Heat of fusion (ΔH_fus) is the energy required to convert a substance from solid to liquid at its melting point, with the temperature staying constant the entire time. In AP Chem Topic 6.5, you use it in q = n × ΔH_fus to calculate the heat absorbed during melting or released during freezing.
Heat of fusion is the amount of energy you have to put into a solid to melt it at its melting point. The strange-sounding part is that the temperature does not change while this happens. All the energy goes into breaking up the rigid arrangement of particles, not into making them move faster. That's why a heating curve has a flat plateau at the melting point. Energy is flowing in, but the thermometer reading sits still until every bit of solid is gone.
On the AP exam this is a molar quantity, usually written ΔH_fus in kJ/mol, and the working equation is q = n × ΔH_fus (moles times molar enthalpy of fusion). The process is reversible in terms of energy. Melting absorbs exactly as much energy as freezing releases for the same amount of substance, which is essential knowledge 6.5.A.2 in disguise. Melting is endothermic (positive q for the system), freezing is exothermic (negative q).
Heat of fusion lives in Topic 6.5 (Phase Changes and Energy) in Unit 6: Thermochemistry. It directly supports learning objective 6.5.A, which asks you to explain the heat absorbed or released during a phase transition using moles and the molar enthalpy of that transition. It's also one half of every heating-curve calculation. Multi-step problems make you stack q = mcΔT segments (warming) with q = nΔH segments (phase changes), and the heat of fusion handles the solid-to-liquid plateau. If you skip it or accidentally apply mcΔT during melting, the whole calculation collapses. For the full heating-curve walkthrough, the Topic 6.5 study guide is the place to go.
Keep studying AP Chemistry Unit 6
Heat of Vaporization (Unit 6)
Same idea, different plateau. Heat of vaporization is the energy to go liquid to gas, and it's almost always much larger than the heat of fusion. Melting just loosens the particles' fixed positions, but vaporizing has to pull them completely apart, which costs far more energy. AP questions love asking you to compare the two.
Specific Heat Capacity (Unit 6)
These two split a heating curve between them. Specific heat (q = mcΔT) governs the sloped parts where temperature rises, and heat of fusion (q = nΔH_fus) governs the flat melting plateau where it doesn't. Knowing which equation applies to which segment is the whole game in heating-curve problems.
Enthalpy Change (Unit 6)
Heat of fusion is just a specific enthalpy change, the ΔH for the melting process. Its sign follows the same rules as any ΔH. Melting is endothermic so ΔH_fus is positive, and freezing is the reverse process with ΔH equal in magnitude but negative.
Melting Point and Intermolecular Forces (Units 3 and 6)
The strength of a substance's intermolecular forces sets both its melting point and its heat of fusion. Stronger attractions between particles mean more energy is needed to break the solid structure apart. This lets the exam tie particle-level reasoning from earlier units to the thermochemistry math in Unit 6.
Expect heat of fusion mostly in calculation and reasoning questions tied to heating curves and insulated-container scenarios. A classic setup gives you ice and liquid water, both at 0°C, adds the same energy to each, and asks what differs. The answer hinges on the fact that the ice spends that energy melting (temperature stays at 0°C) while the liquid warms up. For 50 g of ice, about 16.7 kJ is exactly what melting costs, since 50 g is roughly 2.78 mol and ΔH_fus of water is 6.01 kJ/mol. Other common stems compare two substances with different heats of fusion (the one with the larger ΔH_fus stays at its plateau longer at the same heating rate) or probe lab error analysis, like what happens to a measured heat of fusion if your ice actually started at -20°C instead of 0°C. No released FRQ has used the phrase verbatim, but multi-step q calculations that combine mcΔT and nΔH segments are standard FRQ territory, and you're expected to show each segment separately with correct signs.
Both are molar enthalpies of phase transitions, but heat of fusion covers solid-to-liquid and heat of vaporization covers liquid-to-gas. Vaporization is typically several times larger because boiling must completely overcome the intermolecular attractions holding particles near each other, while melting only disrupts the fixed solid arrangement. For water, ΔH_fus is 6.01 kJ/mol but ΔH_vap is 40.7 kJ/mol. If an exam question asks why the boiling plateau on a heating curve is longer than the melting plateau, this is the answer.
Heat of fusion (ΔH_fus) is the energy needed to melt a substance at its melting point, calculated with q = n × ΔH_fus using moles, not mass.
Temperature stays constant during melting because the absorbed energy breaks up the solid structure instead of increasing particle kinetic energy.
Melting is endothermic and freezing is exothermic, and the energy absorbed melting a sample exactly equals the energy released when it freezes.
Heat of vaporization is much larger than heat of fusion because vaporizing fully overcomes intermolecular forces while melting only loosens them.
In heating-curve problems, use q = mcΔT for the sloped warming segments and q = nΔH_fus for the flat melting plateau, then add the segments together.
Stronger intermolecular forces mean a higher heat of fusion, which connects Unit 6 thermochemistry back to particle-level reasoning.
It's the energy required to convert a substance from solid to liquid at its melting point, with no temperature change. On the exam you calculate it with q = n × ΔH_fus, where n is moles and ΔH_fus is the molar enthalpy of fusion (6.01 kJ/mol for water).
No. During melting, all the absorbed energy goes into breaking the attractions holding the solid together, so the temperature of a pure substance stays constant until the entire sample is liquid. This is why heating curves show a flat plateau at the melting point.
Heat of fusion is for melting (solid to liquid) and heat of vaporization is for boiling (liquid to gas). Vaporization is much larger, 40.7 kJ/mol versus 6.01 kJ/mol for water, because turning a liquid into a gas means completely separating the particles rather than just loosening their fixed positions.
No. Specific heat capacity tells you the energy to raise temperature within one phase (q = mcΔT), while heat of fusion tells you the energy to change phase at constant temperature (q = nΔH_fus). Heating-curve problems require you to use both, one for each type of segment.
Melting is endothermic, so ΔH_fus is positive and the system absorbs energy. Freezing is the reverse, releasing exactly the same amount of energy per mole, so its ΔH is equal in magnitude but negative.
Connect this key term to the AP exam workflow: review the course, practice questions, and check related study tools.
Review units, study guides, and course resources.
Check this vocabulary in multiple-choice context.
Apply key concepts in written AP responses.
Estimate the exam score you are working toward.
Review the highest-yield facts before practice.
Put the full course together before test day.