How the curves are drawn
Each invented sample is heated (or cooled) at a steady rate. A sloped section lasts m × c × ΔT ÷ (rate), a flat one m × L ÷ (rate), so the length of every section is set by the sample’s hidden specific heats and latent heats. Every corner lands on a gridline, so careful reading gives the exact key. Ice-to-steam questions use the real constants for water and are sketched, not to scale.
Example
In set HEATMF (high school), 25.0 g of substance X is heated. The flat section BC sits at −10 °C, so that is the melting point, and DE at 50 °C is the boiling point. Boiling runs from 12 kJ to 29 kJ on the heat axis, 17 kJ, so the heat of vaporization is 17,000 J ÷ 25.0 g = 680 J/g. The liquid (CD) takes 6 kJ to warm by 60 °C, so c = 6,000 ÷ (25.0 × 60) = 4.00 J/(g·°C). Open set HEATMF
Units at each level
| Level | Units | Phase changes |
|---|---|---|
| High school | g, J, J/(g·°C) | q = m ΔH, ΔH in J/g |
| AP Chem | g, kJ, J/(g·°C) | q = n ΔH, ΔH in kJ/mol |
| GCSE | kg, J, J/kg °C, time in minutes at a stated power | E = m L, L in J/kg |
| A-level | g, kJ, J g⁻¹ K⁻¹ | ΔH in kJ mol⁻¹ |
Also here: calorimetry practice (thermometers before and after, the cooling correction) and specific heat capacity practical questions (GCSE required practical 1).
Questions
How do you calculate the energy for each part of a heating curve?
Use q = m c ΔT for each sloped section, with the specific heat of the solid, liquid or gas, and q = m ΔH (or n ΔH with moles) for each flat section. Add the sections for the total. On a graph of heat added, you can read each section’s energy off the axis.
Why does the temperature stay the same while a substance melts or boils?
The energy goes into separating the particles, raising their potential energy, not their average kinetic energy. The temperature of a pure substance stays constant until the change of state is complete.
How do you find specific heat from a heating curve?
Pick one sloped section. Read the energy it took (from the heat axis, or power × time) and its temperature change, then c = q ÷ (m × ΔT).
Why is one sloped section steeper than another?
The steeper one belongs to the phase with the smaller specific heat: each joule raises its temperature more. For water, ice (2.09 J/(g·°C)) warms about twice as fast as liquid water (4.18).
Why is the boiling plateau longer than the melting plateau?
Turning a liquid into a gas pulls the particles fully apart, which takes much more energy than loosening them from a solid. For water, 40.67 kJ/mol against 6.01 kJ/mol.
What is the difference between specific heat capacity and specific latent heat?
Specific heat capacity is the energy to raise 1 kg (or 1 g) by 1 °C with no change of state. Specific latent heat is the energy to change the state of 1 kg with no change of temperature.
Sources, checked October 25, 2026: College Board, AP Chemistry Course and Exam Description (PDF updated June 2026), topics 6.3 to 6.6; OpenStax Chemistry 2e, 5.1 Energy Basics (Table 5.1), 5.2 Calorimetry and 10.3 Phase Transitions (Flowers et al., Rice University, 2019, CC BY 4.0; specific heats of water and the metals and the enthalpies of water used here); AQA GCSE Physics 8463 specification, version 1.1 (September 30, 2019); AQA GCSE Chemistry 8462 specification, version 1.1 (October 4, 2019); AQA AS and A-level Chemistry specification, version 1.1 (December 1, 2015), 3.1.4.