A trend answer states a pattern: “higher temperature, faster reaction”. A particle-level explanation gives the reason: which particles, what they do differently and what that changes. Exam questions that say “explain” are asking for the second kind.
This skill connects many topics, including particle models and changes of state and rates of reaction. It is the second lesson in the extended explanation and error repair module.
What does a full explanation contain?
A strong answer has three linked parts. Write them in order as short sentences.
- Name the particles involved: atoms, molecules, ions or electrons.
- Say what changes about their behaviour: speed, spacing, energy, freedom to move, number in a given volume.
- State the consequence for the result, in the words of the question.
The link words matter: “so”, “which means”, “therefore”. They show the reasoning and not a list of facts.
Worked repair 1: temperature and rate
Question: Explain why the reaction is faster at 50 °C than at 30 °C.
Trend-only answer: The higher the temperature, the faster the reaction.
This restates the question. It names no particles.
Repaired answer:
- Particles have more kinetic energy at 50 °C, so they move faster.
- They collide more often, and a larger fraction of collisions has at least the activation energy.
- So more successful collisions happen each second, and the rate increases.
Worked repair 2: pressure of a gas
Question: Explain why the pressure of a fixed mass of gas rises when it is compressed at constant temperature.
Trend-only answer: Less volume means more pressure.
Repaired answer: The same number of gas particles is now in a smaller volume, so they hit the container walls more often. More collisions with the walls each second means a larger force on each unit of area, so the pressure rises. The particles do not move faster, because the temperature has not changed.
The last sentence is what a careful answer adds. It rules out the wrong explanation.
The mistake to watch for
Students often fix the trend answer by adding the word “particles” without adding any behaviour.
Mistaken answer: “At a higher temperature the particles react faster.”
The particles do not “react faster”. They move faster, collide more often and more of those collisions succeed. The repair is to replace the vague verb with what the particles physically do.
Check yourself
Rewrite each trend answer as a full particle-level explanation, then open the model answer.
1. “Powdered calcium carbonate reacts faster with acid than a large lump.”
Show answer
Powder has a larger surface area, so more particles of the solid are exposed to the acid. Acid particles can collide with more of them each second. More successful collisions per second means a faster reaction.
2. “Gases are easy to compress but solids are not.”
Show answer
In a gas the particles are far apart with large empty spaces between them, so they can be pushed closer together. In a solid the particles are already touching in a fixed arrangement, so there is almost no space to reduce.
3. “Molten sodium chloride conducts electricity but solid sodium chloride does not.”
Show answer
In the solid, ions are held in fixed positions by strong attractions, so they cannot move. When molten, the ions are free to move and carry charge through the liquid, so it conducts.
Where this leads next
Next, check the equations your explanations rely on in checking a balanced equation independently. The mixed practice set includes several repair questions, and the mistake log and retest queue helps you track which explanations keep needing repair.
Some students can say the right words aloud but write a shorter version under pressure. A teacher in online one-to-one Chemistry tuition can read your written answers and show exactly which link is missing.