Contents
1 · Command Words Decoded
Every mark in Paper 2 is tied to a command word. The command word tells you exactly what format of answer is required. Writing the right content in the wrong format is one of the most common reasons students lose marks they "knew" the answer to.
| Command word | What you must do | What NOT to do |
|---|---|---|
| State | Give a brief factual answer. One sentence maximum. No explanation needed. | Do not write "because" or explain the reason. |
| Define | Give the scientific definition, word for word if possible. Marks go to specific key words. | Do not paraphrase loosely — "osmosis is when water moves" scores zero. |
| Describe | Account for what happens — what is observed, in order. Use data from the question if provided. | Do not explain why it happens unless asked. |
| Explain | Give the reason (cause) AND the scientific mechanism (how/why). Always use correct vocabulary. | Do not just describe what happens — state why. |
| Suggest | Apply your knowledge to a new or unfamiliar context. The answer is logical, not a guess. | Do not leave blank — there is always a justifiable answer. |
| Compare | Make a direct comparison between two things. Must address BOTH items explicitly. | Do not describe only one item and ignore the other. |
| Deduce | Draw a conclusion that follows logically from the data or information given. | Do not introduce information not found in the question. |
| Calculate | Show all working. Include units at every step. Round to appropriate significant figures. | Do not write only the final answer with no working — working earns partial credit. |
| Sketch | Draw a clearly labelled diagram. Accuracy matters less than correct key features and labels. | Do not draw without labels — unlabelled diagrams earn few marks. |
2 · Definition Questions
Definitions are the most reliable mark-earners in Paper 2. A correct definition scores the full marks with no ambiguity. The mark scheme is looking for specific biological or chemical terms — not general descriptions. Memorise the high-frequency definitions below exactly.
Biology definitions (exact wording)
Osmosis: The net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
Diffusion: The net movement of molecules or ions from a region of higher concentration to a region of lower concentration down a concentration gradient.
Enzyme: A biological catalyst (protein) that speeds up the rate of a specific biochemical reaction without being used up in the process.
Chemistry definitions (exact wording)
Oxidation: The loss of electrons (or gain of oxygen, or increase in oxidation number).
Reduction: The gain of electrons (or loss of oxygen, or decrease in oxidation number).
Catalyst: A substance that increases the rate of a chemical reaction without being used up in the reaction.
Electrolysis: The decomposition of an ionic compound, in molten form or aqueous solution, by the passage of an electric current.
3 · Explanation Questions (the most important skill)
Explanation questions are the most common high-mark questions in Paper 2. They typically start with "Explain why…" or "Explain how…" and carry 2–4 marks. Students who can write explanation chains — cause → mechanism → effect — consistently outscore those who can only describe.
[Cause] → [Scientific mechanism using key vocabulary] → [Effect/outcome]. Every mark point is either the cause, a step in the mechanism, or the final effect.
Mark 1 (cause): Increasing temperature increases the kinetic energy of the reactant particles.
Mark 2 (mechanism): This leads to more frequent collisions between particles, and a greater proportion of collisions have energy equal to or greater than the activation energy.
Mark 3 (effect): Therefore, the number of successful (effective) collisions per unit time increases, so the rate of reaction increases.
Mark 1: The villus has a large surface area due to microvilli (brush border), increasing the rate of absorption.
Mark 2: The epithelial layer of the villus is only one cell thick, reducing the diffusion distance for glucose.
Mark 3: Dense capillary networks continuously remove absorbed glucose, maintaining a steep concentration gradient to drive further diffusion and active transport.
"Molecules move faster" — this is the cause, not the explanation. You must continue: faster movement → more frequent collisions → more with sufficient energy → higher rate. Each arrow is a potential mark point.
4 · Describe and Compare Questions
Describe questions
Describe what is observed or what happens — without explaining why. If data (a graph, table, or results) is provided, you must reference it with specific values. An answer like "the rate increases" scores one mark. An answer like "the rate increases from 0.2 cm³/s at 20°C to 0.8 cm³/s at 40°C, then decreases sharply above 50°C" scores three.
1 — Overall trend: "As X increases, Y increases/decreases." 2 — Specific values: Quote at least two data points with units. 3 — Any anomaly or turning point: "The rate peaks at 37°C then decreases sharply." Always include units.
Compare questions
Compare questions demand that you address both items in the same sentence or clearly paired sentences. The word "whereas" or "however" is your friend — it forces you to cover both sides.
Arteries have thick muscular walls to withstand high blood pressure, whereas veins have thinner walls as blood pressure is lower. Arteries do not contain valves, whereas veins have valves to prevent backflow of blood.
5 · Suggest and Deduce Questions
"Suggest" questions are the most feared — but they are the most fair. They always have a logically derivable answer based on the science you know applied to the new context. The examiners are testing whether you can transfer knowledge, not whether you have memorised a specific fact.
How to approach a "suggest" question
- Identify which syllabus topic is being tested (the unfamiliar context is always testing a familiar concept).
- Apply the relevant principle to the new context.
- Write your suggestion with a brief justification — "because" is your friend here.
Large eyes allow more light to enter the eye, enabling the fish to detect the small amounts of light available at depth. This increases the stimulation of photoreceptors (rod cells) in the retina, improving vision in dim light conditions.
"Deduce" questions require you to draw a specific conclusion from data provided. State the conclusion clearly and reference the evidence: "Since the mass decreased by 2.4 g and the volume of gas increased, I deduce that the reaction is producing a gaseous product."
6 · Data-Based Questions
Data-based questions (DBQs) give you a table, graph, or experimental result and ask you to interpret it. The context may be unfamiliar, but the science being tested is always from your syllabus.
The 3-step DBQ approach
- Read the question before the data. Know what you are looking for before you study the graph or table.
- Extract specific values. Cite numbers, units, and conditions from the data in your answer. Never describe a graph vaguely.
- Link to syllabus content. Identify which concept the data is illustrating and apply it explicitly.
If two variables increase together in the data, write "there is a positive correlation" — not "X causes Y" unless the question or context explicitly supports causation. Claiming causation from correlation is a common A-level error that starts at O-Level.
7 · The 6 Most Costly Mistakes in Paper 2
Vague language
"The cell gets bigger" → 0 marks. "The cell increases in volume as water enters by osmosis due to a lower water potential inside the cell" → full marks.
Missing units
Every physical quantity needs a unit. "The rate is 0.4" is incomplete. "The rate is 0.4 cm³/s" is correct.
One-sided compare
"Arteries have thick walls" earns 0 for a compare question. You must compare arteries AND veins in the same answer.
Describing instead of explaining
"The rate increases when temperature increases" is a description. The explanation requires collision theory, activation energy, and successful collisions.
Leaving "suggest" blank
There is no negative marking. A logical attempt using syllabus vocabulary always has a chance of earning marks. A blank earns zero.
Wrong significant figures
In Physics calculations, match your answer's significant figures to the data given. "3600.000 N" when data is given to 2 sig fig is wrong. Write "3600 N" or "3.6 × 10³ N".
🎯 Practice Quiz — Test Yourself
8 O-Level-style questions on this topic. Select an answer to see instant feedback.