Experimental design
Choosing apparatus, measuring accurately, and assessing purity.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 12.1.1Name and use appropriate apparatus for measuring time, temperature, mass and volume.
- 12.1.2Suggest suitable methods of purification and identify sources of error.
5 minute read
Experimental design
Practical chemistry questions are really testing one skill: matching the right piece of apparatus, and the right level of precision, to what is actually being measured.
Choosing apparatus
- Volume of liquid: a measuring cylinder for an approximate volume; a pipette for one precise, fixed volume (e.g. 25.0 cm³); a burette for a precise, variable volume that you read before and after adding (used in titrations).
- Mass: a balance, typically reading to 0.01 g or 0.1 g depending on the precision needed.
- Temperature: a thermometer, ideally reading to at least 0.5 °C, used inside an insulated container (e.g. a polystyrene cup) to minimise heat loss to the surroundings, which would otherwise make a measured temperature change too small.
- Time: a stopwatch, started and stopped consistently at defined, visible end points (e.g. when a cross viewed through a solution disappears).
Assessing purity
A pure substance has a sharp, fixed melting and boiling point; an impure substance melts/boils over a range, with the melting point lowered and the boiling point raised compared with the pure substance. This is the standard way to judge whether a prepared or purified sample has succeeded.
Sources of error
Common sources of experimental error to be ready to identify: heat loss to the surroundings (widen with an insulated container/lid), a reaction that is too fast to time accurately (repeat and average, or slow it down), parallax error in reading a scale (read at eye level, at the bottom of the meniscus for liquids), and human reaction-time error when using a stopwatch (repeat and take a mean).
Think of it like this
Choosing apparatus is like choosing a measuring tool at home: you would not use kitchen scales to weigh a gold ring, and you would not use a jeweller's scale to weigh a sack of flour — the precision of the tool has to match the precision the task actually needs.
Worked examples
Method, step by step
A student wants to measure exactly 25.0 cm³ of hydrochloric acid as accurately as possible. Name the most suitable piece of apparatus and explain your choice.
- 1A measuring cylinder could measure roughly 25 cm³ but is not very precise.
- 2A pipette is designed to measure one specific, fixed volume very accurately.
- 325.0 cm³ is a standard pipette volume.
A pipette, because it measures a single fixed volume much more accurately than a measuring cylinder.
Common misconceptions
- Using a measuring cylinder where the question implies high precision is needed (e.g. in a titration) — a measuring cylinder is far less precise than a pipette or burette and would not be accepted as the method there.
- Forgetting that impure substances melt/boil over a *range*, not just at a shifted single temperature — both the range and the direction of shift (melting point down, boiling point up) matter for full marks.
In the exam
- If a question asks how to improve the accuracy of a temperature-change experiment, "use a lid/insulation to reduce heat loss to the surroundings" is almost always a valid, expected answer.
- When justifying an apparatus choice, name the apparatus AND say why (its precision or suitability for that specific measurement) — naming it alone often only earns partial credit.