AS & A-Level Chemistry 18 — Planning experiments, data and evaluation
PublicIndependent Deckloop A Level Chemistry study material aligned with Cambridge International 9701 (2025–2027). Deck 18 of 18: Planning experiments, data and evaluation. Original explanations, worked applications and practice. Not affiliated with or endorsed by Cambridge International Education.
Chemistry
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A-Level
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Designing chemical investigations
A rigorous chemical investigation starts with a clear, testable prediction based on underlying chemical principles. Variables must be categorised clearly: the independent variable is manipulated by the experimenter, the dependent variable is measured as the outcome, and controlled variables are kept as constant as practicable to ensure a fair test.
A reproducible quantitative procedure specifies suitable apparatus, measured quantities, and a defined range with logical intervals—for example, five or more distinct values over a justified range, with repeats. Choose apparatus with suitable calibration and uncertainty; for instance, a burette or volumetric pipette is chosen over a measuring cylinder for accurate volume measurement of standard solutions.
When planning thermal, rate or equilibrium investigations, controlling or measuring the temperature appropriately is vital, usually achieved with a thermostatically controlled water bath rather than direct heating. For kinetics, the timing mechanism and method of measurement must be explicit, whether using continuous monitoring (like collecting gas in a gas syringe) or a quenching method followed by titration. Equilibration investigations must specify allowing sufficient time for the system to reach chemical equilibrium before taking a measurement. Finally, the plan must state precisely how the raw data will be processed, such as calculating derived values or plotting specific variables on a graph to determine an unknown quantity.
A reproducible quantitative procedure specifies suitable apparatus, measured quantities, and a defined range with logical intervals—for example, five or more distinct values over a justified range, with repeats. Choose apparatus with suitable calibration and uncertainty; for instance, a burette or volumetric pipette is chosen over a measuring cylinder for accurate volume measurement of standard solutions.
When planning thermal, rate or equilibrium investigations, controlling or measuring the temperature appropriately is vital, usually achieved with a thermostatically controlled water bath rather than direct heating. For kinetics, the timing mechanism and method of measurement must be explicit, whether using continuous monitoring (like collecting gas in a gas syringe) or a quenching method followed by titration. Equilibration investigations must specify allowing sufficient time for the system to reach chemical equilibrium before taking a measurement. Finally, the plan must state precisely how the raw data will be processed, such as calculating derived values or plotting specific variables on a graph to determine an unknown quantity.
Key points
- The independent variable is altered, the dependent variable is measured, and controlled variables are kept as constant as practicable.
- Choose enough distinct values to resolve the expected trend, with an appropriate range, intervals and repeats; five is a possible plan, not a universal reliability threshold.
- Apparatus selection depends on required precision (e.g., an analytical balance reading to , a burette to ).
- Use a thermostatically controlled water bath to maintain constant temperatures, avoiding direct heating which causes fluctuations.
- Rate investigations may require quenching to rapidly halt the reaction at specific times before performing a titration.
Worked example
Question
A student plans an investigation to determine how the initial rate of reaction between solid magnesium ribbon and aqueous hydrochloric acid depends on the acid concentration. Identify the independent and dependent variables, and suggest two variables that must be controlled.
Solution
1. Identify the parameter being deliberately changed: this is the concentration of the acid.
2. Measure hydrogen volume at short time intervals from mixing, then estimate the initial tangent of the volume–time graph. Total volume divided by the full reaction time is an average, not the initial rate.
3. Determine factors that could affect the rate but must be kept constant to ensure a fair test: temperature, and the surface area and mass of the magnesium ribbon.
Vary the acid concentration; derive the initial hydrogen evolution rate from the initial volume–time gradient. Keep temperature, acid volume and magnesium mass, exposed surface area and surface preparation consistent.
2. Measure hydrogen volume at short time intervals from mixing, then estimate the initial tangent of the volume–time graph. Total volume divided by the full reaction time is an average, not the initial rate.
3. Determine factors that could affect the rate but must be kept constant to ensure a fair test: temperature, and the surface area and mass of the magnesium ribbon.
Vary the acid concentration; derive the initial hydrogen evolution rate from the initial volume–time gradient. Keep temperature, acid volume and magnesium mass, exposed surface area and surface preparation consistent.
Common pitfalls
- Confusing dependent and independent variables: remember that the independent variable is the one you deliberately change, while the dependent variable is the outcome you measure.
- Suggesting a Bunsen burner to maintain a specific reaction temperature like ; a thermostatically controlled water bath is required for precise thermal regulation.
Prerequisites
- Study AS practical measurement and chemical analysis first.
- Study Thermodynamics and electrochemistry first.
- Study Quantitative equilibria and kinetics first.
- Study Group 2 and transition-metal chemistry first.
- Study Synthesis, chromatography and NMR first.