Chapter Map
States of Matter
Solids, liquids, and gases have different properties. When matter changes state, particles rearrange but the number of particles stays the same.
Molecular Motion
Particles move differently in each state. This motion and arrangement explains density, compressibility, and flow.
Gas Pressure
Gas pressure comes from particle collisions with walls. Temperature and volume control pressure.
Brownian Motion
Visible particles jiggle erratically — proof that invisible atoms/molecules exist and are constantly moving.
Absolute Temperature
Kelvin scale starts at absolute zero (−273 °C). Temperature is a measure of particle kinetic energy.
Gas Laws
Pressure, volume, and temperature are linked. Boyle's Law shows pressure ∝ 1/volume at constant temperature.
States of Matter
The Three States: Shape & Volume
Every substance exists in one of three physical states. The defining feature of each is what happens to shape and volume:
| State |
Shape |
Volume |
Can Flow? |
Compressible? |
| Solid |
Fixed (rigid) |
Fixed |
No |
No |
| Liquid |
Takes shape of container |
Fixed |
Yes |
No |
| Gas |
Takes shape of container |
Fills container completely |
Yes |
Yes |
Changes of State
When matter changes state, the number of particles stays constant — only the energy changes. This is why changes of state are reversible:
SOLID ←—— melting/freezing ——→ LIQUID ←—— boiling/condensing ——→ GAS
(↑heat applied) (↑heat applied)
(↓heat removed) (↓heat removed)
Think of it like a classroom:
In a solid, students sit in assigned seats (fixed positions). In a liquid, students mill around but stay in the same room (fixed number, variable positions). In a gas, students leave the room and spread through the school (no limit to space they occupy). But the total number of students hasn't changed—they've just rearranged their energy and positions.
The Four Processes You Must Know
Melting: Solid → Liquid. Heat breaks apart the rigid structure; particles gain enough energy to move past each other but remain close together. Example: ice melts to water.
Freezing: Liquid → Solid. The reverse: particles lose energy and lock into fixed positions. Example: water freezes into ice.
Boiling (or Evaporating): Liquid → Gas. Particles gain enough energy to escape the liquid completely and move freely. Example: water boils to steam. Note: evaporating is the same process, just slower at the surface.
Condensing: Gas → Liquid. Particles lose energy and come back together as a liquid. Example: steam condenses to water droplets.
Examiner's Favourite Question: "Explain in terms of particles why a solid has a fixed shape but a gas does not." Your answer must mention particle positions, movement, and intermolecular forces. Avoid just saying "a gas is spread out"—explain *why*.
Practice Question
When ice melts to water, the mass stays the same but the volume decreases. Explain this using the particle model.