The big idea: All matter is made of tiny moving particles. Whether a substance is a solid, liquid or gas depends only on how much energy those particles have — which decides how close they are and how freely they move.
- Solid — particles packed in fixed positions, only vibrating. - Liquid — particles touching but able to slide past each other. - Gas — particles far apart, moving fast and randomly.
Solid = packed and vibrating in fixed positions · Liquid = touching but able to slide · Gas = far apart, fast and random.
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Energy decides the state: Add energy (heat) and the particles move more, spread further apart and break loose: solid → liquid → gas. Remove energy and the reverse happens. The particles themselves never change — only their arrangement and movement do.
The kinetic molecular theory (KMT) is the model chemists use to explain the states of matter. 'Kinetic' means moving — the theory pictures every particle as constantly in motion, and uses that motion to explain what we observe.
Kinetic molecular theory: A model that treats matter as small particles in constant random motion, where:
- the particles have kinetic energy (energy of movement); - attractive forces act between them — strong when close (solid/liquid), negligible when far apart (gas); - heating transfers energy to the particles, making them move faster and spread further apart.
| Solid | Liquid | Gas | |
|---|---|---|---|
| Arrangement | regular, packed | close, random | far apart, random |
| Movement | vibrate in place | slide past each other | fast, random in all directions |
| Fixed shape? | yes | no (takes container shape) | no (fills container) |
| Compressible? | no | barely | yes (large gaps) |
| Can flow? | no | yes | yes |
Explaining a property with KMT: Why does a gas fill its whole container? KMT says gas particles are far apart with negligible forces between them and move fast in random directions, so they spread out to occupy all the available space.
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Temperature measures average kinetic energy: Temperature is a measure of the average kinetic energy of the particles. Hotter substance → particles moving faster on average → higher temperature.
Because kinetic energy can never be negative, there is a lowest possible temperature — absolute zero, where particle motion is at a minimum. This is the start of the kelvin (K) scale.
To convert a Celsius temperature into the absolute (kelvin) scale used in gas calculations, add 273.15 (often rounded to 273). The kelvin scale starts at absolute zero, which is 0 K (about −273 °C).
- absolute temperature in kelvin (K)
- temperature in degrees Celsius (°C)
- the offset between the two scales (often rounded to 273)
Worked example — converting °C to kelvin
A reaction mixture is heated from 25 °C to 80 °C. Convert both temperatures to kelvin.
Solution
- Formula first — add 273.15 to each Celsius value:
- Convert the starting temperature:
- Convert the final temperature:
Final answer
25 °C = 298 K and 80 °C = 353 K.
Changes of state — temperature holds steady: While a substance is changing state (melting or boiling), its temperature stays constant even though heating continues. The energy added is used to overcome the forces between particles, not to speed them up — so the average kinetic energy (and temperature) does not rise until the change is complete.
| Change of state | Direction | What the particles do |
|---|---|---|
| Melting | solid → liquid | gain energy; break free of fixed positions and start to slide |
| Freezing | liquid → solid | lose energy; lock into fixed positions |
| Boiling / evaporating | liquid → gas | gain enough energy to break apart and spread out |
| Condensing | gas → liquid | lose energy; come back together |
Heating gives particles more energy: a solid melts to a liquid as the particles gain enough energy to break free of their fixed positions.
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How this is tested: S1.1.2 shows up two ways.
- Paper 1B / Paper 1A (calculate): a one-line °C → K conversion, often filling a results table before a gas calculation. - Paper 2 (explain): 'state and explain a property of a solid, liquid or gas using the kinetic molecular theory' — you must link the observation to what the particles are doing.
Scoring an 'explain' mark: Never just describe the property. For each mark, pair a property (what we see) with the particle reason (arrangement, movement or forces). 'A liquid flows because its particles can slide past each other' = the full mark.
IB-style question — properties of a liquid (a)
Using the kinetic molecular theory, explain why a liquid takes the shape of its container but a solid does not. [2]
How to score the marks
- Mark 1 — the liquid. In a liquid the particles are still close together but are not held in fixed positions, so they can slide past one another and flow to fit the shape of the container.
- Mark 2 — the solid. In a solid the particles are held in fixed positions by stronger forces and can only vibrate, so the solid keeps a fixed shape.
Final answer
Liquid particles can slide past each other (not fixed) so a liquid takes the container shape; solid particles are fixed in position and only vibrate, so a solid keeps its shape.
IB-style question — properties of a liquid (b)
Explain why a liquid cannot be compressed easily, whereas a gas can. [2]
How to score the marks
- Mark 1 — the liquid. The particles in a liquid are already very close together (touching), so there is little space to push them into — a liquid is almost incompressible.
- Mark 2 — the gas. In a gas the particles are far apart with large spaces between them, so they can be pushed closer together, meaning a gas is easily compressed.
Final answer
Liquid particles are already touching with little space between them, so a liquid barely compresses; gas particles are far apart with large gaps, so a gas compresses easily.