IGCSE Chemistry Past Paper Technique

This guide is for IGCSE Chemistry students who want to improve their exam technique and pick up marks they currently lose to poor answering habits. It covers how to read questions properly, structure your answers, and avoid the most common mistakes that cost students marks every year.

1. Before You Start: Reading the Question

The single biggest reason students lose marks in Chemistry exams is not reading the question carefully. Before you write a single word, do this every time:

Circle the command word. "State" means something different from "Explain" or "Describe." If the question says "State," a one-line answer is enough. If it says "Explain," you need a reason.
Underline the key chemical terms. Are they asking about an element, a compound, or a mixture? Ionic or covalent? Exothermic or endothermic? One wrong term can lose you the mark.
Check how many marks the question is worth. A 3-mark question needs three distinct points. If you have written one sentence for a 3-mark question, you are almost certainly missing marks.

Examiner Tip The number of marks almost always equals the number of separate points you need to make. If a question is worth 4 marks, write four clear points. Examiners mark by ticking each correct point against their mark scheme.

2. Command Words and What They Mean

Command Word	What to Do	Example Length
State	Give a fact or name. No explanation needed.	1 line
Define	Give the precise meaning of a term.	1–2 lines
Describe	Say what happens. Give observations, steps, or features.	2–4 lines
Explain	Say what happens and why. Link cause to effect.	3–5 lines
Suggest	Apply your knowledge to an unfamiliar situation. There may not be one "right" answer.	2–4 lines
Calculate	Show working. Write the formula, substitute, then give the answer with units.	3+ lines
Compare	Give similarities and differences. Use "whereas" or "but" to link them.	2–4 lines

3. Worked Example: Approaching a Past Paper Question

Worked Example — Electrolysis Question (4 marks)

Concentrated hydrochloric acid is electrolysed using carbon electrodes.

(a) Name the product formed at the cathode. [1]

(b) Name the product formed at the anode. [1]

Step-by-step approach:

Step 1 — Identify the ions. Hydrochloric acid (HCl) dissociates into H⁺ ions and Cl⁻ ions in solution.

Step 2 — Apply the rule for cathode and anode.

Cathode is negative → attracts positive ions (cations) → H⁺ ions go here
Anode is positive → attracts negative ions (anions) → Cl⁻ ions go here

Answers:

(a) Hydrogen (gas) — H⁺ ions gain electrons at the cathode: 2H⁺ + 2e⁻ → H₂

(b) Chlorine (gas) — Cl⁻ ions lose electrons at the anode: 2Cl⁻ → Cl₂ + 2e⁻

(c) Chlorine is formed at the anode because chloride ions are attracted to the positive electrode [1 mark] where they lose electrons (are oxidised) to form chlorine molecules [1 mark].

Notice how part (c) is worth 2 marks and the answer has exactly two distinct marking points, each highlighted. Always match your number of points to the marks available.

4. Answering Calculation Questions

Calculation questions are where many students either pick up full marks or lose them all. The difference is almost always whether you show your working. Follow this structure every time:

Write the formula you are going to use.
Substitute the numbers from the question into the formula.
Calculate and show the arithmetic clearly.
State the answer with the correct unit.

Worked Example — Moles Calculation (3 marks)

Calculate the number of moles in 11.0 g of carbon dioxide (CO₂). [Relative atomic masses: C = 12, O = 16]

Model Answer:

Step 1: Mr of CO₂ = 12 + (16 × 2) = 44

Step 2: Moles = mass ÷ Mr = 11.0 ÷ 44

Step 3: Moles = 0.25 mol

Common Mistake Students often write just the final number without showing the formula or working. Even if your answer is correct, examiners may only award 1 out of 3 marks if the working is missing. Worse, if you make an arithmetic error but show correct working, you can still earn 2 out of 3 marks through "error carried forward."

5. Common Mistakes That Cost Marks

Mistake 1: Confusing "describe" with "explain"

If a question says "Describe what you would observe," just say what you would see, hear, or smell. Do not explain the science behind it. Conversely, if it says "Explain," you must give the scientific reason, not just the observation.

Example: "Describe what happens when magnesium is added to dilute hydrochloric acid."

Wrong: "The magnesium reacts because it is more reactive than hydrogen in the reactivity series."
Right: "The magnesium dissolves / effervescence (bubbling) is observed / the solution gets warm."

Mistake 2: Not using chemical terminology

Examiners look for specific terms. "The stuff gets hot" will not earn a mark; "the reaction is exothermic" or "the temperature increases" will.

Say "effervescence" or "bubbles of gas," not "it fizzes"
Say "precipitate forms," not "it goes cloudy"
Say "decolourises," not "the colour disappears"

Mistake 3: Forgetting state symbols

When a question says "include state symbols," you will lose marks for every one you omit. The four state symbols are:

Symbol	State	Example
(s)	Solid	NaCl(s)
(l)	Liquid	H₂O(l)
(g)	Gas	CO₂(g)
(aq)	Aqueous (dissolved in water)	HCl(aq)

Mistake 4: Writing unbalanced equations

Always count atoms on both sides of the arrow. A quick check: list each element and count its atoms on the left and right. If the numbers do not match, adjust coefficients until they do.

Mistake 5: Leaving blank spaces

Never leave a question blank. Even a partially correct answer or a reasonable attempt can pick up marks. If you are unsure, write what you do know. A blank answer is always zero marks.

6. Tips for Maximising Your Marks

Tip 1: Use the data booklet You are given a data booklet in the exam. Use it. Check relative atomic masses from the booklet rather than relying on memory. Check the reactivity series. Check the colours of precipitates for ion tests.

Tip 2: Practise with a timer As a rough guide, spend about 1 minute per mark. A 75-mark paper gives you 75 minutes of writing time. If you spend 15 minutes on a 4-mark question, you are running over. Practise under timed conditions to build your pace.

Tip 3: Annotate diagrams When you are given a diagram (apparatus setup, structural formula, graph), label it or annotate it directly. Examiners often ask you to draw on diagrams, and clear labelling earns easy marks.

Tip 4: Review your past papers strategically Do not just redo the same papers. After completing a paper, go through the mark scheme point by point. For every mark you dropped, write a one-sentence note on what you should have written. These notes become your personalised revision guide.

Tip 5: Learn the tests for gases and ions These come up almost every year. Know the test for hydrogen (squeaky pop), oxygen (relights a glowing splint), carbon dioxide (turns limewater milky), and chlorine (bleaches damp litmus paper). Know the flame test colours and the cation/anion test results.

Remember: the mark scheme rewards precise, concise answers. More writing is not better writing. Say exactly what is needed and move on to the next question.

7. Your Revision Plan

Here is a practical sequence for working through past papers in the weeks before your exam:

Weeks 4–3 before the exam: Do one past paper per week, untimed. Focus on understanding the mark scheme fully.
Weeks 2–1 before the exam: Do two past papers per week, timed. Focus on pacing and completing all questions.
Final week: Review your error notes from all the papers you have done. Focus on your personal weak topics.