Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Linked equation: ρ = m/V
Notes
The three states of matter
| Solid | Liquid | Gas | |
|---|---|---|---|
| Arrangement | Regular pattern | Random | Random |
| Spacing | Very close together | Close together | Far apart |
| Movement | Vibrate about fixed positions | Move around each other | Move quickly in all directions |
| Density | High | High | Low |
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- The particle model explains the different densities of solids, liquids and gases: gases have a low density because their particles are far apart.
- Changes of state are physical changes: the material can change back and recover its original properties.
- When a substance changes state, its mass is conserved (the number of particles stays the same).
- Melting (solid → liquid), freezing (liquid → solid), boiling or evaporating (liquid → gas), condensing (gas → liquid), sublimating (solid → gas).
Limitations of the model
The simple model shows particles as solid spheres with no forces between them. Real particles aren’t hard spheres, and there are forces between them, which is why energy is needed to change state.
Questions
Q1 (F) Describe the arrangement and movement of particles in a solid.
Show answer
Very close together in a regular pattern. They vibrate about fixed positions.
Q2 (F) Name the change of state from gas to liquid.
Show answer
Condensing (condensation).
Q3 (F) Use the particle model to explain why a gas has a much lower density than a solid.
Show answer
The particles in a gas are much further apart, so there is less mass in the same volume.
Q4 (F) 200 g of ice melts. What is the mass of the water produced? Explain.
Show answer
200 g. Mass is conserved during a change of state because the number of particles doesn’t change.
Q5 (F/H) Why is a change of state a physical change rather than a chemical change?
Show answer
No new substance is made, and the change can be reversed so the material recovers its original properties.
Q6 (F/H) A 50 cm³ block of wax has a mass of 45 g. Calculate its density in g/cm³.
Show answer
F: ρ = m ÷ V
I: ρ = 45 ÷ 50
A: 0.9 g/cm³
Q7 (H) Give one limitation of the simple particle model.
Show answer
It shows particles as solid spheres with no forces between them. Real particles have forces between them and aren’t hard spheres.
