On a heating curve, a sloping line means the particles are gaining kinetic energy, and a flat line means the energy is being used to overcome forces between particles. Decide slope or flat first, and the explanation follows.
This lesson belongs to matter and atomic structure. It applies the particle picture from distinguishing atoms, ions and molecules.
What are the particles doing in each state?
| State | Arrangement | Movement | Forces |
|---|---|---|---|
| Solid | Close, regular, fixed positions | Vibrate in place | Strong |
| Liquid | Close, irregular | Move past each other | Weaker |
| Gas | Far apart, random | Move quickly in all directions | Very weak |
Worked example: a substance that melts at 80 °C
These original data are for a solid heated steadily.
| Time (min) | 0 | 2 | 4 | 6 | 8 | 10 | 12 |
|---|---|---|---|---|---|---|---|
| Temperature (°C) | 60 | 72 | 80 | 80 | 80 | 88 | 96 |
0 to 4 minutes (sloping). The temperature rises from 60 °C to 80 °C. The solid particles absorb heat, vibrate faster and their kinetic energy increases.
4 to 8 minutes (flat at 80 °C). The solid melts. The heat energy is used to overcome the forces of attraction between particles, so they break free from fixed positions. Kinetic energy does not increase, so the temperature stays at the melting point.
8 to 12 minutes (sloping). All the substance is liquid. The particles gain kinetic energy and move faster, so the temperature rises again.
Reading a cooling curve
A cooling curve is the same graph run backwards. The flat part is the freezing point.
During that plateau, particles are losing heat energy as forces of attraction form and the liquid becomes a solid. The heat released replaces the heat lost to the surroundings, so the temperature stays constant. Slopes show the particles losing kinetic energy and slowing down.
The wording that loses marks
The common wrong sentence is “the particles stop moving” on the flat part. They keep moving. What has stopped is the rise in average kinetic energy.
Another slip is to say the heat “is used to break the bonds inside the particles”. Melting changes the arrangement of whole particles, so write “overcome the forces of attraction between particles”. The graph evidence comparison lab gives you more graphs to describe this way.
Check yourself
Using the data above, state the melting point and explain why the temperature is 80 °C at both 5 and 7 minutes. Is the substance solid or liquid at 10 minutes?
Answer
The melting point is 80 °C.
Between 4 and 8 minutes the heat energy is used to overcome the forces of attraction between the particles, so the average kinetic energy does not increase and the temperature stays constant.
At 10 minutes the temperature is 88 °C, above the melting point, so the substance is a liquid.
What to study next
Test all four lessons with the matter and atomic structure practice set. If the particle counting still needs work, revisit calculating protons, neutrons and electrons.
For a teacher to check your graph explanations, see online one-to-one Chemistry tuition.