Last updated: 1 October 2026
Experimental planning can initially seem difficult because the question does not simply ask you to recall a fact or describe a practical you have already performed. Instead, you must design a sensible experiment that would produce evidence capable of answering the question.
The chemicals may be unfamiliar. The quantities may differ from those used in school. A familiar technique may even appear in a completely different context. This is why memorising one complete model procedure for every experiment is not enough.
A stronger approach is to recognise the experimental problem hidden inside the question. Once you know what must be found, compared, prepared, separated or identified, you can decide what evidence is required and build the method around that evidence.
Diagnostic Quiz
Try these five questions before reading the chapter. They test whether you can recognise what a planning question is actually asking you to design.
- 5 quick questions
- Focus on experimental reasoning
- Every answer option is explained
Planning questions are evidence-design questions
The central idea in experimental planning is simple: your method must produce evidence that answers the question.
That evidence can take very different forms. A rate investigation may require repeated gas-volume readings over time. A titration requires accurate reacting volumes and an endpoint. Qualitative analysis requires characteristic observations. A salt-preparation plan must show how a pure product will actually be obtained.
Begin by asking: What does the question require me to find out, and what experimental evidence would allow me to do that?
Different planning questions require different experimental reasoning
It helps to recognise the kind of experimental problem before you start writing the procedure.
| Type of task | What you should identify | Typical example |
|---|---|---|
| Investigation | Independent variable, dependent measurement and relevant control variables | How concentration affects reaction rate |
| Quantitative determination | What must be measured accurately and how the measurements give the unknown value | Determine an unknown concentration by titration |
| Preparation | Suitable reaction and the sequence needed to obtain a pure product | Prepare pure copper(II) sulfate crystals |
| Identification | Possible identities, discriminating tests, observations and conclusions | Identify unknown aqueous solutions |
| Separation or purification | Relevant physical-property differences and the correct sequence of techniques | Separate salt from an insoluble impurity |
| Product collection or identification | How the product is produced, collected or observed and how it is confirmed | Identify gases produced during electrolysis |
When do independent and dependent variables matter?
They matter when the question is genuinely asking you to investigate how one factor affects another.
Investigation: How does hydrochloric acid concentration affect the rate of reaction with magnesium?
Here, concentration is deliberately changed, reaction rate is measured and factors such as temperature and magnesium surface area should be controlled.
When should you not invent variables?
Quantitative determination: Determine the concentration of an unknown sodium hydroxide solution using standard hydrochloric acid.
The logic is accurate volumes → endpoint → titre → mole ratio → unknown concentration. There is no need to invent an independent variable or dependent variable.
Preparation: Prepare pure dry copper(II) sulfate crystals from copper(II) oxide and dilute sulfuric acid.
The logic is reaction → excess insoluble reactant → filtration → concentration → crystallisation → filtration → washing → drying.
A planning process that works for every question
The exact experimental details change from question to question, but the following thinking process can be used broadly without forcing every task into a fair-test investigation.
Step 1: Identify the aim
Decide exactly what the experiment must achieve.
The question may ask you to:
- find how one factor affects another;
- determine an unknown concentration or amount;
- prepare a pure substance;
- identify an unknown;
- separate components of a mixture;
- measure a gas volume;
- measure a temperature change; or
- identify products formed during a reaction or electrolysis.
Step 2: Identify the experimental reasoning
Ask whether this is mainly an investigation, determination, preparation, identification, separation or product-collection problem.
If it is an investigation, then identify the independent variable, dependent measurement and relevant controls. If it is not an investigation, use the experimental logic that actually fits the task.
Step 3: Decide what evidence is needed
The experiment must produce something measurable or observable that can answer the question.
Possible evidence includes:
- volume;
- mass;
- time;
- temperature;
- titre;
- colour change;
- formation or disappearance of a precipitate;
- effervescence;
- gas-test result;
- mass of product; or
- appearance of material at an electrode.
Step 4: Choose suitable apparatus and reagents
Apparatus should be chosen because it produces the measurement you need.
| Need | Suitable apparatus |
|---|---|
| Fixed accurate liquid volume | Volumetric pipette |
| Accurate variable liquid volume | Burette |
| Gas volume | Gas syringe |
| Mass | Electronic balance |
| Temperature | Thermometer |
| Time | Stopwatch |
| Separate insoluble solid from liquid | Filter funnel and filter paper |
Step 5: Write a reproducible procedure
Another student should be able to perform the experiment from your plan without having to guess important details.
State quantities, sequence, measurements, observations and stopping conditions clearly where they matter.
Step 6: Explain how the evidence answers the question
Do not stop at “record the results”.
You may need to:
- calculate a titre;
- calculate an unknown concentration;
- compare reaction rates;
- plot a graph;
- calculate a temperature change;
- use observations to eliminate possible identities;
- calculate a yield;
- identify a gas from its test; or
- decide which separation gives the required purified substance.
Step 7: Include relevant safety precautions
Safety statements should be specific enough to show that you understand the hazard.
Weak: Be careful with the acid.
Better: Wear eye protection when handling the acid to reduce the risk of eye injury from splashes.
Step 8: Consider reliability and limitations where relevant
If the question asks for improvements or evaluation, identify a specific weakness before suggesting an improvement.
aim → type of experimental reasoning → evidence → apparatus/reagents → procedure → data treatment/conclusion → safety → evaluation
8 common types of O-Level Chemistry planning questions
The following eight families are useful patterns to recognise. They are not eight fixed questions that must be memorised. The substances and exact wording can change, and one question may combine several practical techniques.
| Planning family | What the question is really asking | Main experimental logic |
|---|---|---|
| 1. Titration | Determine an unknown concentration or amount accurately | Accurate volumes → endpoint → calculation |
| 2. Rate of reaction | Determine how changing one factor affects reaction speed | Change one factor → measure reaction progress → control others |
| 3. Temperature-change experiment | Measure a temperature change associated with a process | Initial temperature → reaction → maximum/minimum temperature → ΔT |
| 4. Gas collection | Measure or identify a gas produced in a reaction | Airtight apparatus → suitable collection → measurement/test |
| 5. Salt preparation | Prepare a pure dry sample of a salt | Suitable reaction → separation → crystallisation/drying |
| 6. Qualitative analysis | Identify unknown substances | Discriminating test → observation → conclusion |
| 7. Separation and purification | Recover a required component from a mixture | Physical-property difference → suitable technique → recover product |
| 8. Electrolysis | Determine what is formed at the electrodes | Electrolyse → collect/observe separately → identify products |
Type 1: Titration planning questions
Hydrochloric acid of known concentration is provided together with sodium hydroxide of unknown concentration. Plan an experiment to determine the concentration of the sodium hydroxide.
How to recognise one
Think titration when an accurate reacting volume can be used with a known concentration and balanced equation to determine an unknown concentration or amount.
Do you need independent and dependent variables?
No. This is a quantitative determination, not an investigation into the effect of one variable on another.
Suggested answer framework
- Use a volumetric pipette to transfer a fixed accurate volume of one solution into a conical flask.
- Add a suitable indicator.
- Fill a burette with the other solution.
- Record the initial burette reading.
- Add solution from the burette while swirling the flask.
- Near the endpoint, add the solution dropwise.
- Stop at the required permanent indicator colour change.
- Record the final burette reading and calculate the titre.
- Repeat the titration to obtain consistent titres.
- Use a suitable mean titre and the balanced equation to calculate the unknown concentration.
What your answer must make clear
- which solution is measured accurately;
- how the endpoint is detected;
- how the titre is obtained;
- that the experiment is repeated appropriately; and
- how the titre leads to the unknown concentration.
Common trap
Do not invent an independent variable simply because the word experiment appears in the question.
Worked Planning Question 1: A determination with no IV or DV
A student is given 25.0 cm³ portions of sodium hydroxide solution of unknown concentration and hydrochloric acid of known concentration. Describe how the student can determine the concentration of the sodium hydroxide.
How to decode the question
The student is not comparing different sodium hydroxide concentrations or testing the effect of another factor. The unknown concentration is simply something to be determined.
Suggested answer
Use a volumetric pipette to transfer 25.0 cm³ of the sodium hydroxide solution into a conical flask. Add a few drops of a suitable indicator.
Rinse and fill a burette with the hydrochloric acid and record the initial burette reading. Add the acid to the sodium hydroxide while swirling the conical flask. Near the endpoint, add the acid dropwise until the indicator shows the required permanent colour change.
Record the final burette reading and calculate the titre. Repeat the titration to obtain consistent titres and calculate a suitable mean titre.
Use the concentration and volume of hydrochloric acid to calculate its amount in moles. Use the balanced equation to determine the amount of sodium hydroxide in 25.0 cm³, then calculate the concentration of the sodium hydroxide.
Why variables are not needed
Nothing is being deliberately changed to observe an effect. The evidence comes from accurately measured reacting volumes, so the useful planning language is about measurement, endpoint, repetition and calculation rather than IV, DV and controls.
Type 2: Rate-of-reaction planning questions
Plan an experiment to investigate how temperature affects the rate of reaction between calcium carbonate and hydrochloric acid.
How to recognise one
These questions ask how changing one factor affects how quickly a reaction occurs.
Common independent variables include:
- concentration;
- temperature;
- surface area; and
- presence or absence of a catalyst.
Suggested answer framework
Possible measurements include:
- gas volume against time;
- loss of mass against time;
- time taken to produce a fixed gas volume; or
- time taken to reach the same visible endpoint.
What your answer must make clear
- what factor is changed;
- how reaction rate is measured;
- which other rate factors are kept constant;
- how the experiment is repeated; and
- how the results are compared.
Common trap
Too vague: Observe which reaction is faster.
Better: Record the volume of gas produced every 10 s using a gas syringe and compare the resulting volume–time data.
Worked Planning Question 2: Rate of reaction investigation
A student wants to investigate how the concentration of hydrochloric acid affects the rate at which magnesium reacts with the acid. Plan an experiment that would allow the student to obtain suitable results and determine how changing the acid concentration affects the reaction rate.
Step 1: Recognise the type of planning task
This is a genuine investigation because one factor is deliberately changed to determine its effect on another.
Step 2: Identify the variables
| Part of the investigation | What it is here |
|---|---|
| Independent variable | Concentration of hydrochloric acid |
| Dependent variable | Rate of reaction |
| Suitable measurement | Volume of hydrogen collected at regular time intervals |
| Relevant controls | Volume of acid, amount and surface area of magnesium, temperature and apparatus |
Step 3: Choose the apparatus
A suitable setup could contain:
- a conical flask;
- a rubber bung;
- a delivery tube;
- a gas syringe;
- suitable measuring apparatus for the acid;
- a stopwatch; and
- equal pieces or equal masses of magnesium.
Suggested answer
Prepare several hydrochloric acid solutions of different concentrations. For each experiment, measure the same volume of acid into a conical flask and ensure that the acid is at the same temperature.
Use the same mass and surface area of magnesium for each experiment. Add the magnesium to the acid, immediately seal the flask with a bung connected to a gas syringe and start the stopwatch.
Record the volume of hydrogen in the gas syringe at regular time intervals, for example every 10 s, until no further gas is produced.
Repeat the procedure using each acid concentration while keeping the volume of acid, amount and surface area of magnesium, temperature and apparatus constant. Repeat trials where appropriate and obtain suitable mean results after considering any anomalous readings.
Plot gas volume against time for the different acid concentrations, or compare the time taken to produce the same fixed volume of hydrogen. The reaction that produces hydrogen more rapidly has the greater reaction rate.
Wear eye protection when handling hydrochloric acid.
Why this answer works
| Part of the answer | Why it matters |
|---|---|
| Different HCl concentrations | Changes the factor being investigated |
| Same magnesium amount and surface area | Prevents another rate factor from changing |
| Same temperature | Temperature also affects reaction rate |
| Gas syringe | Provides a quantitative measurement of reaction progress |
| Readings at regular time intervals | Allows gas production to be followed as the reaction proceeds |
| Repeats | Help assess consistency and reduce the effect of random variation |
| Graph or fixed-volume comparison | Turns raw results into evidence about reaction rate |
This question deliberately changes one factor and measures its effect. Therefore independent, dependent and control variables are an essential part of the plan.
Type 3: Temperature-change planning questions
Plan an experiment to determine the temperature change when two specified solutions react.
How to recognise one
The key evidence is a measured temperature change. The experiment should therefore allow the initial and highest or lowest temperatures to be measured while reducing unwanted heat transfer to or from the surroundings.
Suggested answer framework
- Measure suitable known quantities of the reactants.
- Place them in an insulated arrangement such as a polystyrene cup where appropriate.
- Measure the initial temperature.
- Mix the reactants.
- Stir the mixture so that its temperature is reasonably uniform.
- Record the highest or lowest temperature reached.
- Calculate the temperature change.
- Repeat where appropriate.
Do you always need IV and DV?
No. If the task simply asks for the temperature change of one reaction, there may be no deliberately changed independent variable.
However, if the question asks how concentration affects temperature change, then concentration becomes an independent variable and the temperature change becomes the dependent measurement.
Common trap
Do not simply write “measure the temperature”. State which temperatures are measured and how they are used.
Type 4: Gas-collection planning questions
Calcium carbonate reacts with excess hydrochloric acid to produce carbon dioxide. Plan an experiment to measure the total volume of carbon dioxide produced.
How to recognise one
The central requirement is to collect a gas without significant loss and obtain the required gas measurement or identification.
Suggested answer framework
- Place known quantities of reactants in a suitable reaction flask.
- Connect the flask to a suitable gas-collection apparatus using an airtight bung and delivery tube.
- Start the reaction while minimising gas loss at the beginning.
- Collect the gas.
- Measure the required final gas volume, or record gas volume with time if the question requires rate data.
- Repeat where appropriate.
Gas syringe or collection over water?
A gas syringe allows the volume to be read directly and avoids losing gas through dissolution in water.
Collection over water may be suitable when the gas is insoluble or only slightly soluble in water and the experimental purpose allows it.
Do you need variables?
It depends on the aim.
Measure the total CO₂ produced: no independent variable is necessarily required.
Investigate how acid concentration affects CO₂ production rate: concentration is the independent variable and rate is the dependent variable.
Common trap
If the apparatus is not airtight, some gas can escape and the measured volume may be lower than the amount actually produced.
Type 5: Salt-preparation planning questions
Plan an experiment to prepare pure dry copper(II) sulfate crystals from copper(II) oxide and dilute sulfuric acid.
How to recognise one
The question is asking for a product. The plan must therefore show both how the product is formed and how it is obtained pure and dry.
Suggested answer
Warm dilute sulfuric acid gently. Add copper(II) oxide a little at a time while stirring until some copper(II) oxide remains undissolved.
Filter the mixture to remove the excess copper(II) oxide. Heat the filtrate gently to concentrate the copper(II) sulfate solution, then allow it to cool so that crystals form.
Filter the crystals, wash them with a small amount of cold distilled water and dry them between filter papers.
Why use excess copper(II) oxide?
Copper(II) oxide is insoluble. Adding it until some remains ensures that enough has been added to react with the acid. The unreacted excess can then be removed by filtration.
Do you need variables?
No. This is a preparation problem. The important reasoning is:
Common trap
Do not mechanically write IV, DV and controls for a preparation question. They do not help explain how a pure product is obtained.
Type 6: Qualitative-analysis planning questions
Several unlabelled aqueous solutions are provided together with suitable reagents. Plan a sequence of tests that would allow the solutions to be identified.
How to recognise one
The evidence comes from characteristic observations, not from measuring a dependent variable.
The planning logic
Suggested answer framework
- Consider the possible identities.
- Choose an initial test that separates the possibilities efficiently.
- Use a fresh portion where necessary.
- Add the stated reagent under suitable conditions.
- Record the actual observation.
- Use that observation to narrow the possible identities.
- Carry out further distinguishing tests until each unknown is identified.
Observation first, conclusion second
Observation: A red-brown precipitate forms when aqueous sodium hydroxide is added.
Conclusion: Fe³⁺ ions may be present.
Why use fresh portions?
A reagent added during an earlier test changes the composition of the sample and may interfere with a later test. Fresh portions help keep tests independent where necessary.
Do you need IV and DV?
Normally no. This is an identification problem.
Worked Planning Question 3: Qualitative analysis
Three colourless solutions contain hydrochloric acid, sodium hydroxide and sodium chloride, one in each test tube. Using suitable tests, describe how you could distinguish the three solutions.
How to decode the problem
This is not an investigation. The evidence must come from observations that distinguish the three possibilities.
Suggested answer
Test separate portions of each solution using a suitable indicator.
The hydrochloric acid produces the characteristic acidic indicator result, while the sodium hydroxide produces the characteristic alkaline result. The remaining neutral solution can therefore be identified as sodium chloride.
If further confirmation is required, suitable ion tests can be performed on fresh portions using the correct reagents and conditions, with observations recorded before conclusions are made.
Why this is a different type of planning
There is no need to identify an independent variable, dependent variable or controls. The experimental reasoning is instead: choose a distinguishing test → observe → deduce identity.
Type 7: Separation and purification planning questions
A mixture contains sodium chloride and sand. Plan a method to obtain pure dry sodium chloride.
How to recognise one
The correct procedure comes from a difference in physical properties.
Suggested answer
Add water to the mixture and stir so that the sodium chloride dissolves. Sand remains insoluble.
Filter the mixture. The sand remains as the residue and the sodium chloride solution passes through as the filtrate.
Recover the sodium chloride from the filtrate using a suitable evaporation or crystallisation procedure, then dry the product.
Common physical-property differences
| Difference used | Possible technique |
|---|---|
| One solid is insoluble in the liquid | Filtration |
| A dissolved solid is required from solution | Crystallisation or evaporation as appropriate |
| A solvent is required from a solution | Simple distillation |
| Solids differ significantly in how solubility changes with temperature | Fractional crystallisation |
| Dissolved substances travel differently between stationary and mobile phases | Chromatography |
The order matters
A separation problem may require several techniques. State what happens to the desired substance at each stage.
Do you need variables?
Normally no. The reasoning is based on physical-property differences and sequence.
Type 8: Electrolysis planning questions
Dilute sulfuric acid is electrolysed using inert electrodes. Plan an experiment to identify the gases formed at the two electrodes.
How to recognise one
The question requires products to be formed at electrodes and then observed, collected or identified experimentally.
Suggested answer framework
- Place the electrolyte in a suitable electrolysis apparatus.
- Insert inert electrodes.
- Connect the electrodes to a d.c. power supply.
- Collect the gases produced at the two electrodes separately.
- Test each collected gas using a suitable gas test.
- Use the observations to identify the products.
Example observations
- Hydrogen: gives a squeaky pop with a lighted splint.
- Oxygen: relights a glowing splint.
Do you need independent and dependent variables?
Not if the task simply asks you to identify the electrode products. The relevant experimental reasoning is:
Common trap
Predicting the electrode products is not the same as identifying them experimentally. If the question asks how you would identify the products, state the actual observations or tests.
How to adapt when the question looks unfamiliar
The chemicals and exact wording can change while the underlying experimental reasoning stays familiar.
Question A: Investigate how hydrochloric acid concentration affects its reaction with magnesium.
Question B: Investigate how temperature affects the reaction between calcium carbonate and an acid.
These use different substances and different independent variables, but both are rate investigations.
In both cases, you still need to decide:
- what factor is changed;
- what quantity represents reaction progress;
- which other rate factors must be kept constant;
- what apparatus produces the required measurement;
- how the results will be compared; and
- how reliability can be improved.
Question C: Determine the concentration of an unknown acid using a standard alkali.
Question D: Determine the concentration of an unknown alkali using a standard acid.
These again use different substances, but both follow:
Recognising the logic is more useful than memorising one exact model procedure.
Turning the experimental idea into an exam-quality plan
Once you know what evidence is required, write the method in a logical sequence.
Use quantities where they matter
Too vague: Add some acid to the magnesium and measure the gas.
Better: Measure 25.0 cm³ of hydrochloric acid into the conical flask, add the fixed amount of magnesium, seal the flask immediately and record the gas-syringe reading every 10 s.
Define what is actually measured
Too vague: Measure the rate.
Better: Record the volume of hydrogen collected every 10 s.
Explain what happens to the results
Too vague: Record the results in a table.
Better: Plot gas volume against time for each concentration and compare how rapidly the gas volume increases.
Use repeats for the right reason
Repetition mainly helps you assess consistency and reduce the influence of random variation when a suitable mean is calculated.
Repeating the same method does not automatically remove a systematic problem such as a leaking gas apparatus or a balance that is incorrectly zeroed.
Common experimental-planning mistakes
| Common mistake | Why it is a problem | Better approach |
|---|---|---|
| Writing IV, DV and controls for every planning question | Many planning tasks are determinations, preparations, identifications or separations rather than fair-test investigations | Identify the type of experimental reasoning first |
| Memorising one exact procedure | The chemicals or required measurement may change | Recognise the underlying experimental family and adapt it |
| Beginning with a long apparatus list | The apparatus may not actually provide the evidence required | Decide what evidence is needed before choosing apparatus |
| Writing “keep everything constant” | It does not identify which factors actually matter | Name relevant control variables when the task is an investigation |
| Writing “observe which reaction is faster” | Reaction rate needs a measurable quantity or defined endpoint | State exactly what is measured and against what time |
| Using unsuitable measuring apparatus | The precision may not match the experimental purpose | Match the apparatus to the required measurement |
| Stopping after “record the results” | Raw readings may not directly answer the question | Explain the calculation, graph, comparison or deduction |
| Writing “repeat for accuracy” | Repeats mainly address random variation rather than every source of error | Repeat appropriately, identify anomalies and calculate a suitable mean where relevant |
| Writing “use better apparatus” as an improvement | The specific experimental weakness has not been identified | State the problem and explain how the improvement reduces it |
| Mixing observation and conclusion in qualitative analysis | The experimental evidence is unclear | State what is observed first, then the chemical conclusion |
| Naming a separation technique without explaining the sequence | Many separation problems require several stages | State what becomes the residue, filtrate, distillate or crystals at each stage |
Experimental-planning checklist
Understand the task
- What is the experiment trying to find, compare, prepare, separate or identify?
- Is this really an investigation with variables?
- What evidence would answer the question?
- Which experimental family best matches the task?
Design the method
- Have I chosen suitable apparatus and reagents?
- Are important quantities stated?
- Is the sequence clear?
- Could another student reproduce the experiment?
Make the evidence valid
- If it is an investigation, are relevant control variables kept constant?
- Is the measurement or observation clearly defined?
- Are repeats included where useful?
- Have I stated relevant safety precautions?
Finish the reasoning
- Have I explained how the results will be processed?
- Will the evidence actually answer the original question?
- Have I distinguished observations from conclusions?
- Are any suggested improvements linked to specific weaknesses?
Mastery Quiz
Apply the planning framework to unfamiliar situations. Some questions involve variables and others deliberately do not.
- 10 application-focused questions
- Questions span different planning styles
- Every answer option is explained
Frequently asked questions
Questions students often ask about experimental planning
Strong planning answers come from matching the experimental reasoning to the question rather than using one fixed template mechanically.
Do all planning questions have independent and dependent variables?
No. Independent and dependent variables are particularly relevant when the question is an investigation into how changing one factor affects another. A titration used to determine an unknown concentration, a salt-preparation question, a separation problem or qualitative analysis may not require an independent or dependent variable at all.
What should I identify before thinking about variables?
Identify the aim first. Ask whether the experiment must find, compare, prepare, separate or identify something. Then decide what evidence would answer that question. Only identify IV, DV and controls if the task is genuinely an investigation.
Are there exactly eight possible planning questions?
No. The eight types in this note are useful experimental families, not eight fixed questions. Examiners can change the chemicals, measurements and circumstances, and one planning problem can combine more than one practical technique.
Do I need to memorise all the suggested answers?
No. The suggested answers show how the experimental logic becomes a written plan. Understanding why each step is necessary is more useful than memorising the wording of a complete answer.
What should I do if the chemicals look unfamiliar?
Focus on the experimental purpose and the relevant chemical behaviour. A rate question is still a rate question when different reactants are used, provided one factor is changed and reaction progress must be measured.
How detailed should my procedure be?
It should be detailed enough that another student could carry out the important parts of the experiment without guessing the required quantities, measurements, observations or sequence.
Do I always need to repeat the experiment?
Repetition is useful when repeated measurements help assess consistency or reduce the effect of random variation. Its value depends on the experiment. Repeats should not be added mechanically when they do not address the experimental purpose.
Why is “repeat for accuracy” not the best explanation?
Repeats mainly help assess consistency and reduce random variation when a suitable mean is calculated. They do not automatically correct a systematic problem such as gas leakage or incorrectly calibrated apparatus.
How do I write a good experimental improvement?
Identify a specific weakness first, then explain how the suggested change reduces that weakness. For example, if gas is escaping, checking that the bung and delivery tube are airtight directly addresses the source of the low gas-volume readings.
What is the main idea I should remember in the examination?
Do not begin by mechanically searching for an independent variable. Begin with the aim. Decide what evidence would answer the question, then design the experiment that produces that evidence.
Experimental planning is not one fixed recipe. Some questions are fair-test investigations and genuinely require independent, dependent and control variables. Others ask you to determine an unknown quantity, prepare a pure substance, identify an unknown, separate a mixture or identify experimental products. The most reliable approach is therefore: understand the aim → identify the experimental reasoning → decide what evidence is needed → design a method that produces that evidence. Once you can recognise these patterns, unfamiliar planning questions become much easier to break down and answer systematically.