Part 1: Plan a useful experiment

Start with the chemical question, choose measurements that answer it and explain how each control or improvement makes the evidence more dependable.

Turn the question into a measurement

An experiment tests a relationship or determines a value. First identify what you will change, what you will measure and the chemical link between them. To investigate acid concentration and reaction rate, the independent variable is initial acid concentration; a possible dependent variable is the initial gradient of a gas-volume–time graph. The final volume of gas answers a different question: how much gas formed overall.

Write a prediction with a reason, then choose measurements that could challenge it. A steeper initial graph at higher concentration supports a faster initial rate. It does not by itself establish the reaction order: that requires quantitative comparisons under controlled conditions.

Control the variables that could change the conclusion

Keep temperature, quantities of other reactants, mixing and measurement procedure consistent when testing concentration. For a solid reactant, control exposed surface area as well as mass. The same mass of powder and large chips does not expose the same area. If both concentration and surface area change, a difference in rate cannot be attributed to concentration alone.

A control is not a variable that must be zero. It is a condition held constant or a comparison designed to isolate an effect. In a dilution series, keep total liquid volume constant by adjusting the added water; calculate the initial concentration after mixing, rather than quoting the stock bottle concentration.

Choose a range and repeats for a reason

Use a pilot trial to select a range that gives measurable changes within the available time. A reaction that finishes before the apparatus is sealed cannot provide a dependable initial-rate trace with that setup. An extremely slow trial can allow temperature drift to dominate.

Use several values distributed across the useful range and repeat measurements. Repeats reveal scatter and allow a mean; they do not remove a persistent gas leak or correct a miscalibrated sensor. Investigate an anomalous result before exclusion and record the reason for any exclusion.

Match apparatus to the quantity and tolerance

Choose equipment by the quantity being measured and its required uncertainty. A volumetric pipette delivers one calibrated volume; a burette delivers a variable volume calculated from two readings; a volumetric flask sets a final total solution volume. A measuring cylinder can be appropriate for an approximate excess of reagent even when unsuitable for the accurately measured aliquot.

Read a clear concave meniscus at eye level. Use a pipette filler, follow the pipette calibration and drainage instructions, and remove bubbles from the jet before beginning burette measurements. A diagram should show how gas enters a calibrated collector and why it cannot escape at the connections.

Connect hazard, exposure and a control

A hazard is a potential source of harm; risk also depends on amount, concentration and exposure. A useful control acts on the actual route of exposure. For a volatile flammable solvent, use suitable electrical heating and keep ignition sources away; for harmful vapour, use the appropriate extraction specified by the laboratory assessment. Eye protection is a baseline control, not a complete answer for every chemical.

Plan small workable quantities, appropriate collection of waste and safe apparatus support. These notes support supervised practical learning; the laboratory method and risk assessment determine the actual handling. Do not heat a sealed vessel or create a pressure trap in a gas-producing setup.

Distinguish validity, precision and accuracy

A valid method measures the intended relationship with important competing influences controlled. Precision concerns agreement among repeated measurements. Accuracy concerns closeness to a reference or accepted value. A tight cluster of results can be precise yet displaced by systematic bias.

For example, a gas syringe connection that leaks by a similar amount every run can give closely agreeing volumes that are all too low. More repeats make that agreement more convincing without fixing the wrong answer. Check the seal or choose a better collection method to address the cause.

Evaluate using cause, consequence and remedy

A strong evaluation identifies the specific limitation, explains how it affects the measured quantity and proposes an improvement that targets it. For an exothermic cup experiment: energy loss to air lowers the measured temperature rise, so the calculated magnitude of the exothermic enthalpy is too small; insulation and an appropriate extrapolation method reduce that effect.

Check the direction with an equation. In a gas experiment Vm = V/n, losing gas lowers V while the calculated reacting amount remains the same, so the calculated molar volume is too low. Do not simply write “the answer is inaccurate”. State which result moves and why.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official Edexcel mark allocations.

Sources