States of MatterEdexcel GCSE Chemistry: Revision notes
Section 1
How are particles arranged and moving in solids, liquids and gases?
The three states of matter (solid, liquid, gas) differ in how their particles are arranged, how they move, and how much energy they have:
| State | Arrangement | Movement | Relative energy |
|---|---|---|---|
| Solid | Regular, closely packed | Vibrate about fixed positions | Lowest |
| Liquid | Randomly arranged, close together | Move around each other, sliding past neighbours | Medium |
| Gas | Randomly arranged, far apart | Move rapidly and randomly in all directions | Highest |
As you go from solid to liquid to gas, particles gain more energy, move more, and become more spread out (less ordered).
Always describe all three features together (arrangement, movement, energy) when asked to compare states — a mark scheme usually splits marks across these three aspects.
Section 2
What are the names for changes between states, and are they physical or chemical?
The interconversions between states have specific names:
| Change | Name |
|---|---|
| Solid → liquid | Melting |
| Liquid → solid | Freezing |
| Liquid → gas | Boiling / evaporating |
| Gas → liquid | Condensing |
| Solid → gas (directly) | Subliming |
| Gas → solid (directly) | Depositing |
All of these interconversions are physical changes, not chemical changes: no new substance is formed, and the change is generally reversible with the same mass of substance present at the start and end. This contrasts with chemical reactions, which form new substances and are generally not easily reversible.
State changes are always physical changes — a common error is describing melting or boiling as a 'chemical reaction'; no new substance is ever formed.
Section 3
How do arrangement, movement and energy change during interconversions?
When a substance is heated and changes state, its particles gain energy, which changes their arrangement and movement:
- Melting (solid → liquid): particles gain enough energy to overcome some of the forces holding them in fixed positions; they begin to move around each other rather than just vibrating, and the ordered arrangement breaks down
- Boiling/evaporating (liquid → gas): particles gain enough energy to completely overcome the forces of attraction between them; they move apart to become widely spaced and move rapidly and randomly
- Condensing (gas → liquid) and freezing (liquid → solid) are the reverse: as particles lose energy, their movement slows, forces of attraction pull them closer together, and the arrangement becomes more ordered
Throughout any state change, the energy change happens at a constant temperature (while the state is actually changing) — the added/removed energy overcomes or restores forces of attraction between particles rather than raising/lowering the temperature at that specific moment.
When ice melts at 0°C, energy input breaks down the ordered arrangement, but the temperature stays at 0°C until all the ice has melted.
Section 4
How do you predict the physical state of a substance from data?
Given a substance's melting point and boiling point, and a specified temperature, you can predict its state:
- If the given temperature is below the melting point → the substance is a solid
- If the given temperature is between the melting point and boiling point → the substance is a liquid
- If the given temperature is above the boiling point → the substance is a gas
Always compare the specified temperature carefully against both given values before stating the state.
A substance has a melting point of -39°C and boiling point of 357°C. At 25°C (room temperature), 25°C is above the melting point but below the boiling point, so the substance is a liquid at room temperature — this is mercury.
Must Know
- Solids: particles regular, closely packed, vibrate in place, lowest energy
- Liquids: particles random, close together, move around each other, medium energy
- Gases: particles random, far apart, move rapidly in all directions, highest energy
- Names of state changes: melting, freezing, boiling/evaporating, condensing, subliming, depositing — all are physical changes, no new substance is formed
- During a state change, particle arrangement and movement change as energy is gained/lost, while temperature stays constant during the actual change
- To predict state at a given temperature: compare the temperature to the melting point and boiling point (below MP = solid, between MP and BP = liquid, above BP = gas)
That's the notes covered.
Carry on to the next subtopic.