Evaluating Experiments

Why this skill matters

Evaluation questions appear on both Paper 5 and Paper 6, typically worth 3-5 marks. They test whether you understand the limitations of experimental methods and can suggest specific improvements. Examiners distinguish between students who give generic “human error” answers (0 marks) and those who identify precise sources of error with matched improvements.

Method and technique

Identifying sources of error

For each experiment, consider:

Heat loss: In energetics experiments, heat escapes to the surroundings (the air, the container, the thermometer). This makes temperature changes smaller than they should be, giving inaccurate energy values.

Gas loss: When a gas is produced, some escapes before collection begins (especially during the first moments of the reaction). This makes the measured gas volume less than the actual volume produced.

Incomplete reactions: If the reaction does not go to completion, the actual yield is lower than expected, and percentage yield calculations are affected.

Measurement limitations: Every instrument has a resolution limit. A thermometer reading to 1 degree C cannot detect changes smaller than 1 degree C. A measuring cylinder to the nearest 1 cm3 introduces uncertainty.

Timing errors: Starting and stopping a stopwatch by hand introduces reaction time errors of about 0.2-0.5 seconds. For fast reactions, this is a significant proportion of the total time.

Suggesting improvements

Every error must be paired with a specific improvement:

Source of error	Improvement
Heat loss to surroundings	Use a polystyrene cup (insulated container) / put a lid on the cup
Gas escaping at the start	Use a gas syringe connected before adding reactants / use a sealed system
Incomplete reaction	Use excess of one reagent / allow more time for reaction
Inaccurate volume measurement	Use a burette instead of a measuring cylinder
Timing errors in fast reactions	Use a data logger / light sensor instead of manual timing
Impure product (wet crystals)	Dry crystals in an oven and reweigh until constant mass

Assessing reliability

To improve reliability:

• Repeat the experiment at least twice at each condition
• Calculate a mean from concordant results
• Identify and exclude anomalous results
• Use the same equipment and method each time

Assessing accuracy

To improve accuracy:

• Calibrate instruments before use
• Use more precise apparatus (burette instead of measuring cylinder)
• Reduce systematic errors (insulate, seal, control variables better)

Common errors and how to avoid them

Saying “human error”: This is too vague and scores zero. Examiners want specific errors: “parallax error when reading the measuring cylinder” is specific; “human error” is not.

Not matching improvements to errors: Listing errors and improvements separately, without pairing them, makes it unclear which improvement addresses which problem.

Suggesting unrealistic improvements: “Use a computer to do the experiment” or “buy better equipment” are not specific enough. Name the actual apparatus or technique.

Confusing systematic and random errors: A thermometer that reads 2 degrees too high is a systematic error (consistently wrong in one direction). A hand slightly unsteady when reading the meniscus is a random error (varies each time).

Marking points examiners look for

• Specific, named sources of error (not “human error”)
• Improvements matched to specific errors
• Repeat readings mentioned for reliability
• Appropriate apparatus upgrades suggested (with names)
• Understanding of accuracy vs reliability
• Anomalous results identified and explained