Heating and cooling curves show how the temperature of a substance changes as heat is added or removed. They matter because they connect energy, temperature, and phase changes in one simple graph. A typical heating curve rises during warming within one phase, then becomes flat during melting or boiling.
A cooling curve has the same ideas in reverse as a substance releases energy and changes from gas to liquid to solid.
During a sloped part of the curve, added or removed heat changes the average kinetic energy of the particles, so the temperature changes. During a flat plateau, the energy changes the particle arrangement and intermolecular forces instead of changing temperature. The heat needed for a phase change is calculated with q = mΔHfus or q = mΔHvap, while heating within one phase uses q = mcΔT.
Reading the curve carefully helps identify the phase, the phase change, and which equation to use.
Understanding Chemistry: Heating and Cooling Curves
A curve is a record of an energy transfer process, so its axes need careful reading. The horizontal axis may show heat energy added or removed, often in joules or kilojoules. It may instead show time if a heater supplies energy at a steady rate.
The vertical axis shows temperature. If time is used, a wider section does not automatically mean a larger energy change unless the heating rate is known. A steep slope means the temperature changes a lot for each unit of energy.
A shallow slope means more energy is needed for the same temperature change. This difference comes from specific heat capacity. Water has a high specific heat capacity, which is why lakes warm and cool more slowly than many land surfaces.
At a melting point, solid particles do not suddenly become a liquid all at once. For a time, solid and liquid are present together. Energy is used to loosen the attractions holding particles in the solid structure.
At a boiling point, liquid and gas exist together while particles gain enough energy to escape attractions in the liquid. The temperature remains fixed only while the pressure stays constant and the substance is pure.
This is why a heating curve is an ideal model. In a classroom experiment, heat can escape to the surroundings, the thermometer may lag behind the sample, and the graph may have a slightly tilted plateau instead of a perfectly flat one.
Pressure changes the position of phase changes. At higher pressure, particles in a gas are pushed closer together, so boiling usually needs a higher temperature. Pressure cookers use this idea to cook food faster because water can become hotter before it boils.
Melting behaves differently for some substances. Ice takes up less space than liquid water, so greater pressure can lower its melting point slightly. Most solids become harder to melt when pressure increases.
Students should not assume every material follows water’s unusual pattern. A substance can also be heated above its normal boiling point without boiling if there are few places for bubbles to form. This unstable state is called superheating.
Multi-step problems require the energy changes from each part of a journey to be found separately. For example, changing ice below its melting point into steam above its boiling point involves warming the ice, melting it, warming the liquid, boiling it, then warming the gas. Each section uses the property that matches what the particles are doing.
The total energy is the sum of all five amounts. Units matter throughout.
Mass must match the units used for specific heat capacity or latent heat, and temperature differences can be measured in degrees Celsius or kelvin because the size of one degree is the same on both scales. A useful habit is to label the state of the substance at every point before choosing a calculation method.
Key Facts
- Temperature changes only on sloped segments of a heating or cooling curve.
- Temperature stays constant during a phase change because energy changes potential energy, not average kinetic energy.
- Heating within one phase uses q = mcΔT.
- Melting or freezing uses q = mΔHfus.
- Boiling or condensing uses q = mΔHvap.
- For the same substance, ΔHvap is usually greater than ΔHfus because separating particles into a gas requires more energy.
Vocabulary
- Heating curve
- A graph showing how the temperature of a substance changes as heat is added.
- Cooling curve
- A graph showing how the temperature of a substance changes as heat is removed.
- Heat of fusion
- The energy required to melt a unit mass or mole of a substance at its melting point.
- Heat of vaporization
- The energy required to vaporize a unit mass or mole of a substance at its boiling point.
- Plateau
- A flat part of a heating or cooling curve where temperature stays constant during a phase change.
Common Mistakes to Avoid
- Using q = mcΔT during a plateau is wrong because ΔT = 0 during a phase change, so latent heat equations must be used instead.
- Thinking the substance stops absorbing heat on a flat segment is wrong because heat is still being used to overcome or form intermolecular attractions.
- Confusing melting point with boiling point is wrong because the lower plateau is usually melting or freezing and the higher plateau is usually boiling or condensing.
- Ignoring the direction of the curve is wrong because heating adds energy while cooling removes energy, even if the phase-change temperatures are the same.
Practice Questions
- 1 A 50.0 g sample of ice at 0°C melts completely. If the heat of fusion of water is 334 J/g, how much heat is absorbed?
- 2 A 25.0 g sample of liquid water warms from 20.0°C to 80.0°C. If c = 4.18 J/g°C, how much heat is absorbed?
- 3 On a heating curve, a substance reaches a flat segment at 100°C while heat continues to be added. Explain what is happening to the particles and why the temperature does not rise.