IGCSE Science Practical Skills
Practical questions are marked on the same small set of ideas in every subject: which variable is changed, which is measured, which are held constant, and how the method makes the result trustworthy.
The practical paper rewards a fixed vocabulary. Once you can identify the variables and explain why a control is needed, most of the marks in an experimental question are reachable regardless of the specific experiment. This sheet sets out that vocabulary with the phrasing examiners accept.
Variables
| Term | Meaning | How it appears in a question |
|---|---|---|
| Independent variable | The one thing you deliberately change | Usually named in the title, as in the effect of temperature on... |
| Dependent variable | The thing you measure | The quantity recorded in the results table |
| Control variables | Everything kept the same so the comparison is fair | Name them specifically and say how each is held constant |
| Control experiment | A parallel run with the factor under test removed | Shows the effect is caused by that factor and nothing else |
| Fair test | Only the independent variable changes between runs | Marks are for naming the controlled variables, not the phrase itself |
Quality of results
| Term | Meaning | How to improve it |
|---|---|---|
| Reliability | Whether repeating the experiment gives similar results | Repeat and calculate a mean, discarding anomalies |
| Accuracy | How close a measurement is to the true value | Use a more precise instrument, check for zero error |
| Precision | How closely repeated measurements agree with each other | Reduce measurement uncertainty, for example by measuring many items |
| Anomalous result | A reading that does not fit the pattern | Identify it, exclude it from the mean, and repeat that run |
| Resolution | The smallest change an instrument can detect | Choose an instrument with finer divisions |
| Systematic error | A consistent offset in every reading | Check and correct for zero error before starting |
Method and safety phrasing
| Situation | What to write | Why it earns the mark |
|---|---|---|
| Measuring a small quantity | Measure many and divide | Reduces the percentage uncertainty |
| Timing a short interval | Time several repetitions and divide | Reduces the effect of reaction time |
| Reading a scale | Read at eye level, from the bottom of the meniscus for liquids | Avoids parallax error |
| Heating a flammable liquid | Use a water bath rather than a naked flame | Ethanol and other solvents catch fire easily |
| Handling acids or alkalis | Wear eye protection | Corrosive substances damage eyes |
| Using a thermometer | Wait for the reading to stabilise before recording | The bulb needs time to reach the temperature of the surroundings |
How to use this sheet
- Name control variables specifically. Keep everything else the same scores nothing.
- For every improvement you suggest, say what it improves: reliability, accuracy or precision. The link is the mark.
- A control experiment is not the same as a control variable. Say what the control proves, not just that it exists.
- Anomalies should be identified, excluded from the mean and investigated, not silently ignored.
- Safety answers must name the specific hazard and the specific precaution. Be careful is not an answer.
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Frequently asked questions
What is the difference between a control variable and a control experiment?
A control variable is a factor held constant so the comparison is fair, such as temperature or volume. A control experiment is a parallel run with the factor under test removed, such as boiled peas alongside germinating ones, which shows the effect is caused by that factor.
What is the difference between accuracy and precision?
Accuracy is how close a measurement is to the true value. Precision is how closely repeated measurements agree with one another. A set of readings can be precise but consistently wrong, which is what a systematic error such as a zero error produces.
How do I improve reliability?
Repeat the experiment several times under the same conditions and calculate a mean, excluding any anomalous results. Reliability concerns repeatability, so a single careful measurement improves accuracy rather than reliability, and the two are marked separately.
How should I answer a safety question?
Name the specific hazard and the specific precaution that addresses it. Wear eye protection because the acid is corrosive scores, whereas be careful with the acid does not. One hazard and one matched precaution per mark.
Practise the topics this sheet supports
Other reference sheets
Written to the published Cambridge IGCSE syllabuses. Always check the formula list and data sheet issued with your own paper, since the material provided differs between subjects and syllabus versions. Last reviewed 2026-08-12.