AP Chemistry Heating Curve
Why temperature plateaus during phase changes, with a labeled diagram and a complete worked example.
A heating curve plots temperature against heat added as a substance moves from solid to liquid to gas. It's not a straight line, it alternates between sloped segments (temperature rising) and flat plateaus (temperature holding steady during a phase change). Here's why that pattern happens, a labeled diagram, and a full calculation from ice to steam.
The Heating Curve, Labeled
Three segments slope upward (heating solid ice, heating liquid water, heating steam) and two segments are flat plateaus (melting at 0°C, boiling at 100°C). The boiling plateau is drawn wider than the melting plateau because vaporization takes far more energy than fusion, for the same reason explained below.
Why Temperature Plateaus During a Phase Change
Temperature measures average kinetic energy, how fast particles are moving. During melting or boiling, heat energy goes entirely into breaking intermolecular attractions and reorganizing how particles are arranged, not into speeding them up. No kinetic energy increase means no temperature increase, even though heat keeps flowing in the entire time. Once the phase change finishes and every particle has made the transition, additional heat goes back to raising kinetic energy, and temperature starts climbing again.
Two Formulas, Depending on Which Part of the Curve You're On
- On a sloped segment (temperature changing): q = mcΔT, using the specific heat of whichever phase you're in, ice, liquid water, or steam each have a different specific heat.
- On a flat plateau (phase changing): q = nΔHfus for melting/freezing, or q = nΔHvap for boiling/condensing.
Full Worked Example: Ice to Steam
How much heat is required to convert 10.0 g of ice at −10.0°C into steam at 110.0°C? Given: cice = 2.09 J/(g·°C), cwater = 4.18 J/(g·°C), csteam = 2.01 J/(g·°C), heat of fusion = 334 J/g, heat of vaporization = 2260 J/g.
Five segments, calculated separately and added together:
- Heat ice from −10.0°C to 0°C: q = (10.0 g)(2.09 J/g·°C)(10.0°C) = 209 J
- Melt ice at 0°C: q = (10.0 g)(334 J/g) = 3340 J
- Heat liquid water from 0°C to 100°C: q = (10.0 g)(4.18 J/g·°C)(100.0°C) = 4180 J
- Boil water at 100°C: q = (10.0 g)(2260 J/g) = 22,600 J
- Heat steam from 100°C to 110.0°C: q = (10.0 g)(2.01 J/g·°C)(10.0°C) = 201 J
Total: 209 + 3340 + 4180 + 22,600 + 201 = 30,530 J ≈ 30.5 kJ. Notice that boiling (step 4) uses more energy by itself than every other step combined, exactly why the boiling plateau is drawn wider on the diagram above.
Common Mistakes
- Using q = mcΔT across a plateau. Temperature isn't changing there, so ΔT = 0 and that formula gives zero, use q = nΔH instead.
- Using the wrong specific heat for the wrong phase. Ice, liquid water, and steam each have a different specific heat, don't reuse liquid water's 4.18 J/(g·°C) for the ice or steam segments.
- Forgetting to add up every segment. The total heat is the sum of all five steps, not just the phase-change steps or just the temperature-change steps.
- Mixing up heat of fusion and heat of vaporization. Fusion applies at the melting/freezing plateau; vaporization applies at the boiling/condensing plateau, and vaporization is always the larger number.
Related Resources
- Unit 6 Review: Thermochemistry
- AP Chemistry Limiting Reactant & Percent Yield
- How to Solve ICE Table Equilibrium Problems
- AP Chemistry Significant Figures Rules
- AP Chemistry Reference Sheet
- AP Chemistry Study Guide
Frequently Asked Questions
Why does temperature stay flat during a phase change on a heating curve?
Because the heat energy is going into breaking (or forming) intermolecular attractions instead of increasing average kinetic energy. Temperature is a measure of average kinetic energy, so as long as energy is being spent reorganizing the particles' arrangement rather than speeding them up, temperature doesn't change, even though heat keeps flowing in the whole time.
What is a cooling curve?
A cooling curve is the mirror image of a heating curve, temperature plotted as heat is removed instead of added. It has the same five segments in reverse: cooling gas, condensation plateau, cooling liquid, freezing plateau, cooling solid. The plateau temperatures (boiling point and melting point) are identical on both curves for the same substance.
What's the difference between heat of fusion and heat of vaporization?
Heat of fusion is the energy needed to melt a solid into a liquid at its melting point (or released when it freezes). Heat of vaporization is the energy needed to boil a liquid into a gas at its boiling point (or released when it condenses). Vaporization always takes more energy than fusion for the same substance, because it requires fully separating particles rather than just letting them move past each other.
Do you need to memorize water's heat of fusion and vaporization values for AP Chemistry?
No. Values like specific heat, heat of fusion, and heat of vaporization for any substance other than liquid water's specific heat (4.18 J/(g·°C)) are given in the problem itself. What you need to memorize is the method, which formula applies on a slope versus a plateau, not the numeric constants.
Why is the boiling plateau longer than the melting plateau on water's heating curve?
Because water's heat of vaporization (about 2260 J/g) is roughly seven times larger than its heat of fusion (about 334 J/g). Boiling requires fully separating water molecules into a gas, a much bigger jump in intermolecular distance than melting, which only needs to loosen the rigid arrangement of a solid.
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