IGCSE Chemistry Practical Skills 4: Planning, Safety and Evaluation
Cambridge IGCSE Chemistry 0620 and 0971 practical notes on variables, method design, ranges, risk, data processing, conclusions and improvements.
Practical Skill 4 owns the complete planning and evaluation process shared by Cambridge IGCSE Chemistry Papers 5 and 6. The official AO3 contract requires variables, range, apparatus justification, reproducible procedure, risk control, recording, processing, prediction, conclusion, anomaly handling, source-of-error analysis and targeted improvement.
Turn the question into variables
Write the aim as a relationship: how changing the independent variable affects the dependent variable.
- Independent variable: deliberately changed.
- Dependent variable: measured response.
- Control variable: relevant factor held constant.
Operational definitions matter. “Rate” is too broad until defined as initial gas-volume gradient, mass lost per second, time to a fixed endpoint or another measurable proxy.
For acid concentration against reaction rate, specify concentration values and a dependent measure. Keep acid volume, solid mass, particle size, temperature, apparatus and mixing method constant where relevant.
Choose a useful range and intervals
Use enough independent-variable values to reveal a trend, commonly at least five where resources and context permit. Spread them sensibly across a safe, measurable range.
Two values permit only a comparison, not a well-defined trend. Closely clustered values may produce changes smaller than instrument resolution. An extreme value that reacts before the apparatus is sealed is also unhelpful.
Pilot evidence can refine a range. If every endpoint is under two seconds, reduce concentration or temperature. If change is barely detectable, increase the range or select a more sensitive dependent measure.
Do not claim one universal number of repeats or values. Justify the choice from pattern definition, time, safety and measurement resolution.
Select apparatus and justify it
Name capacity or resolution where it affects quality. A gas syringe is preferable to bubble counting when gas-volume rate is required because bubble sizes vary. A burette measures a changing delivered volume more precisely than a measuring cylinder. An insulated cup with lid reduces energy transfer in temperature-change work.
Apparatus must fit the predicted range. A 100 cm³ gas syringe cannot record 150 cm³ without stopping or losing gas. A thermometer must cover the expected temperatures.
“Use accurate apparatus” is incomplete. State the instrument and why its range or resolution suits the measurement.
Write a reproducible procedure
A reader should be able to repeat the investigation without guessing.
