Latent heat: Q = mL, constant temp
Specific latent heat () is the energy to change the state of 1 kg of a substance without a temperature change. : = energy (J), = mass (kg), = specific latent heat (J/kg). Two types: fusion (solid ↔ liquid) and vaporisation (liquid ↔ gas). Water: J/kg, J/kg. always — separating particles into a gas takes far more energy than freeing them into a liquid.
Constant temperature during a phase change
At the melting or boiling point, the energy supplied breaks intermolecular bonds rather than raising the particles' average kinetic energy — so the temperature stays constant while energy flows in. On a temperature–time graph this is a horizontal plateau (zero gradient). Below the melting point temperature rises (kinetic energy up); at the melting point it is flat (fusion); it rises again in the liquid; flat at the boiling point (vaporisation); then rises as a gas.
Vaporisation needs more energy than fusion
Melting only frees particles from fixed lattice positions — they stay close and attracted, so relatively little energy is needed. Vaporising must completely overcome all remaining attractions and move particles far apart, needing much more energy per kg (~7× more for water: vs J/kg). This is why sweating cools so well — a small mass of evaporating sweat removes a large amount of thermal energy from the skin.
Drawn from real examiner reports.
Heating curve drawn with no plateau
When sketching a temperature–time (heating) curve, the horizontal plateau at the melting or boiling point is essential. Many candidates draw a smooth rise from start to finish with no flat section, implying temperature keeps rising during the change of state — wrong, and it loses the plateau mark.
June 2023 Paper 2P Q2(b)(iii): the most common misunderstanding was to omit the flat plateau, drawing a smooth uninterrupted rise from start to finish; the plateau was the differentiating mark.
Vague definition; boiling vs evaporation
"The heat needed to melt something" scores zero — the definition needs per 1 kg AND at constant temperature. Also distinguish boiling (throughout the liquid at the boiling point) from evaporation (surface only, any temperature) — June 2024 Paper 2P Q8(c)(ii) gave zero to answers confusing them.
June 2024 Paper 2P Q8(c)(ii): candidates who confused the definitions of boiling and evaporation scored zero; the examiner advised learning definitions from the specification.
Leaving time in minutes for Q = Pt
When the energy comes from a heater, needs in seconds. A 500 W heater for 3 minutes supplies J, not . Using minutes makes (and the derived ) 60 times too small. June 2024 Paper 2P Q8(b)(ii): the minutes→seconds omission was the only common error.
June 2024 Paper 2P Q8(b)(ii): "the only common error was to omit the conversion from minutes into seconds," a recurring trap across sessions.
Using mcΔT for a change of state
Do not use for a change of state — during melting or boiling , so it wrongly gives zero. A phase change uses . In a two-stage problem (warm, then melt) use for the sloped part and for the flat part, then add them.
Using the wrong latent heat (fusion vs vap.)
Use the right latent heat: fusion for melting/freezing, vaporisation for boiling/condensing. For water, mixing them up (using 334,000 for a boiling problem) makes the answer about 7× wrong. Read the state change in the question before choosing .
(general exam technique)
Vague particle descriptions
Be precise about particles: a solid's particles are "fixed in regular positions", not just "in a structure". June 2023 Paper 2P Q2(a) gave no credit for vague phrasing. When explaining a state change, name what happens to the bonds and the arrangement, not just that "particles move".
June 2023 Paper 2P Q2(a): candidates who described solid particles as "in a structure" rather than "fixed in regular positions" did not score — particle-description precision was emphasised.
Draw both plateaux, labelled and horizontal
On a heating-curve sketch: label axes ("time / s", "temperature / °C"), add a temperature scale, draw both plateaux (melting and boiling) exactly horizontal, and do not start with a flat section (that implies a fictitious earlier phase change).
Split two-stage energy problems
For "total energy" problems spanning a temperature change and a state change, split into stages: for each sloped part and for each flat part, then add. Label each stage so you use the right formula and earn the method marks separately.
Convert units and sense-check L
Before substituting, convert time to seconds and mass to kg, and check whether energy comes from . Sense-check : water's , J/kg — a value 60× too small usually means minutes were used.
Define latent heat with both elements
State the latent heat definition with both elements: "the energy to change the state of 1 kg without a temperature change, unit J/kg", and name the type (fusion or vaporisation) if asked. Dropping "per 1 kg" or "constant temperature" costs a mark.
| Quantity | Formula | Symbol definitions | Unit |
|---|---|---|---|
| Energy for phase change | = energy (J), = mass (kg), = specific latent heat (J/kg) | J | |
| Specific latent heat | rearrangement of above | J/kg | |
| Energy from heater | = power (W), = time (seconds, not minutes) | J |
Water values to memorise:
| Type | Value |
|---|---|
| Specific latent heat of fusion (ice water) | |
| Specific latent heat of vaporisation (water steam) |
Note: always — vaporisation needs far more energy than melting.
Define specific latent heat.
A student places 0.40 kg of ice in a beaker. The ice is at its melting point. The specific latent heat of fusion of water is 334 000 J/kg.
Calculate the energy needed to melt all the ice completely. State the formula you use.
