How to Answer Science Questions Properly: Mastering Exam Command Words (with Real O-Level Physics Examples)
Danbo struggling with science exam command words like 'Explain', 'Calculate', and 'Deduce' — a relatable moment for every O-Level Physics and Chemistry student in Singapore.
Do you ever look at a science question in an exam and think,
"I know this topic… but how exactly am I supposed to write the answer?"
That’s not a content issue. That’s a command word issue.
Command words like describe, explain, and calculate are used in almost every question in both O-Level Physics and Chemistry. If you misunderstand what these words are asking you to do, you can lose marks—even when your facts are correct.
This guide will show you how to understand and respond to command words properly, using real examples from O-Level Pure Physics papers. The best part? These tips work for Chemistry too. Mastering them will instantly level up your science answers across subjects.
What Are Command Words, and Why Do They Matter in Science Exams?
Command words are the instructions in a question. They’re usually the first word—like “Explain,” “Calculate,” or “State.” Each word tells you exactly what kind of answer is expected.
These command words are standardized across Physics and Chemistry, especially in SEAB O/N-Level exams. That means once you know how to handle them, you’ll be prepared for both subjects.
Command Words Used in Physics & Chemistry (with Real Examples)
Let’s break down the most common command words you’ll encounter and show you how to answer them using real O-Level Pure Physics examples. But remember—these exact same question types show up in Chemistry too!
1. State / Give / Name / Identify
What it means:
Write a short, specific answer. No explanation needed.
Physics Example:
State the principle of moments.
Answer:
The sum of clockwise moments equals the sum of anticlockwise moments.
Chemistry Tip:
Use this for stating chemical symbols, formulae, or test results (e.g. State the colour change when litmus is added to acid).
2. List
What it means:
Provide multiple items, usually without explanations.
Example:
List two types of energy stored in a moving object.
Answer:
Kinetic energy
Gravitational potential energy
Also appears in Chemistry when listing types of bonding, properties, or reactions.
3. Describe
What it means:
Say what is happening. Focus on patterns or observations.
Physics Example:
Describe the motion of the ball between points A and B.
Answer:
The ball rises with decreasing speed until it stops momentarily.
Chemistry Example:
Describe what happens when sodium reacts with water.
4. Explain
What it means:
Say why something happens, using science.
Physics Example:
Explain why the pressure increases when the gas is heated.
Answer:
Particles move faster, collide more frequently and forcefully, increasing pressure.
Chemistry Example:
Explain why increasing temperature increases the rate of reaction.
5. Compare
What it means:
Mention both similarities and differences.
Physics Example:
Compare the pressure in the cylinder before and after heating.
Answer:
Before heating, pressure is lower due to slower particles. After heating, pressure is higher due to faster collisions.
Chemistry Example:
Compare the properties of ionic and covalent compounds.
6. Contrast
What it means:
Only mention differences.
Example:
Contrast solids and gases.
Answer:
Solids have fixed shapes; gases do not. Particles in solids vibrate; in gases they move freely.
Also used in Chemistry: Contrast metals and non-metals.
7. Calculate
What it means:
Use formulas. Show working. Include units.
Physics Example:
Calculate GPE of a 3 kg object lifted 2 m (g = 10).
Answer:
GPE = mgh = 3 × 10 × 2 = 60 J
Chemistry Example:
Calculate the relative formula mass of NaCl.
8. Determine
What it means:
Use data (from a graph, table, or setup) to work out a value.
Physics Example:
Determine the resultant force on a 2 kg object with 3 m/s² acceleration.
Answer:
F = ma = 6 N
Chemistry Tip:
Used in titration questions, gas volume analysis, etc.
9. Measure
What it means:
Take a value from a diagram or setup.
Physics Example:
Measure the spring extension from the diagram.
Answer:
4.5 cm
Chemistry Example:
Measure the temperature change during a reaction.
10. Predict
What it means:
Use logic or trends to forecast what happens next.
Example:
Predict what happens to pressure if temperature drops.
Answer:
Pressure decreases due to fewer, slower collisions.
Chemistry Tie-in:
Used in energy profile diagrams or reaction trends.
11. Suggest
What it means:
Give a possible explanation, even if it's not directly in the syllabus.
Physics Example:
Suggest why the ammeter reading fluctuates.
Answer:
The wire may be heating up, increasing resistance.
Chemistry Example:
Suggest a reason why the reaction rate suddenly decreased.
12. Deduce
What it means:
Draw a conclusion from the given information.
Physics Example:
Deduce if the spring obeys Hooke’s Law based on the graph.
Answer:
Since the graph is a straight line through the origin, it obeys Hooke’s Law.
Chemistry Example:
Deduce the identity of a gas from test results.
13. Define
What it means:
Write the exact textbook definition.
Physics Example:
Define moment of a force.
Answer:
The product of force and perpendicular distance from the pivot.
Chemistry Example:
Define activation energy.
14. Outline
What it means:
List the main steps or key ideas briefly.
Example:
Outline how to determine density of a solid.
Answer:
Measure mass
Find volume using water displacement
Calculate: Density = Mass ÷ Volume
Used in Chemistry for outlining experiments like chromatography or neutralization.
15. Construct / Draw
What it means:
Create a graph or diagram. Use labels and correct scale.
Physics Example:
Draw a distance-time graph for the motion described.
Chemistry Example:
Draw the setup for electrolysis of copper chloride.
16. Label
What it means:
Add correct names to diagram parts.
Example:
Label the forces acting on the object.
Also used in Chemistry: labeling apparatus, atom structure, or reaction diagrams.
17. Plot
What it means:
Place data points on a graph accurately.
Example:
Plot a graph of force vs extension using the table.
Used across both sciences in experiments involving data collection.
18. Devise / Plan
What it means:
Design a full method for an experiment.
Example:
Devise an experiment to find spring constant.
Answer:
Add weights
Measure extension
Plot graph
Calculate gradient (spring constant)
Chemistry Example:
Plan an experiment to compare the reactivity of Group I metals.
Quick Reference Table
| Command Word | You Should... |
|---|---|
| State | Give a one-line fact |
| Describe | Say what is happening |
| Explain | Say why it happens |
| Compare | List similarities and differences |
| Calculate | Use formula and show working |
| Predict | Forecast based on trend |
| Suggest | Use logic and reasoning |
| Deduce | Conclude from information |
| Define | Give textbook definition |
| Draw/Label | Make accurate diagrams |
Final Tips to Score in Physics and Chemistry
Underline the command word in every question.
Match your answer style to the command word: short, long, calculation, etc.
Always show your working for maths-based questions.
Practice answering using past paper questions.
Whether it’s Physics or Chemistry, knowing what the examiner wants is half the battle. Master these command words, and you’ll unlock more marks across all your science subjects.
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