Changes of state and specific latent heat — AQA GCSE Physics
Test yourself on Changes of state and specific latent heat with AQA GCSE practice questions.
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Changes of state and specific latent heat explained
Internal energy is the total kinetic energy plus the total potential energy of the particles.
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During melting or boiling the energy supplied does not speed the particles up; it pulls them apart against the forces holding them together, so the potential energy store rises while the kinetic energy store, and therefore the temperature, stays the same. That is why a heating graph shows a flat section at the melting point and another at the boiling point even though the heater is still running. Internal energy is increasing the whole time. Reverse the process and the same energy is given out: condensing steam releases a great deal of energy at a constant 100 °C, which is why steam scalds far more severely than boiling water.
The specific latent heat of a substance is the amount of energy required to change the state of one kilogram of the substance with no change in temperature.
Specific latent heat is the energy needed per kilogram to change state, measured while the temperature stays put, so its unit is J/kg with no degrees in it. Two versions are named. Specific latent heat of fusion covers solid to liquid, and for water it is about 334 000 J/kg. Specific latent heat of vaporisation covers liquid to vapour, and for water it is about 2 260 000 J/kg. Vaporisation is far larger because melting only loosens the particles enough to let them slide past one another, while boiling has to separate them completely against all the remaining forces. The same energy per kilogram is released when the change runs the other way, during freezing or condensing.
energy f or a change o f state = mass × speci f ic latent heat E = m L energy, E, in joules, J mass, m, in kilograms, kg specific latent heat, L, in joules per kilogram, J/kg Specific latent heat of fusion – change of state from solid to liquid Specific latent heat of vaporisation – change of state from liquid to vapour Students should be able to interpret heating and cooling graphs that include changes of state.
E = m L has no temperature term, as temperature is constant during a change of state. Boiling 0.25 kg of water needs E = 0.25 × 2 260 000 = 565 000 J. On heating and cooling graphs, sloping parts belong to ΔE = m c Δθ and flat plateaus belong to E = m L. Heating ice at -10 °C to steam at 100 °C involves four stages: warming ice, melting, warming water, and boiling. Cooling graphs show the reverse, with plateaus for condensing and freezing. Two standard practical errors exist. If thermal energy escapes to the surroundings, less energy reaches the water, making the calculated L too high. If water evaporates before boiling, the measured mass change is too large for the energy supplied, making L too low.
Students should be able to distinguish between specific heat capacity and specific latent heat.
The two quantities answer different questions. Specific heat capacity is the energy per kilogram to change the temperature by one degree with no change of state, unit J/kg °C, and it governs the sloping parts of a heating graph. Specific latent heat is the energy per kilogram to change state with no change of temperature, unit J/kg, and it governs the flat parts. Read the question for the giveaway words: warms, cools or rises from one temperature to another means c; melts, freezes, boils, condenses or evaporates means L. A problem that says ice at 0 °C is turned into water at 20 °C needs both, E = m L first at the plateau, then ΔE = m c Δθ up the slope.
Your focus
- Explain, in terms of potential and kinetic energy of particles, why the temperature stays constant while a substance boils.
- Identify the melting point and boiling point from the flat sections of a heating graph and state what is happening to the particles in each.
- Explain why steam at 100 °C causes a worse scald than water at 100 °C.
Show all 12 objectives
- Determine the specific latent heat of fusion of a substance from the energy supplied and the mass melted, and give the unit of the answer.
- Distinguish specific latent heat of fusion from specific latent heat of vaporisation by naming the change of state each describes.
- Explain why the specific latent heat of vaporisation of water is about seven times the specific latent heat of fusion.
- Calculate the energy needed to melt or boil a stated mass of a substance using E = m L.
- Interpret heating and cooling graphs by naming which equation applies to each sloping and flat section, then calculate the total energy supplied or removed.
- Explain how energy lost to the surroundings and evaporation before boiling each shift the experimental value of L, stating the direction of each shift.
- Decide from the wording of a question whether ΔE = m c Δθ or E = m L is required, and justify the choice.
- Compare specific heat capacity and specific latent heat in terms of what changes, what stays constant, and the units of each.
- Calculate the total energy needed to take ice at 0 °C to water at 20 °C by combining both equations in the correct order.
Changes of state and specific latent heat exam tips
Marking Points
- one mark for stating that the potential energy of the particles increases as the forces between them are overcome
- one mark for stating that the kinetic energy of the particles does not change, because the temperature does not change
- one mark for concluding that the internal energy of the substance increases
- one mark for identifying a flat section of a heating or cooling graph as a change of state
- one mark for the energy required to change the state of one kilogram of the substance
- one mark for stating that there is no change in temperature during the change of state
- one mark for naming fusion as the solid to liquid change and vaporisation as the liquid to vapour change
- one mark for the unit J/kg
- one mark for choosing E = m L for a change of state and ΔE = m c Δθ for a temperature change
- one mark for correct substitution with the mass that actually changed state, in kilograms
- one mark for the answer in joules
- one mark for stating that thermal energy transferred to the surroundings makes the experimental value of L too high
- one mark for stating that water evaporating before boiling makes the measured mass change too large, so the value of L is too low
- one mark for specific heat capacity involving a temperature change with no change of state
- one mark for specific latent heat involving a change of state with no temperature change
- one mark for contrasting the units, J/kg °C against J/kg
- one mark for matching each quantity to the sloping or flat section of a heating graph
Examiner Tips
- 💡Split any internal energy answer into two sentences, one about potential energy and one about kinetic energy, then state the effect on internal energy.
- 💡If a graph question mentions a plateau, name the change of state and the temperature at which it happens.
- 💡Do not say heat when you mean internal energy; examiners expect the phrase internal energy in these answers.
- 💡Write the unit as J/kg; the absence of °C is the quickest reminder that no temperature change is involved.
- 💡State which change of state you mean, because fusion and vaporisation have very different values for the same substance.
- 💡If a question gives an unfamiliar substance, quote the definition in general terms rather than water's numbers.
- 💡Label each section of a heating or cooling graph with the equation you will use before you calculate anything.
- 💡When asked about an error, say whether the value of L is too high or too low, and give the reason in the same sentence.
- 💡Check the mass: it is the mass that changed state, often a mass loss read from a balance, not the starting mass.
- 💡Underline the verb in the question: melts and boils point to L, warms and cools point to c.
- 💡State both differences when asked to distinguish them, what changes and what stays the same, not just the units.
- 💡In a multi-stage calculation, set the work out as separate labelled stages so an examiner can award each part.
Common Mistakes
- saying the temperature keeps rising while a liquid boils
- saying the energy supplied during a change of state is lost, wasted or destroyed
- claiming the kinetic energy of the particles increases during melting
- reading the flat part of a heating graph as a period when no energy is being supplied
- leaving out the one kilogram, so the definition could apply to any amount of substance
- omitting the condition that the temperature does not change
- swapping fusion and vaporisation, so quoting 334 000 J/kg for boiling water
- reading fusion as the joining of nuclei rather than as melting
- multiplying by a temperature change in E = m L, which has no temperature term
- reading the melting point from a sloping section of the graph instead of the plateau
- using the total mass of the sample when only part of it changed state
- adding the two stages of a graph question together but using c for both, or L for both
- using ΔE = m c Δθ across the plateau of a heating graph, where the temperature is constant
- inserting a temperature change into E = m L
- quoting J/kg °C as the unit of specific latent heat
- assuming a substance sitting at its melting point is receiving no energy because the thermometer reading is steady