Particle model of matter: density, states and gas pressure — Edexcel GCSE Physics revision

Density and states of matter

• Density = mass ÷ volume; solids are usually densest because particles are packed tightly with strong forces between them.
• In solids particles vibrate about fixed positions; in liquids they move around each other while staying close; in gases they move freely and quickly, far apart.
• Changes of state (melting, freezing, evaporating, condensing, sublimating) are physical changes — the substance can be recovered by reversing the change, and mass is conserved.

Internal energy and heating

• Internal energy is the total kinetic and potential energy of all the particles in a system.
• Heating a substance raises its temperature and increases the kinetic energy of particles, unless it is changing state.
• During a change of state, energy goes into breaking or forming bonds between particles rather than raising temperature, so temperature stays constant on a heating graph while it changes state.
• Specific heat capacity is the energy needed to raise 1 kg of a substance by 1°C: change in thermal energy = mass × specific heat capacity × change in temperature.
• Specific latent heat is the energy needed to change the state of 1 kg of a substance without changing its temperature: energy = mass × specific latent heat.

Key terms

Density

Mass per unit volume of a substance, in kg/m³.

Internal energy

The total kinetic and potential energy of particles in a substance.

Specific heat capacity

Energy needed to raise the temperature of 1 kg of a substance by 1°C.

Specific latent heat

Energy needed to change the state of 1 kg of a substance without a temperature change.

Latent heat of fusion

Specific latent heat for melting or freezing.

Latent heat of vaporisation

Specific latent heat for boiling or condensing.

Common exam traps

• Forgetting to convert cm³ to m³ (÷1,000,000) when calculating density in kg/m³.
• Thinking temperature rises during a change of state — it stays constant while energy changes bonds, not kinetic energy.
• Confusing specific heat capacity (temperature change) with specific latent heat (state change).

Practice questions with answers

1. 1. What is the equation for density?

   • • mass ÷ volume
   • • mass × volume
   • • volume ÷ mass
   • • mass + volume

   Answer: mass ÷ volume

   Density = mass ÷ volume, measured in kg/m³ or g/cm³.

2. 2. A block has a mass of 540 g and a volume of 200 cm³. Calculate its density in g/cm³.

   Answer: 2.7 g/cm³

   Density = mass ÷ volume = 540 ÷ 200 = 2.7 g/cm³.

3. 3. In which state are particles arranged with the most random motion and greatest separation?

   • • Solid
   • • Liquid
   • • Gas
   • • Plasma

   Answer: Gas

   Gas particles move rapidly and randomly with large spaces between them.

4. 4. What happens to temperature during a change of state?

   • • It rises steadily
   • • It stays constant
   • • It falls sharply
   • • It rises then falls

   Answer: It stays constant

   Energy supplied during a change of state breaks/forms bonds rather than increasing kinetic energy, so temperature is constant.

5. 5. Give the equation linking thermal energy change, mass, specific heat capacity and temperature change.

   Answer: ΔE = m × c × ΔT

   Change in thermal energy = mass × specific heat capacity × change in temperature.

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