A state observation is a question that gives you a temperature, a graph or a description and asks you to explain it with particles. A full answer names the arrangement, the movement and the energy, and links them to the evidence.
This lesson belongs to chemical reasoning in mixed tasks. The data below is invented for teaching and is labelled as such.
What does the particle model say?
In a solid the particles are close together in fixed positions and only vibrate. In a liquid they are still close but can slide past each other. In a gas they are far apart and move quickly in all directions.
Heating gives particles more kinetic energy. Cooling takes it away. During a change of state the energy changes the arrangement and the attractions, not the speed, so the temperature stays constant.
Worked example (invented data)
A student records the temperature of a hot liquid wax-like substance, called substance Z, as it cools in a room.
| Time (min) | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 |
|---|---|---|---|---|---|---|---|---|---|
| Temperature (°C) | 80 | 70 | 62 | 56 | 54 | 54 | 54 | 49 | 43 |
Question: Describe the cooling and explain why the temperature stays at 54 °C from 8 to 12 minutes.
Step 1, describe with numbers. The temperature falls from 80 °C to 56 °C in the first 6 minutes. The mean rate is (80 − 56) ÷ 6 = 24 ÷ 6 = 4.0 °C/min. It then stays at 54 °C for 4 minutes, and falls again after 12 minutes.
Step 2, name the state change. The plateau is where substance Z changes from liquid to solid, so 54 °C is its freezing point (the same value as its melting point).
Step 3, explain with particles. Before 8 minutes the particles in the liquid lose kinetic energy, so they move more slowly and the temperature falls. During the plateau the particles move into fixed positions, and attractions form between them. Energy is released as this happens, and this balances the energy lost to the room, so the average kinetic energy stays constant.
Step 4, link back. After 12 minutes all the substance is solid. The particles only vibrate and lose kinetic energy again, so the temperature falls.
The mistake to watch for
Mistaken answer: “The temperature stays the same because the substance has stopped losing heat.”
The data shows a substance in a cooler room, so it is still losing energy. The point is that the energy released by the new attractions replaces what is lost. A second slip is writing “the particles freeze”, which gives the particles a property of the whole substance.
The correction is to name the state change, say what happens to the particles’ arrangement and attractions, and say why the average kinetic energy does not change.
Check yourself
1. Use the table to find the mean cooling rate from 14 to 16 minutes.
Show answer
Fall in temperature = 49 − 43 = 6 °C over 2 minutes. Rate = 6 ÷ 2 = 3.0 °C/min.
2. What state or states is substance Z in at 4 minutes, at 10 minutes and at 16 minutes?
Show answer
At 4 minutes the temperature (62 °C) is above 54 °C, so it is a liquid. At 10 minutes it is on the plateau, so it is a mixture of liquid and solid changing state. At 16 minutes the temperature (43 °C) is below 54 °C, so it is a solid.
3. Explain, using particles, why a gas can be compressed much more easily than a liquid.
Show answer
The particles in a gas are far apart with a lot of empty space between them, so they can be pushed closer together. The particles in a liquid are already touching, so there is almost no space left to squeeze into.
Where this leads next
Next, connect an equation with a supplied mass change to bring in a calculation. When you are ready, try the mixed practice set, and use the scientific investigation critic to review how you would improve a cooling investigation.
If your explanations lose marks because a link in the chain is missing, our teachers can build that habit with you in online one-to-one Combined Science tuition.