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How does the particle model explain density, internal energy and changes of state?

The particle model and the states of matter, density and the equation for density, internal energy, specific heat capacity, and changes of state including specific latent heat.

A focused answer to Edexcel GCSE Combined Science Topic 14 (CP14), covering the particle model and the states of matter, the density equation, internal energy, specific heat capacity, and changes of state including specific latent heat.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The particle model and density
  3. Internal energy and specific heat capacity
  4. Changes of state and latent heat
  5. Try this

What this dot point is asking

Edexcel wants you to use the particle model to describe the states of matter, calculate density, explain internal energy and specific heat capacity, and explain changes of state including specific latent heat.

The particle model and density

Solids are usually the densest state because their particles are most tightly packed; gases are the least dense because their particles are far apart.

Internal energy and specific heat capacity

The specific heat capacity is the energy needed to raise the temperature of 1 kg1\,kg of a substance by 1 ∘C1\,^{\circ}C. A substance with a high specific heat capacity (such as water) needs a lot of energy to warm up.

Changes of state and latent heat

During a change of state (melting or boiling), the temperature stays constant even though energy is still being supplied. This is because the energy is used to break the forces holding the particles together, not to raise their kinetic energy. The specific latent heat is the energy needed to change the state of 1 kg1\,kg of a substance without a change in temperature. Changes of state are physical: the mass is conserved and the change can be reversed.

A heating curve (temperature against time as a substance is heated steadily) makes this clear. The temperature rises during heating of the solid, liquid or gas (the energy increasing the kinetic energy of the particles), but stays flat during melting and boiling (the energy breaking the forces between particles). The flat sections are where the latent heat is being supplied, and the sloping sections are where the specific heat capacity determines how fast the temperature rises.

To measure density you find the mass with a balance and the volume by a suitable method: for a regular solid you measure its dimensions and calculate the volume, while for an irregular solid you lower it into a measuring cylinder (or a displacement can) and measure the volume of water it pushes aside. Dividing the mass by the volume gives the density. This is the core practical for density, and questions often ask you to identify the best method for measuring the volume of a particular object.

Try this

Q1. State the equation for density. [1 mark]

  • Cue. Density = mass / volume.

Q2. Explain why the temperature stays constant while a substance melts. [2 marks]

  • Cue. The energy supplied breaks the forces between particles rather than raising their kinetic energy.

Exam-style practice questions

Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Edexcel 20193 marksA block of metal has a mass of 600 g600\,\text{g} and a volume of 80 cm380\,\text{cm}^3. Calculate its density, stating the equation you use and the unit of your answer.
Show worked answer →

A 3-mark density calculation.

Use density = mass / volume, ρ=mV\rho = \dfrac{m}{V} (1 mark). Substitute: ρ=60080\rho = \dfrac{600}{80} (1 mark). So ρ=7.5 g/cm3\rho = 7.5\,\text{g/cm}^3 (1 mark).

Markers reward stating and using the density equation, the substitution, and the correct value with a unit (g/cm3 or kg/m3 if converted). A common error is dividing volume by mass.

Edexcel 20214 marksExplain, using the particle model, why energy must be supplied to melt a solid even though the temperature does not change during melting.
Show worked answer →

A 4-mark question on specific latent heat and changes of state.

During melting the temperature stays constant because the energy supplied is not increasing the kinetic energy of the particles (so the temperature does not rise) (1 mark). Instead, the energy is used to break the forces (bonds) holding the particles in their fixed positions in the solid (2 marks). Once these forces are overcome, the particles can move past each other and the substance becomes a liquid (1 mark). This energy is the latent heat.

Markers reward the constant temperature, the energy going into breaking the forces between particles rather than raising kinetic energy, and the change to a liquid.

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