GCSE Physics Particle Model of Matter Notes
The particle model explains solids, liquids and gases by how their particles are arranged and how much energy they have. It underpins density and heating calculations.
Key concepts
- Three states
- Solid: particles packed in a regular pattern, vibrating. Liquid: close but able to move past each other. Gas: far apart, moving quickly and randomly.
- Density
- The mass per unit volume of a substance, in kg/m³. Solids are usually densest and gases least dense.
- Internal energy
- The total kinetic and potential energy of all the particles in a substance. Heating increases internal energy.
- Changes of state are physical changes — mass is conserved and the change can be reversed.
- During a change of state the temperature stays constant while bonds are broken or formed.
- Gases have the lowest density because their particles are far apart.
Key formulas
Density
ρ = m ÷ V
density (kg/m³) = mass (kg) ÷ volume (m³).
Change in thermal energy
ΔE = m × c × Δθ
mass × specific heat capacity × temperature change.
Required practicals
- Determine the density of regular and irregular solids and of liquids.
- Investigate the specific heat capacity of a material.
Worked example
A block has a mass of 600 g and a volume of 200 cm³. Find its density in g/cm³.
- 1Write the equation: ρ = m ÷ V.
- 2Substitute: ρ = 600 ÷ 200.
- 3Divide: ρ = 3.
- 4Add the unit: ρ = 3 g/cm³.
Answer: ρ = 3 g/cm³
Common mistakes
✗ Thinking mass changes during a change of state.
✓ Mass is conserved — only the arrangement and energy of particles change.
✗ Mixing up mass and volume in the density equation.
✓ Density = mass ÷ volume; keep the units consistent.
✗ Expecting temperature to rise during melting or boiling.
✓ Temperature stays constant during a change of state as energy breaks bonds.
Quick check
Try these, then reveal the answers.
In which state are particles arranged in a regular pattern?Reveal answer
Solid.
A liquid has mass 500 g and volume 250 cm³. Find its density.Reveal answer
ρ = 500 ÷ 250 = 2 g/cm³
What happens to temperature during a change of state?Reveal answer
It stays constant while bonds are broken or formed.
Exam tips
- Keep units consistent — don't mix grams with cubic metres.
- For irregular solids, find volume by displacement in a measuring cylinder.
- Explain state changes in terms of particle arrangement and energy.
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Frequently asked questions
Why is a gas less dense than a solid?+
In a gas the particles are spread far apart, so the same mass occupies a much larger volume, giving a lower density.
Why does temperature stay constant during boiling?+
The energy supplied is used to break the bonds between particles rather than to increase their kinetic energy, so the temperature does not rise until the change of state is complete.
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