Experimental programme and skills in the study of chemistry (including the internal assessment investigation)
The experimental programme, including the internal assessment, tests whether you can design, carry out, process and evaluate an investigation. The exam and IA reward precise measurement, correct uncertainty treatment and evidence-based evaluation.
The IA investigation
- A focused research question identifies independent, dependent and controlled variables and is answerable with available equipment.
- Background should use chemistry to justify the method and predict an outcome.
- The method must allow sufficient relevant data, repeated where appropriate, with safety and ethical considerations addressed.
- Analysis includes processed data, uncertainties, graphs with gradients and a conclusion linked to the research question.
- Evaluation distinguishes random from systematic error and suggests specific improvements.
Measurement and uncertainty
- For an analogue scale, the reading uncertainty is usually half the smallest division; for a digital display it is ± the last digit.
- Absolute uncertainty has the same unit as the measurement; percentage uncertainty = (absolute/measurement) × 100.
- When adding or subtracting, add absolute uncertainties. When multiplying or dividing, add percentage uncertainties. For a power, multiply the percentage uncertainty by the exponent.
- Report a final answer to the same number of significant figures as the least precise measurement, and match decimal places of uncertainty.
Worked example
A 2.500 ± 0.001 g sample is dissolved and made to 250.0 ± 0.2 cm³. Percentage uncertainty in mass = 0.04%; in volume = 0.08%. Concentration involves division, so add percentages: 0.12%.
Traps
- Calling a systematic error "human error" without identifying its effect.
- Quoting a mean to more significant figures than the data support.
- Confusing precision with accuracy.