TL;DR:
Start with the claim, a direct answer in one sentence.
Back it with specific evidence from the data you were given, with numbers and units.
Finish with reasoning: the scientific idea that links the evidence to the claim.
"Explain" and "justify" questions lose marks for a simple reason: the student knows the answer but writes a description. They restate what the graph shows, or write everything they remember about the topic, and never connect the two. A marker looking for a reason does not find one.
There is a simple three-part structure that fixes this: claim, evidence, reasoning.
What each part does
Claim: a direct answer to the question, in one sentence.
Evidence: specific facts from the data or situation, with numbers and units.
Reasoning: the rule, mechanism or principle that explains why the evidence supports the claim.
The reasoning is the part most often missing. Evidence says what happened. Reasoning says why that means the claim is true.
The steps
Read the command word: "Explain" asks why or how. "Justify" asks you to give reasons for a conclusion. "Describe" only asks what you see.
Write the claim first: Answer the question in a sentence before anything else.
Add evidence: Quote values from the table or graph, and compare them, with units.
Add reasoning: Name the scientific idea and link it to the evidence with a word like because.
Reread against the question: Does every sentence help answer what was asked?
Worked example: a biology answer using claim, evidence, reasoning
Here is an invented investigation. A student places a piece of pondweed at different distances from a lamp and counts the bubbles of gas released per minute. At 10 cm there are 24 bubbles per minute. At 40 cm there are 6. The question: Explain how the distance from the lamp affects the rate of photosynthesis.
A weak answer: The pondweed at 10 cm had more bubbles. The bubbles are oxygen. Photosynthesis makes oxygen. It has facts but no connection and no claim.
A strong answer, built in three parts:
Part | Sentence |
|---|---|
Claim | The closer the pondweed is to the lamp, the faster the rate of photosynthesis. |
Evidence | At 10 cm the pondweed released 24 bubbles per minute, while at 40 cm it released 6, so the rate at 10 cm was |
Reasoning, first link | Bubbles are oxygen, a product of photosynthesis, so more bubbles means more photosynthesis. |
Reasoning, second link | Light supplies the energy for the process, and more light reaches the plant when the lamp is closer, so the rate increases. |
Check the arithmetic: 24 ÷ 6 = 4. The word "because" is hidden in the reasoning row, which links bubbles to oxygen to photosynthesis, and light to rate. Each link answers the question "so what?" about the evidence.
Put together, the answer is three sentences. It starts with the claim, uses numbers from the data, and ends with the science. Notice how short it is. A good explanation is not long; it is connected.
A note on what the data can support
A good answer does not claim more than the data show. Two distances are enough to show a difference, but not the exact shape of the relationship. If the question asks for it, you can add that more distances would help. It is also reasonable to note that other things, such as temperature, should be kept constant for a fair comparison.
Using the marks as a guide
Look at how many marks the question carries. Two marks usually means two distinct points, so a claim and one piece of evidence with a reason. Four marks asks for more development, perhaps two pieces of evidence and a clear chain of reasoning. Write the amount of detail the marks suggest. A one-line answer to a four-mark question leaves marks unclaimed, and a page for two marks wastes time you need elsewhere.
Practise writing the three parts on separate lines in your notes. In the exam, you can write them as a paragraph, but the habit of thinking in three parts helps you notice when one is missing.
Justify questions
"Justify" often asks you to defend a choice or a conclusion. The structure is the same: state the conclusion, give the evidence that supports it, and explain why that evidence is enough. If the evidence is mixed, say so and say which part weighs more.
Checking your answer against the command word
Before you move on, reread the question and the first word. If it says explain and your answer only says what happened, add a because. If it says justify and you only gave a conclusion, add the evidence that makes it hold. This ten-second check is the cheapest improvement you can make to written answers.
Mistakes to avoid
Describing the data without saying what it means.
Writing everything you know about the topic instead of answering the question.
Using words like "affects" with no direction. Say increases or decreases.
Quoting evidence without numbers or units.
Leaving out the reasoning, which is where the reasons are.
Your plan for this week
Take three past "explain" questions from your class.
Write each answer as claim, evidence, reasoning on separate lines.
Underline the connecting word in the reasoning.
Compare with the mark scheme or your teacher's feedback.
Frequently asked questions
How long should an explanation be?
As long as it takes to include a claim, evidence and reasoning, usually two to four sentences. Use the marks available as a guide to how much to write.
What if I do not know the reasoning?
Write the claim and evidence anyway, then give your best scientific link. Part of the answer earns more than none.
Get started
