Planning — OCR A-Level Chemistry
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Planning explained
Planning covers 3 specification points, including (a) experimental design, including to solve… and (b) identification of variables that must be….
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Open a point below for its explanation and revision guidance.
(a) experimental design, including to solve problems set in a practical context
Planning an experiment means turning a problem into a workable procedure. Start by stating the aim and the variable you will measure (the dependent variable) and the one you will change (the independent variable). Choose apparatus that suits the quantities involved, for example a burette for accurate titration volumes or a gas syringe for measuring gas produced. Decide how many readings or repeats to take and how to vary the independent variable across a sensible range. Identify what must be held constant so the comparison is fair. Describe how you will collect and record data, for example in a suitable table, and how you will process it, such as plotting a graph or calculating a mean. Finally, consider safety and reliability: what hazards exist, what control measures apply, and how repeats and careful technique reduce error.
(b) identification of variables that must be controlled, where appropriate
A controlled variable is a factor that could affect the outcome but is deliberately kept the same across all trials so that any change in the dependent variable can be attributed to the independent variable. In a rates experiment, for example, temperature, concentration and particle size might all influence the rate, so the ones not being investigated must be fixed. Identifying which variables need controlling requires you to think about every factor that could influence the measurement, then decide which are being varied and which must be held constant. Some factors cannot be controlled, such as ambient room temperature, and these should be acknowledged as limitations. The phrase 'where appropriate' matters: you control only variables that could realistically affect the result, not every conceivable factor.
(c) evaluation that an experimental method is appropriate to meet the expected outcomes.
Evaluation of method suitability means judging, before or after carrying out a plan, whether the chosen apparatus, technique and measurements can actually deliver the expected outcome. A learner should state the expected outcome (for example a rate constant at a stated temperature, or a mass of product), then test the method against it: does the apparatus measure the required quantity to sufficient precision, does the technique control the variables that matter, and are the hazards manageable? For instance, a titration to find an acid concentration suits a burette reading to ±0.05 cm³, but a gas-collection method suits measuring a volume of gas evolved. The learner weighs strengths against limitations and reaches a justified judgement, not a list of pros and cons.
Your focus
- Design a procedure that solves a problem set in a practical context.
- Identify independent, dependent and controlled variables for a planned experiment.
- Explain how data will be recorded, processed and checked for reliability.
Show all 9 objectives
- Define a controlled variable and distinguish it from the independent variable.
- Identify which factors must be held constant in a given practical context.
- Select the option that names a genuine confounding factor and a realistic control method.
- State the expected outcome of a planned experiment
- Evaluate whether apparatus and technique can deliver that outcome
- Reach and justify an overall judgement on method suitability
Planning flashcards
Card 1 of 11
Controlled variable
Planning exam tips
Marking Points
- States the aim and identifies the independent and dependent variables for the problem set.
- Selects apparatus appropriate to the quantities and precision required by the practical context.
- Describes a procedure that varies the independent variable across a sensible range with repeats.
- Identifies the variables to be controlled and explains how each is held constant.
- Explains how data will be recorded and processed, for example in a table, graph or mean calculation.
- Addresses hazards and control measures relevant to the chemicals and apparatus chosen.
- Defines a controlled variable as a factor kept constant so it cannot affect the dependent variable.
- Identifies factors that could influence the outcome in the given practical context.
- Distinguishes the variable being investigated from those that must be held constant.
- Selects the option that names a genuine confounding factor and a realistic method of controlling it.
- Recognises that some factors cannot be controlled and should be acknowledged as limitations.
- States the expected outcome of the experiment explicitly before judging the method
- Links each chosen piece of apparatus to the quantity it is required to measure
- Explains how the technique controls the key variable that would otherwise affect the outcome
- Identifies a specific limitation of the method and explains its effect on the expected outcome
- Reaches a justified overall judgement on whether the method is appropriate
- Suggests a realistic improvement that would make the method better suited to the outcome
Examiner Tips
- 💡Read the practical context carefully and tailor the method to the named chemicals and equipment.
- 💡Structure the answer as aim, variables, apparatus, procedure, data handling and safety so nothing is missed.
- 💡Justify choices, for example why a burette suits accurate volume measurement, rather than listing equipment.
- 💡State how repeats and controlled variables improve the reliability and fairness of the results.
- 💡Ask which factors could change the dependent variable, then decide which are being varied and which must be fixed.
- 💡Check that each suggested control includes a practical method of keeping it constant.
- 💡Remember that 'where appropriate' means controlling only realistic confounding factors.
- 💡Acknowledge factors that cannot be controlled as limitations rather than ignoring them.
- 💡Open by naming the expected outcome so every later judgement has a reference point
- 💡Use the phrase 'this is appropriate because...' or 'this is not appropriate because...' to force a judgement
- 💡Quantify where possible, for example the burette reading uncertainty, to strengthen the evaluation
- 💡Finish with one concrete improvement and say how it improves suitability
Common Mistakes
- Lists apparatus without linking it to the quantities or precision needed; justify each choice against the measurement.
- Omits controlled variables; name each one and state how it is kept constant.
- Describes a single reading with no repeats; include repeats and explain how they improve reliability.
- Ignores safety; identify the specific hazards of the chosen chemicals and give control measures.
- Confuses the controlled variable with the independent variable; the independent variable is changed deliberately, the controlled variable is held constant.
- Names a factor that cannot affect the result; choose only factors that could genuinely influence the measurement.
- Suggests controlling every conceivable factor; control only those that could realistically affect the outcome.
- Gives a control without a method; state how the variable is kept constant, for example by using a water bath.
- Lists advantages and disadvantages without reaching a judgement; state clearly whether the method is appropriate and why
- Judges the method against a vague aim rather than a stated expected outcome; name the outcome first
- Treats precision of apparatus as the only criterion; also consider control of variables and hazard management
- Describes a different experiment as the improvement; keep the suggested change within the same method