IB Chemistry HLTopic 1 — The Particulate Nature of MatterPaper 1 & 2Core idea~12 min read
Changes of State
Ice, water and steam are the same substance three times over. Nothing chemical happens between them — the only thing that changes is how much energy the particles have, and therefore how well the forces between them can hold on.
📚 What you need to know
The kinetic molecular theory describes matter in terms of particle energy, arrangement and motion.
Solids have fixed shape and volume; particles vibrate in place in an ordered arrangement.
Liquids have fixed volume but take the container’s shape; particles are close but can slide past.
Gases have neither fixed shape nor volume; particles are far apart and move fast and randomly.
The six changes: melting, freezing, vaporisation, condensation, sublimation, deposition.
Changes that separate particles absorb energy; changes that bring them together release it.
State changes are physical and reversible. State symbols are (s), (l), (g) and (aq).
The three states at particle level
Every property in the table below comes from one balance: the kinetic energy of the particles pulling them apart against the intermolecular forces holding them together. In a solid the forces win, in a gas the energy wins, and a liquid is the awkward middle where neither quite does.
Look at the middle box carefully. Liquids are often drawn with big gaps, but they are almost as densely packed as solids — the real difference is order, not spacing.
The six changes of state
There are three pairs, and each pair is the same change run in opposite directions. Which direction you are going tells you immediately whether energy goes in or comes out.
You never have to memorise which changes are endothermic. Separating particles always costs energy, because you are working against the forces pulling them together.
Change
What happens
Energy
Melting
solid to liquid
absorbed (endothermic)
Freezing
liquid to solid
released (exothermic)
Vaporisation
liquid to gas
absorbed (endothermic)
Condensation
gas to liquid
released (exothermic)
Sublimation
solid straight to gas
absorbed (endothermic)
Deposition
gas straight to solid
released (exothermic)
Boiling and evaporation are not the same thing
Both turn a liquid into a gas, and students use the words interchangeably. They are different processes and questions do test the difference.
Evaporative cooling is the reason sweating works. The particles carrying the most kinetic energy are exactly the ones that escape, so the average energy of what stays behind falls.
State symbols
Every substance in an equation should carry its physical state, and marks are given for them. There are four:
(s) — solid
(l) — liquid, meaning the pure substance in its liquid form
(g) — gas
(aq) — aqueous, meaning dissolved in water
The pair that gets confused is (l) and (aq). Water itself is H2O(l), because it is the pure liquid. Sodium chloride solution is NaCl(aq), because the salt is dissolved in water. If something is dissolved, it is (aq); if it is the pure substance sitting there as a liquid, it is (l).
WORKED EXAMPLE
Explain, in terms of particles, why a gas can be compressed easily but a liquid can barely be compressed at all.
In a gasThe particles are separated by distances much larger than the particles themselves, so most of the volume of a gas is empty space. Pushing the particles closer simply removes some of that space.easily compressedIn a liquidThe particles are already in contact with their neighbours. There is almost no empty space to remove, and squeezing further means forcing electron clouds into each other, which is strongly resisted.almost incompressibleThe scale of it1 mol water: about 18 cm³ as liquid, about 22 700 cm³ as gasA gas takes up roughly a thousand times the volume of the same amount of liquid, and nearly all of that extra volume is nothing at all.
WORKED EXAMPLE
Name the change of state in each case and say whether energy is absorbed or released. (a) Dew appears on grass overnight. (b) A block of solid carbon dioxide produces white fog and leaves no puddle. (c) Frost forms on a car windscreen directly from water vapour in the air. (d) A puddle disappears on a cool day at 15 °C.
(a) dew on grassWater vapour in the air becomes liquid on a cold surface. Particles come closer together.condensation — energy released(b) solid carbon dioxideIt goes straight from solid to gas without melting, which is why there is no puddle.sublimation — energy absorbed(c) frost on a windscreenGas to solid directly, with no liquid stage in between.deposition — energy released(d) a puddle at 15 °CWell below the boiling point and only from the surface, so this is evaporation rather than boiling.evaporation — energy absorbedPart (d) is the one to watch. Liquid turning to gas does not automatically mean boiling.
WORKED EXAMPLE
Write a balanced equation with full state symbols for magnesium reacting with dilute hydrochloric acid, and justify each symbol you use.
The equationMg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)Mg(s)Magnesium ribbon is a solid metal, added as a solid.HCl(aq)“Dilute hydrochloric acid” means hydrogen chloride dissolved in water, so it is aqueous, not liquid.MgCl2(aq)The salt formed is soluble and stays dissolved in the solution.H2(g)Hydrogen is a gas at room temperature, which is why you see bubbles.Writing HCl(l) would describe pure liquid hydrogen chloride, which boils at around −85 °C and is not what is in the bottle.
💡 Exam tip
Describe states using all three of arrangement, motion and energy, not just “close together”.
Explain endothermic changes as overcoming forces of attraction between particles.
Distinguish boiling (throughout, fixed temperature) from evaporation (surface, any temperature).
Say state changes are physical and reversible — no new substance is made.
Put state symbols on every species in an equation, and check (l) against (aq).
Remember liquids are nearly as densely packed as solids; the difference is order.
⚠️ Common mix-up
Calling any liquid-to-gas change “boiling” when it is evaporation.
Drawing liquid particles with big gaps between them.
Saying particles “melt” or “expand” — particles do neither; the substance does.
Using (l) for a dissolved substance instead of (aq).
Thinking sublimation is rare or exotic — it is just solid to gas, missing out the liquid.
Treating a state change as a chemical change. The substance is unaltered.
Up next: Average Kinetic Energy — you have said energy goes in during melting and boiling. The next page asks the obvious follow-up: if energy is going in, why does the thermometer stop rising?
Want this explained one-to-one?
Book a free session with an experienced IB Chemistry tutor and get your trickiest topics made simple.
