Experimental design and laboratory skills
This section tests whether you can plan a fair test, choose and use apparatus correctly, record raw data properly, and identify the type of error present.
Core rules
- A hypothesis must be testable and falsifiable; change one independent variable, measure one dependent variable, hold everything else constant.
- Include a control group and replicate measurements (usually three) to improve reliability; randomise or standardise where bias could enter.
- Accuracy is closeness to the true value; precision is closeness of repeated values to each other. Systematic error shifts all readings the same way (zero error, miscalibration); random error scatters them.
- Technique: read the meniscus at eye level; zero a balance before weighing; use a standard curve of absorbance against known concentrations for colorimetry; in titration accept concordant titres within about 0.10 cm³.
Worked example
To test light intensity on Elodea photosynthesis, set lamp distances of 10, 20, 30 and 40 cm and count oxygen bubbles per minute. Control temperature with a 25 °C water bath, use equal masses of plant, identical NaHCO₃ concentration and the same counting time; place a heat filter between lamp and beaker because a hot lamp changes temperature as well as light. Repeat each distance three times and average. If counts at one distance vary widely, extend the counting period rather than discarding results.
Traps that cost marks
Changing two variables at once; omitting a control; taking a single measurement; using a mean of non-concordant titres; discarding apparent anomalies without justification; forgetting parallax when reading scales.
Key skills to drill
Write a method as a numbered sequence with quantities, then state one improvement and the error it addresses.