5 Experimental skills
- Syllabus
- 2024
- Section
- 5
- Level
- —
Problem-solving and application
- Syllabus
- 2024
- Topic
- —
- Level
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Solve a problem set in a practical context
A practical-context problem combines a chemical goal with information about an experiment. Solve it by translating the apparatus, quantities and observations into the chemistry they represent.
| Step | Action | Check |
|---|---|---|
| 1 | identify exactly what must be found, chosen or explained | distinguish the target from background detail |
| 2 | extract the relevant measurements, units, observations and conditions | include labels from diagrams and tables |
| 3 | connect each relevant fact to a chemical idea, relationship or equation | state why the connection applies here |
| 4 | carry out the reasoning or calculation in a clear sequence | keep units and use data consistently |
| 5 | check the result against the experiment | ask whether its sign, size, trend and units are plausible |
For a numerical problem, show the relationship, substitution, answer and unit. For a choice problem, compare the options against the experimental goal and justify the choice using the supplied evidence.
Do not copy every number from the stem or start calculating before identifying the target. A correct chemical fact earns its place only when it is connected to the particular experiment and question.
Apply scientific knowledge to a practical situation
To apply chemistry in a practical context, select the scientific idea that controls the situation and use it to explain the observed or intended result.
| Practical detail | Relevant science | Result |
|---|---|---|
| a reagent is deliberately in excess | the other reagent is completely used up | the limiting reagent controls the amount formed; an exact excess volume or mass may be unnecessary |
| a solid reactant is in excess when preparing a soluble salt | all acid reacts and surplus solid can be filtered off | the filtrate contains the soluble salt rather than unreacted acid |
| gas in an apparatus is heated before the reaction begins | gases expand when heated | expanding trapped air can make early bubbles that are not reaction-product gas |
Build an explanation as a causal chain: experimental detail → relevant chemical or physical principle → consequence. Name the actual substance, measurement or observation wherever possible.
Do not give a memorised fact without applying it. For example, saying only “it is in excess” is incomplete when the question asks why: state what is fully reacted, why precision changes, or how the excess is removed.
Investigation planning and methods
- Syllabus
- 2024
- Topic
- —
- Level
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Devise a testable investigation plan
An investigation plan turns a scientific question into a method that can produce relevant evidence. Every choice of technique should follow from what must be changed, observed or measured.
| Planning decision | What to state | Why it matters |
|---|---|---|
| question and prediction | the relationship being tested and the expected outcome | keeps the evidence tied to a chemical idea |
| change and measurement | what will be changed and what result will be observed or measured | makes the investigation testable |
| conditions | which other relevant conditions will be kept consistent | prevents a second cause changing the result |
| technique and apparatus | a method with suitable range, precision and chemical compatibility | produces measurements or observations fit for the question |
| range and repetition | enough values to reveal a pattern and repeated measurements where appropriate | makes the proposed conclusion better supported |
| evidence decision | the comparison, graph or calculation that will answer the question | shows how results will test the prediction |
To test whether a deduction applies beyond one substance, repeat the same investigation with additional suitable substances while keeping the comparison method consistent. A wider evidence set can test the proposed general pattern.
Do not write a list of apparatus before defining the evidence needed. A valid plan is specific enough to follow, but detailed variable classification, recording conventions and evaluation belong to their own later skills and should not be substituted for the plan itself.
Choose and justify a safe experimental method
An appropriate experimental method uses apparatus and techniques that achieve the chemical purpose while controlling the specific hazards and avoidable losses in that experiment.
| Purpose | Appropriate choice or technique | Reason |
|---|---|---|
| measure a chosen liquid volume | measuring cylinder; use a pipette or burette when their precision or delivery function is required | match capacity and precision to the task |
| collect and measure a gas | gas syringe connected without leaks | records gas volume directly |
| evaporate solvent to form crystals | evaporating basin with controlled heating | provides a wide heated surface |
| show heating or reaction is complete | cool and weigh, then repeat heating, cooling and weighing until constant mass | unchanged mass shows no further measurable change |
| prevent droplets or solid leaving while gas escapes | a loose cotton-wool plug or suitable cover | reduces material loss without sealing a gas-producing vessel |
| Hazard | Control | Why the control fits |
|---|---|---|
| flammable liquid | remove naked flames; use indirect heating such as a water bath | reduces ignition risk |
| toxic gas | use a fume cupboard | contains and extracts the gas from the breathing zone |
| corrosive or irritating liquid | wear eye protection and avoid skin contact | reduces exposure to splashes |
Write safety as a linked pair: name the specific hazard, then the control that reduces exposure or ignition. Write apparatus choices as purpose → apparatus → reason, not as an unlabelled equipment list.
Never tightly seal an apparatus that produces or heats a gas. Do not claim that goggles control toxic inhalation or that a fume cupboard replaces careful handling; each control must match the actual route of harm.
Observations, measurements and variables
- Syllabus
- 2024
- Topic
- —
- Level
- —
Observe, measure and record with appropriate precision
A useful experimental record states what was directly observed or measured, uses precision supported by the instrument, and presents values so their meaning and units are unambiguous.
| Evidence type | Record | Avoid |
|---|---|---|
| qualitative observation | exact colour, precipitate, effervescence, state or other visible change | explaining the cause in place of the observation |
| scale reading | read at eye level, identify the scale interval, and record to the required supported precision | adding unsupported decimal places |
| change in a quantity | final reading minus initial reading, with a sign or stated rise/fall where needed | reversing readings or dropping the unit |
| repeated or ordered data | use a table with one row per condition or trial | prose that hides which value belongs to which condition |
| Condition | Initial temperature / ∘C | Highest temperature / ∘C | Temperature rise / ∘C |
|---|---|---|---|
| trial 1 | 16.0 | 32.4 | 16.4 |
Put the unit in the heading rather than repeating it in every data cell. Use a consistent number of decimal places for measurements made with the same instrument; calculate a difference from unrounded recorded readings and report it at matching precision.
Precision is the fineness and consistency of recording, not a guarantee that a value is close to the true value. Do not write more digits than the scale or stated requirement supports.
Identify independent, dependent and control variables
In a fair investigation, the independent variable is deliberately changed, the dependent variable is measured or observed, and control variables are kept constant so they do not provide alternative causes for the result.
| Variable type | Diagnostic question | Example when comparing reactions of carboxylic acids with magnesium |
|---|---|---|
| independent | what does the investigator deliberately change? | identity of the carboxylic acid |
| dependent | what result is measured or observed? | reaction time, gas volume in a fixed time, or another stated rate measure |
| control | what else could affect that result and must stay constant? | acid concentration and volume; magnesium mass and surface area; temperature |
First write the investigation as “effect of ___ on ___”: the first blank identifies the independent variable and the second the dependent variable. Then select control variables by asking which other factors could change the dependent result.
Name a measurable quantity and how it is held constant—for example, “use 25.0cm3 of acid each time”—rather than writing only “same acid”. Keep the apparatus arrangement consistent when it could affect the measurement.
A control variable is not the same as a control experiment. Do not call every unchanged feature a control variable: include conditions that could materially affect the dependent result.
Analysis, communication and evaluation
- Syllabus
- 2024
- Topic
- —
- Level
- —
Analyse data and draw an evidence-based conclusion
Analyse experimental data by identifying the pattern, testing it against every result and using relevant scientific knowledge to explain what the evidence supports.
| Move | What to do |
|---|---|
| describe | state how the dependent result changes as the independent variable changes; include direction and any plateau, maximum or minimum |
| check | identify results that do not fit the overall pattern and verify that they are genuinely anomalous |
| interpret | connect the pattern to the relevant chemical idea without claiming more than the data show |
| conclude | answer the investigation question and cite the trend or observation that supports the answer |
A graph may show that increasing acid volume decreases conductivity over a stated range, or that longer carbon chains take longer to produce a fixed gas volume. An observation such as no zinc remaining supports the conclusion that zinc was limiting and acid was in excess.
Do not describe one point as a trend, ignore an inconvenient result, or turn association into unsupported causation. A conclusion must remain consistent with the tested range and experimental evidence.
Communicate findings with calculations and graphs
Communicate findings so another reader can reconstruct what was measured, how values were processed and what the graph or calculation shows.
| Form | Required features |
|---|---|
| results table | descriptive headings with units, consistent precision, one row per condition or trial |
| calculation | relationship or formula, substitution, working, unit and appropriately precise result |
| graph | independent variable on x, dependent variable on y, labelled units, sensible linear scales using the grid, small accurate points |
| best fit | a straight line for an approximately linear relationship or a smooth curve for a changing gradient; balance the overall pattern rather than join point-to-point |
Exclude a justified anomaly when fitting the pattern. Read an intersection, maximum or plateau from the best-fit representation at the precision supported by the grid, and report both value and unit.
A curve of best fit is not a chain of straight segments. Do not force every point onto the line, use uneven unmarked scales, omit units, or hide a calculation behind a bare answer.
Assess whether experimental results are reliable
Reliable results are consistent when a measurement or investigation is repeated under the same conditions.
| Evidence | Reliability judgment |
|---|---|
| repeated values are close together | good repeatability; the result is more reliable |
| repeated values have a wide spread | poor repeatability; the result is less reliable |
| one value is far from the others | investigate it as a possible anomaly before calculating a representative value |
| only one measurement exists | reliability cannot be assessed directly from repeats |
Repeat each condition, identify anomalies using the pattern and repeat evidence, then calculate a mean from justified consistent results. More repeats strengthen the estimate only when the method and conditions remain the same.
A repeated measurement can be tightly grouped yet systematically wrong. Reliability concerns consistency; it does not by itself prove accuracy or validity.
Evaluate accuracy and validity, then improve the method
Evaluate an experiment by identifying a specific weakness, explaining how it changes the measured result, and proposing a change that directly reduces that effect.
| Idea | Question | Example weakness | Targeted improvement |
|---|---|---|---|
| accuracy | how close is the result to the accepted or true value? | gas escapes before the syringe is connected, so measured volume is too low | connect the apparatus quickly and check for leaks |
| validity | does the method measure the intended effect in a fair comparison? | initial temperatures differ, so temperature change is not caused only by the tested variable | equalise or record both initial temperatures and compare changes |
| systematic effect | does the same bias act in one direction? | product retains water, making mass and percentage yield too high | dry to constant mass before weighing |
| random effect | do readings vary unpredictably? | judging a fluctuating maximum temperature | stir for an even temperature and repeat the measurement |
Use the chain weakness → effect on data → improvement. Cotton wool can stop acid spray while allowing gas to escape, so mass loss represents the gas more accurately; a condenser reduces loss of water vapour and raises collected yield.
“Human error”, “be more careful” and “repeat” are not complete accuracy improvements unless the source and direction of error are explained. Repeating improves reliability but does not remove a systematic bias or an invalid design.