What is happening to a substance's energy and particles at each stage of a heating curve?
Heating and cooling curves: interpret heating and cooling curves, distinguishing changes in kinetic energy from changes in potential energy during phase changes.
A focused Regents Chemistry answer on heating and cooling curves: why temperature is constant during a phase change, how kinetic and potential energy change in each segment, and how to read melting and boiling plateaus from the graph.
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What this topic is asking
The Core Curriculum asks you to interpret heating and cooling curves for a pure substance, distinguishing when kinetic energy changes (the sloped parts, where temperature changes) from when potential energy changes (the flat plateaus, during phase changes). This is a classic Part A and Part B-2 graph-reading skill on the Regents.
Reading a heating curve
A typical curve for heating ice to steam has five parts: warming the solid (slope), melting (plateau at the melting point), warming the liquid (slope), boiling (plateau at the boiling point), and warming the gas (slope). The plateaus are the phase changes; the slopes are temperature increases within a single phase.
Kinetic versus potential energy
This is the heart of every heating-curve question. When temperature changes, think kinetic energy. When the temperature holds steady during a phase change, think potential energy. The two never increase at the same time on a heating curve: a segment is either a temperature change or a phase change.
Why temperature is constant during a phase change
During melting or boiling, all the heat being added goes into pulling the particles apart against their intermolecular attractions, raising the potential energy. Because none of the added energy increases the particles' motion, the temperature stays constant until the phase change is complete. Only after the substance has fully changed phase does further heating raise the temperature again.
Cooling curves
A cooling curve is the mirror image: as a substance loses heat at a constant rate, the sloped sections show falling temperature (decreasing kinetic energy) and the plateaus show freezing and condensing, where energy is released as attractions re-form (decreasing potential energy) at constant temperature. The freezing point and melting point of a pure substance are the same temperature.
Try this
Q1. On a heating curve, what does a sloped (rising) segment tell you about the particles' energy? [1 point]
- Cue. Their average kinetic energy is increasing (the temperature is rising) within a single phase.
Q2. State the phase change occurring at the lower plateau of a heating curve. [1 point]
- Cue. Melting (fusion), solid to liquid.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (Part B-2 style)3 marksA heating curve is plotted for a pure substance as heat is added at a constant rate. (a) State what is happening to the average kinetic energy during a sloped (rising) portion of the curve. (b) State what is happening to the temperature during the flat (plateau) portion at the boiling point. (c) Explain why the temperature stays constant during this plateau even though heat is still being added.Show worked answer →
A 3-point constructed-response item interpreting a heating curve.
(a) Sloped portion (1 point): the average kinetic energy of the particles is increasing (the temperature is rising).
(b) Plateau (1 point): the temperature remains constant during the boiling plateau.
(c) Explanation (1 point): during the phase change the added heat is used to overcome the attractive forces between particles (increasing potential energy), not to increase their kinetic energy, so the temperature does not change until the phase change is complete.
Markers reward linking the slope to kinetic-energy change, recognizing the constant temperature at the plateau, and explaining that the energy goes into potential energy (breaking attractions) during the phase change.
Regents (Part A style)1 marksDuring which process does the potential energy of a substance increase while its temperature stays constant? (1) heating a solid below its melting point (2) melting at the melting point (3) heating a gas (4) cooling a liquidShow worked answer →
A 1-point Part A item on phase changes. The answer is (2) melting at the melting point.
During melting, the temperature stays constant (so the average kinetic energy is unchanged) while the added heat increases the potential energy by separating the particles from their fixed solid arrangement. Heating a solid, a gas or a liquid (without a phase change) raises the temperature, so the kinetic energy increases instead.
Markers reward identifying a phase change (melting) as the process where potential energy rises at constant temperature.
Related dot points
- States of matter and kinetic molecular theory: describe the particle arrangement and energy in solids, liquids and gases, and state the assumptions of the kinetic molecular theory of an ideal gas.
A focused Regents Chemistry answer on the three states of matter and kinetic molecular theory: how particle arrangement and motion differ across solids, liquids and gases, the assumptions of an ideal gas, and how real gases deviate from ideal behavior.
- Heat and calorimetry: calculate heat changes using q = mC(delta-T) for temperature changes and q = mH for phase changes, with constants from Table B and formulas from Table T.
A focused Regents Chemistry answer on heat and calorimetry: the q = mC(delta-T) equation for warming or cooling, q = mH for melting and boiling, the water constants on Table B, and the difference between exothermic and endothermic changes.
- The gas laws: use the combined gas law to relate the pressure, volume and Kelvin temperature of a fixed mass of gas, with STP from Table A.
A focused Regents Chemistry answer on the gas laws: the qualitative pressure-volume and volume-temperature relationships, the combined gas law from Table T, the use of Kelvin temperature, and STP values from Table A, with a worked calculation.
- Solutions and solubility curves: classify solutions as unsaturated, saturated or supersaturated, and use the Table G solubility curves to determine how much solute dissolves at a given temperature.
A focused Regents Chemistry answer on solutions and the Table G solubility curves: solute and solvent, saturated, unsaturated and supersaturated solutions, the factors that affect solubility, and how to read grams of solute per 100 g of water from the curve.
- Intermolecular forces: describe hydrogen bonding, dipole-dipole forces and weak dispersion forces, and use them to explain trends in boiling point and the properties of water.
A focused Regents Chemistry answer on intermolecular forces: hydrogen bonding, dipole-dipole attractions and weak dispersion (van der Waals) forces, how they differ from chemical bonds, and how they explain boiling points and water's high boiling point and surface tension.
Sources & how we know this
- Physical Setting/Chemistry Core Curriculum — New York State Education Department (2002)
- Reference Tables for Physical Setting/Chemistry, 2011 Edition — New York State Education Department (2011)