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Extended response questions — typically worth 6 marks — are the most challenging questions on the Edexcel Chemistry papers. They require sustained reasoning, correct scientific terminology, and quality of written communication (QWC). Many students lose marks not because they lack knowledge, but because they fail to structure their answers effectively. This lesson teaches you exactly how to plan, write, and check extended responses for maximum marks.
Extended response questions use levels-based marking with three levels:
| Level | Marks | Description |
|---|---|---|
| 3 | 5-6 | Detailed, coherent answer with correct terminology and logical structure |
| 2 | 3-4 | Some relevant points but incomplete or partially disorganised |
| 1 | 1-2 | Basic points with limited detail or significant errors |
| 0 | 0 | No relevant content |
To reach Level 3, your answer must:
The examiner first decides which level your answer belongs to, then decides whether it is at the top or bottom of that level. This means the difference between 3 marks and 5 marks often comes down to organisation and terminology, not additional content.
Spend 2 minutes planning before you start writing. This is not wasted time — it is the most efficient way to organise your thoughts and ensure you cover all the key points.
graph LR
A[No plan] --> B[Rambling answer]
B --> C[Repeat points]
C --> D[Miss key content]
D --> E[Level 1-2]
F[2-minute plan] --> G[Logical structure]
G --> H[All points covered]
H --> I[No repetition]
I --> J[Level 3]
Without a plan, you write what comes to mind first, then think of something else, then realise you have already said it. The result is a disorganised answer that loops back on itself. With a plan, each paragraph makes a new point in logical sequence.
"Compare the bonding in sodium chloride and diamond."
Structure: discuss one, then the other, then explicitly compare. Use words like "whereas," "in contrast," "however," "both."
Model answer: Sodium chloride has ionic bonding — strong electrostatic attractions between oppositely charged ions (Na⁺ and Cl⁻) arranged in a regular lattice structure. Each ion is surrounded by six oppositely charged ions, giving a coordination number of 6.
Diamond has covalent bonding — each carbon atom forms four strong covalent bonds to four other carbon atoms in a tetrahedral arrangement, creating a giant covalent (macromolecular) structure extending in three dimensions.
Both have very high melting points because a large amount of energy is needed to break the many strong bonds. However, sodium chloride can conduct electricity when molten or dissolved (because the ions are free to move), whereas diamond cannot conduct electricity in any state because it has no free electrons or ions. Both are hard solids at room temperature, but for different reasons: in NaCl, the strong ionic bonds in all directions resist deformation; in diamond, the rigid tetrahedral network of covalent bonds gives exceptional hardness.
Analysis: This answer reaches Level 3 because it discusses both substances, uses correct terminology (electrostatic attractions, coordination number, macromolecular, tetrahedral), makes explicit comparisons ("whereas," "however," "both"), and follows a logical structure.
"Describe and explain the mechanism of nucleophilic substitution of 1-bromopropane with hydroxide ions."
Structure: name the mechanism, describe each step, explain why each step occurs using electronic theory.
Model answer: This is an SN2 (nucleophilic substitution, bimolecular) mechanism. The C-Br bond is polar because bromine is more electronegative than carbon, creating a δ+ charge on the carbon atom and a δ- charge on bromine.
The hydroxide ion (OH⁻) acts as a nucleophile because it has a lone pair of electrons that it can donate. The lone pair on the oxygen of OH⁻ attacks the δ+ carbon atom from the side opposite to the C-Br bond (backside attack).
As the new C-O bond forms, the C-Br bond breaks heterolytically, and Br⁻ leaves as the leaving group. This occurs in a single concerted step — bond forming and bond breaking happen simultaneously. The transition state has five groups partially bonded to the central carbon.
The reaction is bimolecular because both the substrate (1-bromopropane) and the nucleophile (OH⁻) are involved in the rate-determining step, giving a rate equation: rate = k[C₃H₇Br][OH⁻].
"Explain how a buffer solution maintains a nearly constant pH when small amounts of acid are added."
Structure: define the buffer, identify the equilibrium, explain the response to added acid step by step.
Model answer: A buffer solution contains a weak acid and its conjugate base (e.g., ethanoic acid, CH₃COOH, and sodium ethanoate, CH₃COONa). The weak acid establishes an equilibrium: CH₃COOH ⇌ CH₃COO⁻ + H⁺.
When a small amount of acid (H⁺ ions) is added, the excess H⁺ ions react with the conjugate base (CH₃COO⁻) to form the undissociated weak acid: H⁺ + CH₃COO⁻ → CH₃COOH. This removes the added H⁺ from solution, so the pH barely changes.
The equilibrium shifts to the left (by Le Chatelier's principle), converting the added H⁺ into undissociated acid. The large reservoir of CH₃COO⁻ present in the buffer ensures there is sufficient conjugate base to react with the added acid. The pH remains nearly constant provided the amount of acid added is small relative to the buffer capacity.
"Evaluate the use of ethanol as an alternative to petrol as a fuel."
Structure: advantages (with evidence), disadvantages (with evidence), balanced conclusion.
Planning notes: Advantages: renewable (from fermentation of crops), carbon neutral in principle (CO₂ absorbed during growth = CO₂ released during combustion), lower carbon emissions per km Disadvantages: lower energy density than petrol (so more fuel needed per km), land use for crops competes with food production, energy required for distillation Conclusion: viable as a blend or supplement but not a complete replacement due to energy density and land constraints
Examiners specifically look for correct terminology at Level 3. Generic language pulls your answer down to Level 2.
| Generic language | Correct terminology |
|---|---|
| "The bond breaks" | "Heterolytic fission produces..." |
| "The electrons move" | "The lone pair on the nucleophile attacks the δ+ carbon" |
| "It gets harder to remove" | "Ionisation energy increases due to increasing nuclear charge" |
| "The reaction goes faster" | "The rate increases because a greater proportion of molecules exceed the activation energy" |
| "The particles move more" | "The kinetic energy of molecules increases" |
| "The acid makes it dissolve" | "The acid protonates the base, forming a soluble salt" |
| "Heat breaks the bonds" | "The thermal energy overcomes the intermolecular forces" |
Think of terminology as a ladder — each rung gets you closer to Level 3:
Rung 1 (Level 1): "The electrons go somewhere." Rung 2 (Level 2): "The electrons are attracted to the positive carbon." Rung 3 (Level 3): "The lone pair on the nucleophilic oxygen attacks the electrophilic δ+ carbon via donation of the electron pair into the C-Br σ* antibonding orbital."
You do not need rung 3 for A-Level, but you need at least rung 2 with correct chemical terms.
Use short paragraphs, each making one clear point. Avoid long, rambling sentences. Each paragraph should:
End with a concluding sentence that directly answers the question asked.
Comparison:
Explain a process:
Evaluation:
| Mistake | Impact on level | How to avoid |
|---|---|---|
| No plan — rambling structure | Level 2 at best | Spend 2 minutes planning |
| Generic language throughout | Level 2 | Use correct scientific terms |
| One-sided evaluation | Level 1 | Always discuss both sides |
| Not answering the actual question | Level 0-1 | Re-read the question after writing |
| Repeating the same point | Wastes time, stays at same level | Check plan before writing |
| Writing excessively (3+ pages) | No benefit — quality over quantity | Cover key points concisely, then stop |
The best way to improve at 6-mark questions is deliberate practice:
Aim to practise at least 2-3 extended responses per revision session. Over time, the planning process becomes automatic and your terminology improves naturally.
The 6-mark extended-response question is the highest-leverage single item on Edexcel 9CH0. It is level-marked, so a candidate who writes a structured answer with explicit comparison and evaluation can secure 5–6 marks even when individual chemistry details are imperfect. Conversely, a chaotic stream-of-consciousness answer with all the right facts but no structure typically scores 2–3. The skill is structure first, content second — and this deeper strategy section codifies the structure.
Edexcel 9CH0 deploys extended-response questions in two main forms:
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