IB Chemistry SLTopic 1 — Particulate Nature of MatterPaper 1 & 2Core idea~8 min read
Changes of State
Solid, liquid, gas — the same particles, just arranged and moving differently. Once you can picture how the particles are packed and how much energy they carry, every change of state (and every state-symbol mark) becomes straightforward.
📘 What you need to know
The kinetic molecular theory describes matter in terms of particle energy, arrangement and motion.
Solids have a fixed shape and volume; liquids take the shape of their container; gases fill all the space available.
Changes of state are physical and reversible — the chemical identity of the substance doesn’t change.
Melting, vaporising and subliming take in energy (endothermic); freezing, condensing and depositing release energy (exothermic).
State symbols in equations: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).
The kinetic molecular theory
Everything is made of tiny particles that are constantly moving. The kinetic molecular theory explains the three states of matter by looking at three things about those particles: how much energy they have, how they’re arranged, and how they move.
The three states of matter
Let’s compare solids, liquids and gases side by side.
Solids
Fixed shape and volume.
Particles are closely packed in a regular, ordered pattern.
They can only vibrate in place — they don’t move around.
High density, and the lowest energy of the three states.
Liquids
Fixed volume, but take the shape of their container.
Particles are close together but randomly arranged.
They can slide past one another, which lets liquids flow.
Medium density; more energy than solids, less than gases.
Gases
No fixed shape or volume — they expand to fill the container.
Particles are widely spaced and move quickly in all directions.
Easily compressed because of the large gaps between particles.
Very low density; the highest energy of the three states.
The same particles in three states — what changes is how they’re arranged and how much energy they carry.
Naming the changes of state
Each change has a name, and each one is reversible. Notice they come in pairs — one direction takes in energy, the reverse gives it back.
Melting — solid to liquid.
Freezing — liquid to solid.
Vaporisation — liquid to gas. This happens as boiling (throughout the liquid, at a fixed temperature) or evaporation (only at the surface, below the boiling point).
Condensation — gas to liquid.
Sublimation — solid straight to gas.
Deposition — gas straight to solid.
Green arrows take in energy (endothermic); blue arrows release it (exothermic). Energy rises from solid to gas.
💡 Exam tip
Match the process to the energy flow: melting, vaporising and subliming are endothermic (energy in); freezing, condensing and depositing are exothermic (energy out).
Energy is needed to overcome forces between particles, and released when particles come closer together.
State symbols in equations
In a chemical equation, the physical state of each substance is shown with a small letter in brackets:
(s) — solid
(l) — liquid
(g) — gas
(aq) — aqueous (dissolved in water)
Marks are often given for correct state symbols — don’t drop them. They matter most in thermodynamics questions, where the state of a substance changes the energy involved.
WORKED EXAMPLE
Name the change of state in each case and say whether it takes in or releases energy: (a) dew forming on grass overnight, (b) a puddle drying up, (c) dry ice (solid CO2) turning straight to gas.
(a) Dew forming → condensationGas to liquid — releases energy (exothermic).(b) Puddle drying → evaporation (vaporisation)Liquid to gas at the surface — takes in energy (endothermic).(c) Dry ice → sublimationSolid straight to gas — takes in energy (endothermic).
⚠️ Common mix-up
Boiling vs evaporation: boiling happens throughout the liquid at a fixed temperature; evaporation happens only at the surface and at temperatures below boiling.
Sublimation vs deposition: sublimation is solid → gas; deposition is the reverse, gas → solid.
Up next: Average Kinetic Energy — what temperature really measures, and why particles at the same temperature don’t all move at the same speed.
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