HKDSE command words: describe, explain and compare

Read the whole task, not just the topic

Before writing, identify the command word, the object of the question and any conditions. ‘Explain the change in mass’ asks for a mechanism; ‘describe the change in mass’ asks what happened. A familiar topic does not make those tasks interchangeable.

Treat the guidance below as an answer-planning aid. Command words and expected detail depend on the subject, the full question and its marking scheme. Use HKEAA’s framework for your examination year and your school’s subject guidance; there is no single sentence pattern that guarantees marks across every subject.

Describe: say what you observe

Original example: a plant shoot has 10 bubbles per minute at low light intensity and 24 at a higher intensity. Describe the recorded change.

Possible answer: ‘The bubble count increased from 10 to 24 per minute as light intensity increased.’ It states the direction and uses the data. Do not invent a plateau when only these two readings are given.

A count of bubbles is an observation. It is not automatically a precise measurement of oxygen volume, because bubble sizes can vary. Keep your statement at the level the evidence supports.

Explain: connect a cause to an outcome

If asked why increased light might increase photosynthesis, a description of the bubble count is not enough. Explain that light supplies energy for photosynthesis, so an increase can raise the rate while light is limiting and other conditions permit.

Notice the qualification. Increasing light does not guarantee that the rate will keep increasing: another factor can become limiting. Build your answer as condition → mechanism → outcome, while staying within what the question establishes.

Compare: pair the same feature

When comparing diffusion and osmosis, use the same comparison dimension on both sides. For example: diffusion can involve different kinds of particles; osmosis concerns water. Osmosis requires a partially permeable membrane; diffusion does not necessarily require one.

If the question asks for similarities as well as differences, add that both involve net movement down the relevant gradient and are passive processes. Two unrelated descriptions force the reader to find the comparison for you.

• Name the feature being compared.
• Give the statement for A and the matching statement for B.
• Include similarities only when relevant to the question.

Use the marks to budget, not to guess

The mark allocation helps you decide how much time to spend. It does not mean that three marks always require exactly three sentences or three memorised keywords. A point may depend on a relationship, a calculation or evidence.

For calculations, show the formula, substitution and units where appropriate. For an explanation, check that every link in the mechanism is clear. After marking, ask whether the missing element was knowledge, evidence or the requested task.

A final reading check

In TeachAI DSE, use an unofficial practice paper or worksheet to practise one answer type, then compare your response with the explanation. The examples above are original teaching examples, not copied HKDSE questions or official marking schemes.

• Have I answered the command word?
• Have I used the data, units and conditions provided?
• Does each comparison pair the same feature?
• Have I added an unsupported claim?
• Can I explain why each sentence is relevant?