13.2 Experimental skills and investigations
- Syllabus
- 0620–2026–2027
- Topic
- 13.2
- Level
- —
Learning objectives
13.2.1Select and safely use techniques and apparatus• Demonstrate knowledge of how to select and safely use techniques, apparatus and materials, including following a sequence of instructions where appropriate.13.2.2Plan experiments and investigations• Plan experiments and investigations with suitable variables, methods, apparatus and safety controls.13.2.3Record observations and measurements• Make and record observations, measurements and estimates with appropriate precision and units.13.2.4Interpret and evaluate experimental data• Interpret and evaluate experimental observations and data, including patterns, anomalies and limitations.13.2.5Evaluate and improve methods• Evaluate experimental methods and suggest possible improvements.
Choose apparatus and control laboratory risk
Choose a technique or apparatus by the job it must do, the required measurement quality and the substances involved. Safety then follows from the specific hazard and exposure route, not from a generic precaution list.
| Need | Selection reason | Use or safety control |
|---|---|---|
| measure an accurate fixed liquid volume | volumetric pipette is designed to deliver one calibrated volume | use a pipette filler, never mouth pipette |
| deliver a variable liquid volume accurately | burette gives initial and final readings for volume delivered | clamp vertically; remove the filling funnel before readings |
| collect and measure a gas | gas syringe measures volume directly | make connections gas-tight before the reaction starts |
| heat a flammable mixture | avoid a naked Bunsen flame | use an appropriate non-flame heat source and keep ignition sources away |
| use a reactive or corrosive substance | minimise contact and splashing | wear eye protection and use the stated screen, distance or handling tool where justified |
Follow the method in order because timing, mixing, heating and collection steps can affect both safety and the validity of the result. Identify apparatus from its function and explain why it suits the task.
A precaution must reduce the stated risk: eye protection reduces injury from splashes but does not prevent gas escape or ignition. Do not claim that every hazard is removed by gloves alone.
Plan a fair and testable chemistry investigation
A complete plan changes one independent variable, measures one dependent variable and controls other factors that could alter the result, while specifying a safe repeatable method and how the data will address the investigation aim.
| Planning decision | What to specify |
|---|---|
| question and prediction | expected relationship with a chemistry reason |
| independent variable | suitable number and range of values, including how each value is set |
| dependent variable | exactly what is measured, with apparatus and units |
| controlled variables | how each relevant factor is kept constant and why it could otherwise affect the result |
| method | quantities, apparatus, ordered steps, endpoint and repeat strategy |
| risk control | hazard, possible harm and a matching precaution |
| record and process | results-table headings, calculation or graph, and the comparison used for the conclusion |
To investigate how acid concentration affects reaction rate, vary concentration across a suitable range, measure gas volume against time, and keep temperature, acid volume, carbonate mass and carbonate particle size constant.
‘Keep everything the same’ is not a plan. Name the relevant control variables and say how they are controlled. A variable is not controlled merely because it is measured.
Record observations and measurements faithfully
Record what is seen or measured at the time it occurs. Observations describe evidence; measurements pair a numerical reading with a unit; estimates report a justified approximate value.
| Evidence type | Strong record |
|---|---|
| qualitative observation | specific change such as ‘effervescence’, ‘solid disappears’ or ‘solution turns blue’ |
| analogue reading | read at the correct eye position and to the nearest half-scale division where required |
| digital reading | record all displayed digits with the unit |
| repeated measurements | keep every reading, including a possible anomaly, before calculating or selecting concordant results |
| results table | independent variable first; units in headings; consistent precision within each measured column |
Take enough measurements to reveal the relationship and repeat where appropriate. Constant mass is established by heating, cooling and weighing again until successive masses agree, not by assuming one heating was enough.
Do not change a reading to match the expected result and do not write an inference as an observation. ‘Hydrogen formed’ is an interpretation; ‘a gas was produced that gave a pop with a lighted splint’ records the evidence.
Interpret results and judge data quality
Interpretation turns processed observations or data into a conclusion; evaluation asks how strongly the data support it. Keep the observed pattern, the chemical explanation and the quality judgment distinct.
| Stage | Evidence move |
|---|---|
| process | calculate required quantities or prepare values for plotting without premature rounding |
| represent | use an appropriate graph and best-fit line when the variables are continuous |
| interpret | describe the pattern with variables and cite relevant values or observations |
| conclude | answer the investigation question and justify it from the evidence |
| evaluate | assess repeats, spread, resolution, range and anomalies; state how each limits confidence |
| use graph | interpolate within the measured range; treat extrapolation beyond it as less certain; determine requested gradient or intercept |
Concordant repeats are close enough to support a representative result under the stated criterion. If a result lies away from the pattern, keep it visible, repeat that condition if possible and exclude it from a mean only with a recorded reason.
A best-fit line need not pass through every point or the origin. An anomaly is not automatically a mistake, and a trend alone does not prove a proposed mechanism.
Turn method limitations into specific improvements
A useful evaluation links a concrete weakness to its likely effect on the result, then proposes a feasible change that reduces that effect while preserving the investigation's purpose.
| Limitation or error source | Likely effect | Targeted improvement |
|---|---|---|
| gas escapes before the bung is fitted | measured gas volume is too low | assemble and test a gas-tight system and start collection as reactants mix |
| heat transfers to the surroundings | measured temperature change is smaller in magnitude | use an insulated container with a lid |
| liquid level is read from above or below | parallax shifts the volume reading | read the scale at eye level at the correct meniscus |
| one reading is used | random variation cannot be judged | repeat, identify concordant values and calculate a representative result |
| independent-variable range is narrow | the pattern or turning point may be missed | use more suitably spaced values across a wider safe range |
| an uncontrolled factor changes | comparison is not fair | state how that factor will be held constant |
Write evaluation as cause and remedy: ‘Some gas escaped, so the measured volume was too low; make the apparatus gas-tight before mixing.’ This is testable and tied to the result.
Avoid vague labels such as ‘human error’, ‘use better equipment’ or ‘repeat for accuracy’. Name the source, its direction or consequence where known, and how the proposed change addresses it; repeats mainly improve reliability and reveal random variation.