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How the Unit 3 practical paper works

Edexcel IAL Chemistry · WCH13 Practical Skills in Chemistry I

Unit 3 is a written paper about practical chemistry. You do not do experiments in the exam, but you must answer as someone who has: choosing apparatus, describing methods, interpreting observations and handling real data.

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The paper

Time and marks: 1 hour 20 minutes, 50 marks.
Weighting: 20% of the International AS, 10% of the full International A level.
Question types: short-open, open-response and calculation questions. At least 6 marks test maths at Level 2 or above.
Sittings: January, June and October.
Content: the practical skills and techniques of Units 1 and 2, including Core Practicals 1 to 8, and the same skills applied to experiments you have never seen.

What it tests

The specification groups the skills into four areas:

1. Independent thinking: solving problems and applying chemistry to a practical situation.

2. Scientific methods: controlling variables, presenting data, evaluating results, appreciating uncertainty and commenting on methods.

3. Numeracy: plotting and reading graphs, processing data, significant figures, accuracy and precision.

4. Apparatus: recognising and choosing apparatus, using techniques properly, range and resolution, and health and safety.

Questions are built around tests for ions, gases and organic groups, and around molar volume, titration, thermochemistry, simple kinetics and preparations.

What examiners reward

Recent examiners' reports repeat the same advice:

Answer the exact question. If it asks for the gas, write hydrogen chloride, not 'misty fumes'. If it asks what is seen, give an observation, not a name.
Give one answer. A right answer next to a wrong one usually scores nothing.
Explain why. 'More accurate' on its own scores nothing; say why (for example, no gas escapes because the bung is not removed).
Show working and do not round early. Give the final answer to 2 or 3 significant figures, and check it is sensible (a molar volume of 2.4 dm³ mol^-1 should make you look again).
Learn the names and spellings of apparatus (pestle and mortar, anti-bumping granules) and practise drawing set-ups.
Revise the core practicals, especially why each step is done.

Worked example

A student collects carbon dioxide from calcium carbonate and acid. In set-up A the bung is removed to add the solid. In set-up B the solid is held in a small tube inside the flask and tipped in without opening it. Explain why B gives a more accurate molar volume.

Spot the difference between the methods: A is opened after the reaction may have started.
Link it to the measurement: in A some gas escapes before the bung is replaced, so the volume collected is too small.
State the effect: B loses no gas, so the measured volume is closer to the true value.

Answer: No gas escapes in B because the flask is never opened, so the volume of gas (and the molar volume) is not too low.

Key idea: Unit 3: 1 h 20 min, 50 marks, skills from Units 1 and 2. Answer exactly what is asked, give one answer, always say why, show working and revise the core practicals.

Check yourself

How many marks is the Unit 3 paper worth?
- 80
- 50
- 90
- 100
Show answer
50. Unit 3 is a 50-mark, 1 hour 20 minute written paper.
What percentage of the full International A level is Unit 3 worth?
- 20%
- 30%
- 40%
- 10%
Show answer
10%. It is 20% of the AS and 10% of the full A level.
A question asks: 'Identify the gas given off when PCl5 is added to an alcohol.' Which answer scores the mark?
- Misty fumes
- Hydrogen chloride
- Hydrochloric acid
- White smoke
Show answer
Hydrogen chloride. The question asks for the gas, so name it. Hydrochloric acid is a solution, and misty fumes is an observation.
A student answers 'white precipitate, so the ion is chloride or bromide'. Why does this usually score nothing?
- Examiners never accept the word precipitate
- Bromide ions do not form precipitates
- A correct answer next to an incorrect one cancels the mark
- The colour must always be given in capitals
Show answer
A correct answer next to an incorrect one cancels the mark. Giving more than one answer, where one is wrong, loses the mark.
Which answer earns credit for why a gas syringe gives a better volume of carbon dioxide than collecting it over water in an inverted measuring cylinder?
- Carbon dioxide is slightly soluble in water, so some would be lost over water
- A gas syringe is more accurate
- A gas syringe looks tidier
- Measuring cylinders cannot hold gases
Show answer
Carbon dioxide is slightly soluble in water, so some would be lost over water. The answer gives a chemical reason. 'More accurate' with no reason does not score.
State two things examiners advise you to do when setting out a calculation in Unit 3.
Show answer
Show all working, and do not round until the final answer (then give it to 2 or 3 significant figures).. Clear working earns method marks even if the final answer is wrong, and early rounding causes errors.

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Last updated 8 November 2026. Spotted a mistake? Tell us.