States of Matter Notes
Edexcel GCSE Chemistry: Revision notes
Key facts
- Solids: regular, vibrating, lowest energy. Liquids: random, close, sliding. Gases: random, far apart, fastest, highest energy.
- Changes of state are physical changes: no new substance forms.
- Temperature stays constant during a change of state.
- Compare a temperature with the melting and boiling points to predict the state.
Solid
Liquid
Gas
Solids, liquids and gases
The states differ in particle arrangement, movement and energy.
Going from solid to liquid to gas, particles gain energy, move more and become more spread out.
Solid
Liquid
Gas
| Solid | Liquid | Gas | |
|---|---|---|---|
| Arrangement | Regular, closely packed | Random, close together | Random, far apart |
| Movement | Vibrate about fixed positions | Slide around each other | Move rapidly in all directions |
| Relative energy | Lowest | Medium | Highest |
Solid
- Arrangement:
- Regular, closely packed
- Movement:
- Vibrate about fixed positions
- Relative energy:
- Lowest
Liquid
- Arrangement:
- Random, close together
- Movement:
- Slide around each other
- Relative energy:
- Medium
Gas
- Arrangement:
- Random, far apart
- Movement:
- Move rapidly in all directions
- Relative energy:
- Highest
Which state has particles that are far apart and move rapidly in all directions?
Changes of state
Melting, freezing, boiling, condensing, subliming and depositing are all physical changes.
All of these interconversions are physical changes: no new substance forms, and the change is generally reversible. Chemical reactions form new substances.
Solid
Liquid
Gas
Energy in
- Melting: solid to liquid
- Boiling or evaporating: liquid to gas
- Subliming: solid to gas
Energy out
- Freezing: liquid to solid
- Condensing: gas to liquid
- Depositing: gas to solid
Which change is called condensing?
Energy and changes of state
Energy overcomes or restores forces between particles, and the temperature stays constant during the change.
Melting: particles gain enough energy to overcome some of the forces holding them in fixed positions. Boiling: they fully overcome the forces of attraction and move apart. Condensing and freezing are the reverse: particles lose energy and move closer. Temperature stays constant while the state changes. Ice melts at 0°C and stays at 0°C until all of it has melted.
- 1
Solid
particles vibrate in fixed positions
- 2
Melting
particles gain energy and begin to move around each other
- 3
Liquid
particles close together but free to move
- 4
Boiling
forces of attraction fully overcome, particles move apart
- 5
Gas
particles far apart, moving rapidly
What happens to the temperature while ice is melting?
Predicting the state
Below the melting point it is a solid, between the melting and boiling points a liquid, and above the boiling point a gas.
Compare the specified temperature carefully against both the melting point and the boiling point before stating the state.
Worked example
A substance has a melting point of -39°C and a boiling point of 357°C. What state is it in at 25°C?
- 1
Compare 25°C with the melting point: 25 is above -39.
- 2
Compare 25°C with the boiling point: 25 is below 357.
- 3
So 25°C lies between the two.
A substance melts at 80°C and boils at 218°C. What state is it at 50°C?
Try an exam question
Describe what happens to the particles when a solid melts and then boils.
[4 marks]
- [1]Particles gain energy.
- [1]On melting they overcome some forces and begin to move around each other instead of only vibrating.
- [1]On boiling the forces of attraction are fully overcome.
- [1]The particles move apart and move rapidly and randomly.
That's the notes covered.
Carry on to the next subtopic.