How to answer 'explain' questions in SPM Chemistry

More marks are lost on “explain” questions than on any other kind in SPM Chemistry. The reason is simple: an “explain” question is never asking for a fact on its own. It is asking for a fact and the reason behind it, and most students write only the first half. They know the chemistry, they get the observation right, and then they stop one sentence too early. This post gives you a reliable framework for answering “explain” questions, with worked examples across several chapters, so you learn to write the full answer every time.

What “explain” actually demands

When you see “explain”, the examiner wants a cause-and-effect chain. There is a what (the claim or observation) and a why (the reason, mechanism or principle behind it). A complete answer states both and links them, usually with the words “because” or “since”.

Think of it as a simple template:

Claim + because + reason (+ because + deeper reason, if the marks demand it).

The number of marks tells you how many linked reasons to give. A two-mark “explain” usually wants a claim and one clear reason. A three-mark “explain” wants a claim and a chain of two connected reasons. Match the length of your answer to the marks in the bracket. You can see how this maps onto the official scheme in our guide to how the marking scheme works.

The mistake that costs the second mark

The most common failure is the missing link, stating that something happens without saying what it does. Consider:

• Question: Explain why the rate of reaction increases when the temperature is raised. (3 marks)
• Weak answer: Because the temperature is higher, the reaction is faster.

That earns almost nothing, because it just restates the question. The chain of reasoning is missing. Here is the full answer:

• Model answer: When the temperature is raised, the particles gain more kinetic energy and move faster, so they collide more frequently. More importantly, a greater proportion of particles now have energy equal to or greater than the activation energy. This gives more effective collisions per second, so the rate of reaction increases.

Every step earns its mark: faster movement, more frequent collisions, more particles above the activation energy, more effective collisions. That is what “explain” means, you show the machinery, not just the outcome. This kind of reasoning sits at the heart of rate of reaction.

Worked examples across the syllabus

The same claim-because-reason shape works everywhere. Here are four from different chapters.

Ionic conductivity

• Question: Explain why lead(II) bromide conducts electricity when molten but not when solid. (2 marks)
• Model answer: When solid, the ions are held in fixed positions in the lattice and cannot move, so no charge is carried. When molten, the ions are free to move and carry charge, so the compound conducts.

Group 1 reactivity

• Question: Explain why potassium is more reactive than sodium. (2 marks)
• Model answer: A potassium atom has more shells than a sodium atom, so its outermost electron is further from the nucleus and more shielded. The outer electron is therefore lost more easily, making potassium more reactive.

Catalysts

• Question: Explain how a catalyst increases the rate of a reaction. (2 marks)
• Model answer: A catalyst provides an alternative reaction path with a lower activation energy. A greater proportion of the colliding particles now have energy equal to or greater than this lower activation energy, so there are more effective collisions and the rate increases.

Displacement

• Question: Explain why zinc can displace copper from copper(II) sulfate solution. (2 marks)
• Model answer: Zinc is more electropositive (more reactive) than copper, so zinc atoms lose electrons more readily. The zinc is oxidised and the copper(II) ions are reduced to copper, so zinc displaces the copper.

Notice how each answer names the principle (kinetic energy, shielding, activation energy, electropositivity) rather than gesturing at it. That named principle is usually where the second mark lives.

A checklist before you write

Train these four habits and “explain” marks stop leaking away:

1. Underline the command word so you know a reason is required, not just a fact. See our full list of command words.
2. Check the marks and give that many linked points.
3. Use “because” or “since” at least once, it forces you to write the reason.
4. Name the principle, collision theory, metallic bonding, shielding, electronegativity, instead of vaguely saying “it reacts more”.

Build the chain, not just the claim

The difference between a B and an A in the written papers is very often the difference between a student who states what happens and a student who explains why. The chemistry is the same; the second student simply finishes the sentence. If you would like a teacher to mark your written answers against the scheme the way an examiner does, our online one-to-one lessons run in English from RM50 an hour, with a paid one-hour trial. Our experienced SPM Chemistry teachers and our team make “answer the command word in full” one of the first habits we build, because it is one of the fastest ways to lift a Chemistry grade.