A heating curve is a graph of temperature against time for a substance being heated steadily. Sloping sections show the temperature rising in one state. Flat sections show a change of state, where the temperature stays constant.
You are asked to label the states, read melting and boiling points, and explain the flat sections.
This lesson builds on boiling and evaporation and belongs to the module on particle models and changes of state.
How do you read the graph?
Take the graph one section at a time. In each section ask two questions: what state or states are present, and what is the energy doing to the particles?
- First slope up: the substance is a solid. Energy makes the particles vibrate faster, so the temperature rises.
- First flat section: melting. Energy is used to overcome attractions so the particles can move past each other. Solid and liquid are both present.
- Second slope up: the substance is a liquid. The particles move faster and the temperature rises again.
- Second flat section: boiling. Energy is used to separate the particles completely. Liquid and gas are both present.
- Final slope up (if shown): the substance is a gas, and the temperature rises again.
A pure substance has a sharp melting point and a sharp boiling point. That is why the flat sections are flat.
Worked example
The table shows data for a pure substance, called X, heated steadily. The numbers are invented for practice and do not describe a real substance.
| Time (min) | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Temperature (°C) | 20 | 30 | 40 | 50 | 60 | 60 | 60 | 60 | 75 | 90 | 105 | 120 | 135 | 150 | 150 | 150 | 150 |
Question: Find the melting point and boiling point of X. State how long melting lasted and explain why the temperature is constant then.
Step 1, find the flat sections. The temperature is 60 °C from 4 to 7 minutes and 150 °C from 13 to 16 minutes.
Step 2, name them. The lower flat section is melting, so the melting point is 60 °C. The higher flat section is boiling, so the boiling point is 150 °C.
Step 3, time for melting. Melting started at 4 minutes and finished at 7 minutes. 7 − 4 = 3 minutes.
Step 4, explain the flat section. During melting, the energy supplied is used to overcome the attractions between particles so they can move past each other. The average kinetic energy does not increase, so the temperature stays at 60 °C.
Step 5, check the liquid section. The liquid is heated from 7 to 13 minutes. The temperature rises from 60 °C to 150 °C, which is 90 °C in 6 minutes, or 15 °C each minute. This matches the table.
Notice that the flat sections are explained by where the energy goes, not by the heater stopping.
The mistake to watch for
The usual error is to read a flat section as “nothing is happening”.
Mistaken answer: “From 4 to 7 minutes the temperature stays the same because the heating has stopped, or the substance has stopped absorbing energy.”
The heater is still supplying energy. The substance is still absorbing it. The energy goes into changing the arrangement of the particles, so it does not raise their average kinetic energy. The correction: “The energy is used to overcome the forces between the particles, so the temperature stays constant during melting.”
A second slip is to call the substance “a liquid” at 60 °C. During the flat section it is a mixture of solid and liquid.
Check yourself
Use the table from the worked example.
1. What state is X at 2 minutes and at 10 minutes?
Show answer
At 2 minutes it is a solid, because that is in the first slope before melting begins at 4 minutes. At 10 minutes it is a liquid, because that is between the end of melting at 7 minutes and the start of boiling at 13 minutes.
2. What is the temperature at 11 minutes, and what is happening to the particles?
Show answer
The temperature is 120 °C. X is a liquid and the particles are moving faster as the temperature rises.
3. On a cooling curve for X, at what temperature would the graph go flat first, and what is this process called?
Show answer
It goes flat first at 150 °C, when the gas condenses to liquid. The second flat section is at 60 °C, when the liquid freezes. The freezing point is the same as the melting point for a pure substance.
Where this leads next
The next lesson, relate pressure to particle collisions, moves from the state of a substance to the behaviour of a gas. You can also check your reading of graphs with the mixed particle practice.
Students often describe the shape well and still lose the explanation marks. That is the pattern a teacher in online one-to-one Chemistry tuition can work on with new data each time.