What is internal energy, and what happens to it during a change of state?
Internal energy and changes of state: internal energy as the total kinetic and potential energy of particles, how heating changes it, and why changes of state are physical changes.
A focused answer to AQA GCSE Physics 4.3.2, covering internal energy as the total kinetic and potential energy of particles, how heating raises temperature or causes a change of state, and why melting, boiling and the rest are physical changes.
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What this dot point is asking
AQA wants you to define internal energy, explain how heating a system changes the energy stored by its particles, and explain why changes of state are physical changes in which mass is conserved. This is part of topic 4.3.2 of the AQA GCSE Physics (8463) specification.
Internal energy
Heating a system
During a change of state the temperature does not change, even though energy is still being supplied, because the energy goes into breaking the forces between particles rather than speeding them up. This shows up clearly on a temperature-time graph (a heating curve): as a substance is heated steadily, the temperature rises during each single state, but flattens into a horizontal plateau during melting and during boiling. Each plateau corresponds to a change of state happening at a fixed temperature (the melting point or boiling point), while the energy supplied changes the particles' potential energy. Reading these flat sections from a graph is a skill AQA tests directly.
Changes of state
The changes of state are melting (solid to liquid), freezing (liquid to solid), boiling and evaporating (liquid to gas), and condensing (gas to liquid). Boiling and evaporating both turn a liquid into a gas, but they are not quite the same: boiling happens throughout the liquid at the boiling point, while evaporation happens only at the surface and can occur at any temperature below boiling, which is why a puddle dries up on a cool day. There is also sublimation, where a solid turns directly into a gas without passing through the liquid state, as solid carbon dioxide (dry ice) does.
Try this
Q1. Define the internal energy of a system. [2 marks]
- Cue. The total kinetic and potential energy of all the particles in the system.
Q2. Explain why the temperature stays constant while ice is melting. [2 marks]
- Cue. The supplied energy increases the potential energy of the particles (breaking forces between them) rather than their kinetic energy, so the temperature does not rise.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20204 marksA beaker of ice is heated steadily until it has all melted and the water has warmed up. Explain, in terms of the internal energy of the particles, what happens to the temperature during the melting and what happens after all the ice has melted.Show worked answer →
While the ice is melting, the temperature stays constant even though energy is being supplied (1 mark). This is because the supplied energy increases the potential energy of the particles, breaking the forces holding them in the solid structure, rather than increasing their kinetic energy (1 mark). Once all the ice has melted, the supplied energy now increases the kinetic energy of the particles, so they move faster and the temperature of the water rises (1 mark). Throughout, the internal energy of the system increases because heating always adds energy, but it is shared between potential and kinetic energy in different proportions before and after melting (1 mark). Markers reward the constant temperature during melting linked to potential energy, and the rising temperature afterwards linked to kinetic energy.
AQA 20213 marksExplain why a change of state, such as water boiling to form steam, is described as a physical change rather than a chemical change.Show worked answer →
A change of state is physical because the substance is still made of exactly the same particles before and after the change; no new substance is formed (1 mark). The change can be reversed (for example steam can condense back to water) to recover the original material, which is a feature of physical changes (1 mark). Mass is also conserved, so the mass of steam equals the mass of water it came from, and no atoms are created or destroyed (1 mark). Markers reward "same particles, no new substance", reversibility, and conservation of mass. A common error is to call boiling a chemical change because it looks dramatic.
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Sources & how we know this
- AQA GCSE Physics (8463) specification — AQA (2016)