Changes of State and Internal EnergyAQA GCSE Physics: Revision notes
Section 1
What is internal energy and how does it relate to particle motion?
Internal energy is the total kinetic and potential energy of all the particles in a substance. It has two components:
- Kinetic energy: energy due to the random motion of particles. Hotter substances have particles moving faster, so higher kinetic energy.
- Potential energy: energy stored in the bonds between particles due to their relative positions.
When you heat a substance, you transfer energy to it. This energy increases the internal energy by making particles move faster (increasing kinetic energy) or by pushing particles further apart (increasing potential energy).
Examiners expect you to explain internal energy as having two parts. Always state that heating increases internal energy by increasing kinetic energy (particles move faster) or potential energy (particles move further apart).
Think of internal energy like a savings account: kinetic energy is like cash in your wallet (easy to spend/use), and potential energy is like money in bonds (locked away, but still part of your wealth).
Section 2
What happens to particles during melting and freezing?
Melting and freezing are reversible changes of state between solid and liquid.
During melting:
- Solid particles are held in fixed positions by strong forces
- Heat energy breaks some of these forces
- Particles gain kinetic energy and can move past each other
- The substance becomes a liquid
- Particles are still close together but no longer in a fixed arrangement
During freezing:
- Liquid particles are moving freely
- Heat energy is removed
- Particles slow down and lose kinetic energy
- Forces between particles become strong enough to hold them in fixed positions
- The substance becomes a solid
Both processes require energy transfer but temperature remains constant during the change of state.
Students often think that because temperature doesn't change during melting, no energy is being transferred. Wrong! Energy is being used to break the bonds holding particles in fixed positions—this is latent heat.
Ice at 0°C melts to water at 0°C. The temperature stays at 0°C, but energy is still needed to allow particles to move freely. Once all ice has melted, further heating increases temperature.
Section 3
What happens to particles during evaporation and condensation?
Evaporation and condensation are reversible changes of state between liquid and gas.
During evaporation:
- Liquid particles have enough energy to escape the surface
- Heat energy breaks almost all forces between particles
- Particles spread out and move far apart very quickly
- The substance becomes a gas
- Particles occupy much more space and move randomly at high speed
During condensation:
- Gas particles lose kinetic energy (cooling occurs)
- Particles slow down and move closer together
- Forces between particles become significant again
- The substance becomes a liquid
- Particles move less freely but remain in close contact
Like melting and freezing, temperature remains constant during evaporation and condensation, even though energy is being transferred. The energy goes into breaking or forming bonds, not increasing kinetic energy.
Remember that evaporation requires much more energy than melting because particles must overcome forces and separate widely. Examiners want you to explain that latent heat of vaporisation is larger than latent heat of fusion.
Evaporation is like breaking free from a crowd: at melting, people can shuffle around (liquid); at evaporation, people run away in all directions (gas).
Section 4
How does sublimation differ from other changes of state?
Sublimation is a change of state where a solid changes directly into a gas without becoming a liquid first.
Key features:
- The reverse process is called deposition (gas to solid)
- Sublimation requires significant energy input because particles must overcome strong solid bonds and separate widely like a gas
- Temperature remains constant during sublimation (like other changes of state)
- It occurs in specific substances under certain conditions
Common example: Dry ice (solid carbon dioxide) sublimates at room temperature and atmospheric pressure, turning directly into carbon dioxide gas without melting into a liquid.
Sublimation is less commonly tested than melting/freezing or evaporation/condensation, but you must be able to describe the process and recognise it as a valid change of state.
Dry ice sublimes because CO₂ molecules have weak forces in the solid state and can escape directly to gas form at room temperature. This is why dry ice 'disappears' without melting.
Section 5
What is specific latent heat and how do we calculate it?
Specific latent heat (L) is the energy required to change the state of 1 kilogram of a substance without changing its temperature.
The equation is:
Q = mL
Where:
- Q = energy transferred (joules, J)
- m = mass of substance (kilograms, kg)
- L = specific latent heat (joules per kilogram, J/kg)
Important distinction:
| Type | Meaning | When it applies |
|---|---|---|
| Latent heat of fusion (Lf) | Energy to melt a solid or freeze a liquid | Melting or freezing |
| Latent heat of vaporisation (Lv) | Energy to evaporate a liquid or condense a gas | Evaporation or condensation |
Latent heat of vaporisation is always larger than latent heat of fusion because evaporation requires breaking more bonds and separating particles over greater distances.
Worked example: How much energy is needed to evaporate 2 kg of water? (Latent heat of vaporisation of water = 2,260,000 J/kg)
- Identify: m = 2 kg, L = 2,260,000 J/kg
- Use Q = mL
- Q = 2 × 2,260,000 = 4,520,000 J = 4.52 MJ
Show all steps in Q = mL calculations: state the formula, substitute values with units, and state the final answer with units. Examiners award marks for method, not just the answer.
Students confuse which latent heat to use: fusion for melting/freezing, vaporisation for evaporation/condensation. Check the change of state in the question before choosing L.
Must Know
- Internal energy is the total kinetic and potential energy of particles. Heating increases internal energy by increasing kinetic energy (particles move faster) or potential energy (particles move apart).
- Temperature remains constant during any change of state (melting, freezing, evaporation, condensation, sublimation). Energy transferred during changes of state is called latent heat and breaks/forms bonds, not increase kinetic energy.
- Five changes of state: Melting (solid→liquid), Freezing (liquid→solid), Evaporation (liquid→gas), Condensation (gas→liquid), Sublimation (solid→gas directly).
- Latent heat of vaporisation is always larger than latent heat of fusion because evaporation requires particles to separate widely, not just move freely.
- Use Q = mL: Q is energy (J), m is mass (kg), L is specific latent heat (J/kg). Always identify whether to use latent heat of fusion or vaporisation based on the change of state described.
- Reverse processes exist: Freezing reverses melting, condensation reverses evaporation, deposition reverses sublimation, and they require the same amount of energy but in opposite directions.
That's the notes covered.
Carry on to the next subtopic.