IB Physics: Latent Heat & Changes of State — Every Question Answered

Latent heat is the energy absorbed or released when a substance changes state — solid, liquid or gas — without any change in temperature. In IB Physics it sits in Theme B.1, Thermal energy transfers, examinable at both SL and HL through the equation Q = mL, tested across Paper 1 and Paper 2.

Specific heat capacity (c) is the energy needed to raise 1 kg of a substance by 1 K without changing its state — used in Q = mcΔθ. Specific latent heat (L) is the energy needed to change the state of 1 kg of a substance at constant temperature — used in Q = mL. Different equations, different physical process entirely.

Because the energy you add during a phase change doesn't speed the particles up — it breaks or forms the intermolecular bonds holding them together. Kinetic energy, and therefore temperature, stays fixed until the entire sample has finished melting or boiling. Only then does further heating raise the temperature again.

Latent heat of fusion is the energy per kilogram needed to change a substance between solid and liquid (melting or freezing). Latent heat of vaporisation is the energy per kilogram needed to change between liquid and gas (boiling or condensing). Vaporisation values are almost always much larger, because gas molecules must fully separate.

The IB Physics data booklet gives Q = mL, where Q is thermal energy in joules, m is mass in kilograms, and L is specific latent heat in J kg⁻¹. Rearranged as L = Q/m, this appears in almost every latent-heat calculation question you'll meet across SL and HL.

Use Q = mL for the phase change itself, and Q = mcΔθ for any temperature change either side of it — then add the results together if the question spans both stages. Always check whether the substance starts below its melting or boiling point, since that adds an extra heating stage before the change of state begins.

Internal energy is the total kinetic and potential energy of a substance's particles. Adding latent heat raises potential energy — by breaking intermolecular bonds — while kinetic energy, and therefore temperature, stays fixed. Adding heat via specific heat capacity does the reverse: kinetic energy rises while potential energy stays roughly constant.

The three mistakes I mark every exam season: confusing L (specific latent heat) with c (specific heat capacity), forgetting to convert grams to kilograms before substituting, and treating a multi-stage problem as if one equation covers the whole thing. Each one costs an otherwise easy mark.

Not really — the specific latent heat content in Theme B.1, Thermal energy transfers, is identical for SL and HL. The difference shows up in question style: HL papers are more likely to embed latent heat inside a longer, multi-step scenario, sometimes combined with the gas laws content that's HL-only.

Latent heat can appear as a Paper 1 multiple-choice question, a short calculation worth roughly 2 to 4 marks within a longer Paper 2 structured question, or embedded inside a data-based scenario in Paper 3. It's rarely the sole focus of an extended-response question on its own.

Most students find thermal energy transfers, latent heat included, one of the more approachable topics in the DP Physics syllabus — the maths is simpler than mechanics or fields, and there are really only two core equations to master. Where marks get lost is sloppy unit conversion, not the underlying physics.

Effective revision combines two things: genuinely learning the two core equations, Q = mcΔθ and Q = mL, and practising past-paper style calculations that combine both in one problem — exactly how the IB likes to test this subtopic. Sketching a heating curve from memory is also a fast way to check real understanding rather than surface familiarity.

Work through a mixed set of calculation questions that combine specific heat capacity and latent heat in the same problem, since that's the format IB examiners favour — it tests whether you know which equation applies at each stage of a substance's heating. Timed practice under real exam conditions matters more than re-reading notes.