In short: Start every explain answer with a statement of fact or a description, then add the reason. The first, simplest marking point is the one students miss most often.
Very often the marks on an explain question are lost on the first and most simple bit. Students know the chemistry, jump straight to "because…", and never write down the fact the explanation hangs on. It happens on 2, 3 and 4-mark questions alike.
What "explain" asks you to do
An explain question wants two things: what happens and why it happens. A describe question only wants the first. So an explain answer is really a description with a reason tagged on the end, and both halves carry marks.
The method: fact first, then the explanation
- Write a statement of fact or a description: what is true about the particles, the structure or the trend.
- Add the explanation: the reason, linked to the fact with words like "so" or "because".
- Check that each point is a separate idea, and that you have about one point for each mark.
Rich's tip: Statement of fact first, then tag the explanation on the end.
Worked example: ionisation energy down a group
Question: Explain why first ionisation energy decreases down Group 2. (3 marks)
Start with the facts about the atoms:
- Going down the group, the atoms have more shells, so the outer electron is further from the nucleus.
- There is more shielding from the inner shells.
Then the explanation:
- So there is a weaker attraction between the nucleus and the outer electron, even though the nuclear charge increases, and less energy is needed to remove it.
Notice the first two points are simple facts about the atom. They are easy marks, and they are exactly the ones that go missing when an answer starts with "it's easier to remove". For the same idea across a period, see First ionisation energy across Period 3.
The same method on other topics
- Boiling points of alkanes: fact first: longer chains have a larger surface area, so there are stronger van der Waals forces between the molecules. Then the explanation: more energy is needed to overcome them, so the boiling point is higher.
- Temperature and rate: fact first: at a higher temperature more particles have energy greater than or equal to the activation energy. Then the explanation: so there are more successful collisions per second, and the rate increases.
Common mistakes on explain questions
- Starting with the reason and never stating the fact.
- Writing "it's harder to remove" or "the forces are stronger" without saying which attraction or which forces.
- Saying the electron is "further away" without saying from what: the nucleus.
- Writing one long point for a 3-mark question, when the mark scheme is looking for three separate ideas.
Think in bullet points, write in sentences
Plan the answer as bullet points: the fact, then each link in the reason. Then write each bullet as a full sentence in the exam. Sentences let you join the fact to its reason, and that link is usually where the explanation mark sits.
Rich's tip: Think in bullet points, write in sentences.
If you want to practise this with feedback, Dexter, the A Level Chemistry tutor I built, writes new explain questions on the topics in your course and shows you the mark scheme, so you can tick off the points you made.