liquid
Close together, irregular
Move around each other, slide past
Medium
Gas
Far apart, random
Move rapidly in straight lines until they collide
Highest
Changes of state
Melting (solid→liquid) and boiling/evaporating (liquid→gas) both absorb energy, breaking the forces holding particles in place. Freezing (liquid→solid) and condensing (gas→liquid) both release energy, as particles come closer together and forces re-form. Sublimation is a solid changing directly to a gas (and vice versa), without passing through a liquid state — e.g. solid carbon dioxide (dry ice).
2. Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, as a result of random particle motion.
Worked example — comparing diffusion rates
Ammonia gas (Mr=17) and hydrogen chloride gas (Mr=36.5) are released from opposite ends of a glass tube at the same time. A white ring of ammonium chloride forms closer to the hydrogen chloride end. Explain why.
Ammonia has a lower relative molecular mass, so its molecules move faster (at the same temperature) and diffuse further in the same time than the heavier, slower-moving hydrogen chloride molecules — so the two gases meet closer to the HCl end.
Common mistakes to watch for
Saying particles “expand” when heated — the particles themselves don’t get bigger, they just move faster and further apart.
Confusing evaporation (happens at any temperature, only at the surface) with boiling (happens throughout the liquid, only at the boiling point).
Forgetting that diffusion happens in liquids too, not just gases — just more slowly, since liquid particles are more closely packed.
Key Vocabulary Recap
Diffusion — net movement of particles from high to low concentration
Sublimation — solid changing directly to gas, or gas directly to solid
Brownian motion — random, jittery movement of small particles, caused by collisions with faster-moving surrounding molecules
Syllabus Reference
This note covers IGCSE Chemistry 0620 Topic 1, States of Matter: the particle model of solids, liquids and gases, changes of state, and diffusion.
Practice questions
IGCSE Chemistry 0620 · Topic 1: States of Matter — Practice Question Bank (no answers)
States of Matter — Practice
Covers: particle arrangement, changes of state, and diffusion. 10 questions.
Q1 Core
Describe the arrangement, motion, and relative energy of particles in a solid, a liquid, and a gas.
Q2 Core
Name the change of state that occurs when: (a) a liquid becomes a gas, (b) a gas becomes a liquid, (c) a solid becomes a liquid.
Q3 Core
Explain, in terms of particles, why a gas can be compressed much more easily than a liquid.
Q4 Core
Define diffusion.
Q5 Extended
Ammonia (Mr=17) and hydrogen chloride (Mr=36.5) are released simultaneously from opposite ends of a tube. Explain why the white ring of ammonium chloride forms closer to the hydrogen chloride end.
Q6 Core
Explain the difference between evaporation and boiling.
Q7 Core
What is sublimation? Give one example of a substance that sublimes.
Q8 Core
Explain, in terms of particles and energy, what happens when a solid melts.
Q9 Extended
Explain why increasing the temperature of a gas increases the pressure it exerts on a sealed container, in terms of particle behaviour.
Q10 Core
Describe Brownian motion, and explain what it provides evidence for.
Worked solutions
Show worked solutions
IGCSE Chemistry 0620 · Topic 1: States of Matter — Solutions
States of Matter — Practice: Solutions
Q1
Particle arrangement/motion/energy in solid, liquid, gas.
✓Solid: regular, tightly packed, vibrate about fixed positions, lowest energy. Liquid: close together but irregular, move around each other, medium energy. Gas: far apart, random, move rapidly in straight lines until colliding, highest energy.
✓In a gas, particles are far apart with large empty spaces between them, so those spaces can be reduced. In a liquid, particles are already close together with little empty space, so there is very little room to compress further.
Q4
Define diffusion.
✓Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, as a result of their random motion.
Q5
Why does the ring form closer to the HCl end?
✓Ammonia has a lower relative molecular mass (17) than hydrogen chloride (36.5), so at the same temperature its molecules move faster. Faster-moving ammonia molecules travel further in the same time than the heavier, slower HCl molecules, so they meet closer to the HCl end.
Q6
Evaporation vs boiling.
✓Evaporation happens only at the surface of a liquid, at any temperature below the boiling point. Boiling happens throughout the whole liquid, and only at one specific temperature (the boiling point).
Q7
Define sublimation, give an example.
✓Sublimation is the direct change of state from a solid to a gas (or gas to solid), without passing through the liquid state. Example: solid carbon dioxide (“dry ice”).
Q8
Explain melting in terms of particles and energy.
✓As a solid is heated, its particles gain energy and vibrate more vigorously about their fixed positions. At the melting point, the particles gain enough energy to overcome the forces holding them in a fixed arrangement, so they can move around each other — the solid becomes a liquid.
Q9
Why does heating a gas increase pressure?
✓Heating increases the average kinetic energy of the gas particles, so they move faster. Faster particles collide with the container walls more frequently and with greater force, increasing the pressure exerted.
Q10
Describe Brownian motion; what does it show?
✓Brownian motion is the random, jittery movement of small particles (e.g. smoke particles or pollen grains) suspended in a fluid, caused by being struck unevenly by fast-moving, invisible molecules of the surrounding gas or liquid. It provides evidence that matter is made of moving particles too small to see directly.
