Density and States of MatterAQA GCSE Physics: Revision notes
Section 1
Defining and Calculating Density
Density is defined as mass per unit volume: ρ = m/V (density in kg/m³, mass in kg, volume in m³). When a substance changes state, its mass is conserved (stays the same), but its volume usually changes, so its density changes too.
A 2 kg block with volume 0.001 m³ has density ρ = 2 / 0.001 = 2000 kg/m³.
Section 2
The Particle Model and States of Matter
The particle model explains the different states of matter (solid, liquid, gas) in terms of how atoms or molecules are arranged and how they move:
| State | Arrangement | Movement | Relative density |
|---|---|---|---|
| Solid | Closely packed, regular pattern | Vibrate in place | Highest |
| Liquid | Closely packed, random arrangement | Move around each other | Middle |
| Gas | Far apart, random arrangement | Move quickly in all directions | Lowest |
You should be able to recognise and draw simple particle diagrams to model the difference between solids, liquids and gases.
Section 3
Changes of State
When a substance changes state - melting, freezing, boiling, evaporating, condensing, or subliming - its mass is conserved: no matter is created or destroyed. Changes of state are physical changes: the substance recovers its original properties if the change is reversed (e.g. melted ice can be refrozen back into ice with the same properties).
Don't describe a change of state as a chemical change - no new substance is formed, so it is always physical and reversible.
Must Know
- Density: ρ = m/V (kg/m³)
- Solids have particles closely packed in a regular pattern, vibrating in place
- Liquids have particles closely packed but randomly arranged, able to move past each other
- Gases have particles far apart, moving quickly and randomly
- Changes of state conserve mass and are physical changes - the substance can be recovered with its original properties
That's the notes covered.
Carry on to the next subtopic.