Section 1 of 9Reading a heating curve
Sloping sections
- Sloping sections The substance is in a single state and its temperature changes.
- Energy goes into kinetic energy of the particles.
Flat sections
- Flat sections A change of state is under way and the temperature is constant.
- Energy goes into potential energy as the particles separate.
Flat section positions
- Flat section positions The lower plateau is the melting point; the upper plateau is the boiling point.
Section 2 of 9Walking up the curve
1
Solid warms: particles vibrate more strongly and the line rises.
2
Melting: the first plateau, solid and liquid exist together.
3
Liquid warms: the line rises again, but more slowly for a substance with high specific heat capacity.
4
Boiling: the second, longer plateau, as bonds are fully broken.
5
Gas warms: particles speed up and the line rises once more.
Section 3 of 9Cooling curve differences
| Section | Heating curve | Cooling curve |
|---|---|---|
| Direction | Temperature rises over time | Temperature falls over time |
| Plateaus | Melting and boiling points | Condensing and freezing points, at the same temperatures |
| Energy | Absorbed by the substance | Released to the surroundings |
Section 4 of 9Purity clue
Pure substance check
Sharp plateau
- Sharp plateau A pure substance changes state at one fixed temperature.
Sloping region
- Sloping region A mixture changes state across a range of temperatures.
Section 5 of 9Using the graph
1
Read the temperature of each plateau straight off the vertical axis to find the melting and boiling points.
2
Use the length of a plateau to compare how much energy was needed; a longer plateau means more energy for the same heating power.
3
Compare slopes: a shallower sloping section means more energy is needed for each degree of temperature rise, which suggests a higher specific heat capacity.
4
Always say which state or states are present in each section when you label the graph.
Section 6 of 9Worked reading of a graph
| Section of the graph | State present | What the energy is doing |
|---|---|---|
| First rising line | Solid only | Increasing the kinetic energy of the particles |
| First plateau | Solid and liquid together | Increasing potential energy while the solid melts |
| Second rising line | Liquid only | Increasing the kinetic energy again |
| Second plateau | Liquid and gas together | Increasing potential energy while the liquid boils |
| Final rising line | Gas only | Making the gas particles move faster |
Section 7 of 9Why the second plateau is longer
More bonds to break
- More bonds to break Boiling must overcome all the remaining attractions between particles,
- while melting only loosens them,
- so boiling needs more energy per kilogram.
Constant heating power
- Constant heating power If the heater supplies energy at a steady rate, a longer plateau directly shows that more energy was needed.
Cooling in reverse
- Cooling in reverse On a cooling curve the plateaus appear in the opposite order,
- and the substance gives energy out to its surroundings while its state changes.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Plateau | A flat region of a graph where the temperature does not change. |
| Melting point | Fixed temperature at which a pure solid melts. |
| Freezing point | Temperature at which a liquid becomes solid; equal to the melting point. |
| Heating curve | A graph of temperature against time as a substance is heated. |
| Boiling point | The fixed temperature at which a pure liquid changes to a gas throughout its volume. |
| Cooling curve | A graph of temperature against time as a substance loses energy to its surroundings. |
Section 9 of 9Exam technique
1
Saying no energy is supplied on a plateau. Energy is still transferred, but it changes the state, not the temperature.
2
Labelling the plateau by the starting state only; both states are present during the change.
3
Mistaking the steeper slope for more energy; a steeper line can mean a lower heat capacity.
4
Drawing a plateau as a sloping line for a pure substance; it must be perfectly flat.