Why does a heating curve have slopes and plateaus?
You will be able to: Interpret temperature versus supplied heat using sensible heating and phase transitions.
Why does a heating curve have slopes and plateaus?
A heater first warms ice, then melts it, then warms the resulting water. A single straight line cannot represent all three jobs because some energy raises temperature and some changes phase.
A useful starting point: Where does heat go while a substance melts? →
Words and symbols before equations
- Heating curve
- Temperature plotted against cumulative supplied heat, or against time under stated power conditions.
- Plateau
- A flat region where energy changes phase without raising temperature in the ideal model.
- Sensible heating
- Energy transfer that changes temperature within one phase.
- Cumulative heat
- Total energy supplied since the defined starting point.
What this picture assumes
One mole (18.0 g) water, −20 °C ice to 120 °C steam, approximately 1 atm. Supplied rounded c_ice=2.00, c_liquid=4.00, c_steam=2.00 J/(g·K); ΔHfus=6.00 and ΔHvap=40.0 kJ/mol at transition temperatures. Ideal equilibrium segments, no losses; x axis is heat, not time.
Read the picture in three steps
- Read the species and labels first. Identify what each symbol and line represents. Read the units and fixed conditions before comparing quantities.
- At q=8 kJ: T=17.778 °C; liquid water. This segment changes temperature within one phase. Plateaus absorb energy without a temperature rise.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
In a single-phase segment, q=mcΔT gives a sloped T-versus-q line with slope 1/(mc). A higher heat capacity makes that line less steep.
During a transition, q=nΔH changes phase fraction while T stays fixed. Plateau width on a heat axis equals the energy required for that amount.
A time axis only has the same shape if net heater power is constant and losses negligible. Label the horizontal variable rather than calling every graph a time graph.
The model follows one mole of water from −20 °C to steam at 120 °C at approximately 1 atm, using supplied rounded constants. Each stage keeps its own heat capacity; it is not a molecular-motion trajectory.
| Feature | Single-phase warming | Ideal phase transition |
|---|---|---|
| Temperature | Changes | Constant during coexistence |
| Relation | q=mcΔT | q=nΔHtransition |
| Energy role | Temperature response | Phase/interparticle change |
A worked example, step by step
A pure sample needs 2.0 kJ to warm to its melting point and 6.0 kJ more to melt fully. Locate the melting plateau on a cumulative-heat axis and identify its width.
- Choose cumulative heat zero at the cold starting state.
- Warming ends at q=2.0 kJ.
- Melting occupies q=2.0 through 8.0 kJ at constant temperature.
- Its 6.0 kJ width measures transition heat; it is not a 6.0-degree temperature rise.
A flat temperature line does not mean the heater stopped transferring energy.
Would doubling mass leave plateau widths unchanged?
Compare with an explanation
No. At the same conditions twice the amount needs twice the phase-transition energy.
Predict. Change one thing. Explain.
Move cumulative heat through every segment. Predict the phase(s), temperature trend and energy calculation needed at each point.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
At q=8 kJ: T=17.778 °C; liquid water. This segment changes temperature within one phase. Plateaus absorb energy without a temperature rise.
One mole (18.0 g) water, −20 °C ice to 120 °C steam, approximately 1 atm. Supplied rounded c_ice=2.00, c_liquid=4.00, c_steam=2.00 J/(g·K); ΔHfus=6.00 and ΔHvap=40.0 kJ/mol at transition temperatures. Ideal equilibrium segments, no losses; x axis is heat, not time.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using heat-flow signs, energy conservation, phase changes, bond inventories or the stated thermochemical path. Identify what the representation cannot tell you.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionDescribe a T-versus-q curve for ice warming to 0 °C, melting, then liquid warming. State the calculation for each region and one assumption needed for the melting plateau.
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Compare with the answer and four-point rubric
- 1 point: Ice warming is a slope, using q=mc_iceΔT.
- 1 point: Melting is a plateau, using q=nΔHfus.
- 1 point: Liquid warming is a new slope, using q=mc_liquidΔT.
- 1 point: A pure substance at fixed pressure in equilibrium phase coexistence supports the ideal plateau.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1What does a plateau’s width represent?
Heat needed for the transition in the shown sample.
RECALL 2Why can slopes differ?
Different phases have different heat capacities.
RECALL 3When can time replace heat on the axis?
With constant net heater power and negligible losses, with appropriate labeling.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Why does a heating curve have slopes and plateaus?
- Single phase: slope of T vs q = 1/(mc).
- Plateau width = nΔH_transition; labels must identify the horizontal variable.
Remember: A flat temperature line does not mean the heater stopped transferring energy.
Conditions: One mole (18.0 g) water, −20 °C ice to 120 °C steam, approximately 1 atm. Supplied rounded c_ice=2.00, c_liquid=4.00, c_steam=2.00 J/(g·K); ΔHfus=6.00 and ΔHvap=40.0 kJ/mol at transition temperatures. Ideal equilibrium segments, no losses; x axis is heat, not time.
Refresh Kid · AP Chemistry Unit 6 · Objectives 6.5.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 6.5, objective 6.5.A. CED effective Fall 2024 and June 2026 clarifications checked September 16, 2026. Unit 6: Thermochemistry, Topics 6.1–6.9. Focused lesson names, examples, models and assessments are original Refresh Kid teaching materials, not additional official topics or official AP questions. Official corrections.
The model states its assumptions beside the diagram. Technical enthalpy/internal-energy distinctions and formal state-function terminology are not assessed in the current AP framework. Constant-pressure heat, conservation, phase-specific capacities, reaction amounts and Hess sums are taught here with explicit conditions. Supplied rounded data and original molecular geometry are teaching models, not experimental measurements. A phase transition preserves molecular identity; a bond-energy accounting path is not an actual reaction mechanism.
Teaching resources: The Organic Chemistry Tutor video titles/descriptions and topic coverage were checked for optional links; no claim is made to have watched every video. No creator scripts, examples, worksheets or artwork were copied. GitHub’s 3D website collection and its Three.js camera-control example informed the idea of controllable spatial inspection. Scientific diagrams, geometry and interactions here are original. The self-hosted Three.js runtime retains its MIT license. Camera rotation changes the view, not the chemistry.
Independent teacher review and observation of students remain pending. Implementation checks do not certify scientific accuracy, accessibility or learning effectiveness. This is a review edition.
Optional official resource: Released AP Chemistry questions and scoring guides. This archive contains questions across units; it is not an assignment of every question to this lesson.
The teaching sequence is informed by the IES learning guide; this exact implementation has not been evaluated with learners.
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